Stuck on what costume to wear for this Halloween? Perhaps you should look to nature for some suggestions! Here are a few classics:
Placing sand, kelp, and stinging anemones all over your body
Covering yourself with feces and/or the corpses of your enemies
Urinating in mud and then bathing in it for an especially masculine look
Removing all your clothes and sticking a blade of grass in your ear
Truly, this is the best time of the year.
In the spirit of the season I thought I’d write a bit about animals that like to make costumes for themselves. In a strictly scientific sense, this is called “decorating behavior” and spans from full-body coverings to the most tastefully sparse jewelry. As for why animals decorate, the reasons vary. For example, the juveniles of one species are known to use it to signal the adults to give them sugar-rich substances. (I am referring, of course, to Trick-or-Treating.)
Before we delve deeper into what causes decorating behavior, though, let’s first make sure we understand what decorating behavior isn’t. With the exception of feces, excretions that come from the animal itself aren’t considered decorative. For example, when you brush your hair, it draws oil secretions from your scalp down the follicles- but you wouldn’t think of that as putting something in your hair, would you? Many animals ooze stuff that they rub on their bodies, but for it to count as decoration it must come from the external environment. Feces are an exception because they pretty much become part of the environment as soon as they make their way out of the body.
So, the first part of our decoration definition states that decorations must be something picked up from the environment. But not everything an animal picks up is a decoration. Food items aren’t, obviously. And neither are tools. For something to be considered decoration, an animal must place it on its body and retain it there. No eating, no lock-picking or anything like that. Nuts in a hamster’s cheek pouches do not count as decorations.
Have a good idea of what we’re talking about now? Ok, so let’s look at the many reasons why animals decorate themselves. The most popular and well-studied one is one you’ve already thought of- defense.
But defense against what? Your mind likely immediately jumps to ‘predators,’ but predators are honestly only one of many, many ways to die. I mean, humans don’t have predators, yet roughly 500,000 people managed to do themselves in by tripping over furniture in 2013. The environment is truly deadly.
Sea urchins cannot trip, given that they have no legs, but they do suffer environmental damage when currents smack them around and tangle seaweed up in their poky bits. Many urchins will cover themselves in little rocks in order to shield themselves from these hazards, which is amazing because I bet you never considered the fact that an animal composed nearly entirely of sharp points would need more defense.
This sea urchin has made a charming outfit out of seashells. (Photo by Brocken Inaglory. CC by SA 3.0)
The rocks also act like a very thick layer of sunscreen, protecting the urchin from UV damage. Keep that in mind as a feasible alternative the next time you forget your Coppertone at the beach.
Parasites are another non-predator hazard that many critters face; this is a common explanation for why so many animals roll around in mud. Yet you must also consider the danger of parasitoids, which are like parasites except they kill you in the end and burst out of your gut alien-style. Obviously this is something most animals are interested in avoiding, particularly the larvae of leaf beetles, who are aggressively attacked by wasps looking for a warm gooey place to lay their eggs. The larvae’s avenue of defense is to gather up bits of its shed skin and feces and weave them into a little umbrella using, I kid you not, a part of their anatomy called an ‘anal turret.’ They stick the shield onto another part of their anatomy with the equally charming name ‘anal fork.’
The poop umbrella also shields against weather hazards, and some species actually swing it around to deter predators. Amazing. (Photo by Manfred Kunz. CC BY-SA 3.0)
Now that you’ve digested that bit of information, let’s go into costumes that are strictly antipredator.
The most famous decorating animal is probably the aptly-named decorator crab. Though calling it the decorator crab is a bit of a misnomer, as many species of crab within the superfamily Majoidea (the spider crabs and relatives) decorate themselves, and the trait is not monophyletic. Apparently, the behavior was profitable enough to have evolved separately in multiple lineages. In any case, watching them decorate themselves is fascinating and adorable.
Without decorations, there is nothing particularly remarkable about the looks of a decorator crab species, aside from their being somewhat…. fuzzy. Like all crabs, their bodies are covered by a hard carapace (exoskeleton), but on this carapace they have millions of tiny, hooked setae that act like velcro.
But it is highly unlikely that you will ever see a decorator crab that isn’t costumed. To them, nudity is more than embarrassing, it’s deadly. Multiple studies that all involved disrobing a poor crab have confirmed that they are much more likely to get picked off if in the buff.
Here is a naked spider crab. Let’s, uh, cover that up. (Photo by Hans Hillewaert. CC by SA 4.0)
As to exactly how the crabs’ clothes work as a defense, that really depends on what it wears. Crabs that swathe themselves in sand, small rocks, and bits of kelp are usually going for basic camouflage. But many species also pick up other living animals- that is, sponges, bryozoans, algaes, and anemones- are usually trying to co-opt some chemical defense. All of these creatures can secrete noxious substances, and in the case of the anemone, deliver painful stings. You might think that these sessile organisms wouldn’t be very happy to be picked up and moved around, but it seems the benefits go both ways. The crab gets a potent predator deterrent, while the anemone, by virtue of being transported, is able to feed on diverse food items and gets better water flow through its tentacles, which helps with respiration.
I promise you, there is a crab under there. (Image from divegallery.)
Aside from all that, the decorator crabs get an additional defense boost simply by appearing larger. Many of their predators can’t swallow prey over a certain size limit.
Camouflaging with live animals is fairly unusual outside of the seafloor, however, since most terrestrial animals tend to move around too much to be considered decorations. On the other hand, as many taxidermists know, corpses and castoffs make a fine addition to any collection. The larvae of several species of assassin bugs weave ‘backpacks’ out of the actual dead bodies of their victims- ants, in most cases. After paralyzing the hapless critters and sucking out their internal fluids, the assassin bug bundles up the ant’s remaining exoskeleton in sticky thread it secretes from its abdomen and sticks it there.
This is an effective antipredator defense, as shown in at least one study, because most predators don’t like messing with ants- they bite, sting, and have a nasty habit of ganging up on you and ripping you into bite-sized pieces. Seeing an amorphous cloud of ants trundling around is apt to make any spider run far in the opposite direction. If they do manage to strike up the courage to attack, they’re likely to grab onto a faceful of dessicated ants while the bug itself continues merrily on its murdery way.
I can’t forget my backpack if I’m going to assassin bug school! (Photo by Getty Images.)
Invertebrates are not the only creatures to change their looks via the environment. Some bird species, like the rock ptarmigan, disguise their feathers with mud in order to blend in when the winter snow melts. Curiously, only males display this behavior, which suggests that bright white plumage is attractive to the ladies (who molt into darker colors as soon as the season turns). In fact, males that dirtied themselves usually did so immediately after their mate laid eggs; aka, ‘now that she’s pregnant I no longer have to put effort into looking good!’ Nice one, guys.
Left: Clean ptarmigan. Right: A dirty, dirty bird.
Pigs are another group of animals well-known for getting dirty, or wallowing, as it’s called. This is not to hide, however, but for the dual purpose of suffocating parasites and cooling off via evaporation. Wild boars also use mud for another surprising reason: to look sexy. Males do most of their wallowing in autumn, not a particularly hot or parasite-ridden season- but one that coincides with the mating rut. As to why wallowing coincides with sexytimes, the scientific jury’s still out. But they aren’t the only ungulate (hoofed animal) to do this. The bucks of many deer species have a lovely habit of urinating on the ground and then wallowing in it, coating themselves with manly-stinking mud. Sometimes they even forego the mud and put their heads down to urinate directly on their own faces. Aren’t deer majestic.
Ungulates in general are just obsessed with pee. (Photo from IndiaWilds.)
Bearded vultures (which also go by the name lammergeier) also have a habit of rolling in mud, but it isn’t to disguise themselves, or to get rid of parasites, or even to attract mates. Though many have seen and admired the elegant look of the bearded vulture, with its pinkish-to-rusty chest plumage, few realize that this lovely color is actually not naturally occurring: bearded vultures kept in strict captivity actually have white chests and heads. Wild vultures attain their looks by bathing in iron-enriched mud and soil. Higher-ranking birds within the vulture hierarchy tend to have darker red feathers. This has lead some researchers to believe that the mud is a status symbol like human makeup, indicating that the bird has the leisure of seeking out this specific mud and rolling in it for long periods of time. In fact, they can spend over an hour staining themselves, and prefer to do so without being watched.
Variations in vultures.
You can’t strictly call bearded vulture mud-staining a cultural thing, of course, because even birds raised by humans in captivity will perform the behavior: this means it’s innate, and it has a good evolutionary backing behind it. But animals who dress themselves up purely for its own sake do exist- animals besides humans, I mean. A recent paper on some of our closest relatives found that arbitrary self-decorating ‘fads’ can be passed around in chimpanzee. The fad in question was sticking a piece of grass in their ear.
(Photo by Smithsonian.)
This fashion statement was started by a chimpanzee named Julie for mysterious reasons- maybe she had an itchy ear?- but was quickly picked up by other members of the group. And again: there’s no evidence that having a blade of grass sticking out of your ear confers any actual benefit! Other local groups of chimps didn’t do it. It was purely cultural.
Costuming without a direct defensive or sexual benefit is, of course, pretty rare in the animal kingdom. This is because there are very specific costs to dressing up. The biggest one, of course, is carrying around that added weight. Hermit crabs, for example, can move a whole lot faster if they aren’t carrying a shell (but nobody wants that, because they are actually gross and hideous creatures without them).
PUT IT BACK! PUT IT BACK!!
Likewise, many costumes take a lot of energy just to construct. Caddisfly larvae build very elaborate and time-consuming butt cases out of sand or twigs or whatever detritus is in their environment. If you are an asshole like certain scientists are, you can remove the case and watch them struggle to build the whole thing all over again. Larvae that have to build cases several times end up actually being smaller as adults because of all the effort they have to expend.
Shh…. it’s sleeping now.
So that’s what animals wear! Hope this gave you a few costume ideas to scare the kids with!
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Mondy, N., Rey, B., & Voituron, Y. (2012). The proximal costs of case construction in caddisflies: antioxidant and life history responses. The Journal of experimental biology, 215(19), 3453-3458.
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Van Leeuwen, E. J., Cronin, K. A., & Haun, D. B. (2014). A group-specific arbitrary tradition in chimpanzees (Pan troglodytes). Animal cognition, 17(6), 1421-1425.
Finally, we get to the final post of spider behavior month (ok, so maybe it took THREE months, whatever): the post on spider social behavior!
Social behavior, you say? In MY spiders?
Yes! Indeed there is, though spiders are known (for good reason) for being antisocial loners… sometimes even cannibalistic antisocial loners. One hypothesis for the evolution of so many frantic and flashy spider mating displays is, in fact, that the poor males are just trying to convince the female to let them pass on a bit of sperm before they get chewed into a pulp.
Honestly, you might think that spiders are so successful as solitary hunters that they would have no reason to ever try to team up- and you’d mostly be right. Of the 40,000+ species of spiders that we know of, only around 80 or so are known to some of their lives living together in large groups.
So what is different about these chosen few? What does it mean to be a social spider, and what evolutionary pressures lead to this striking change in behavior?
Take my hand, and I will lead you into a magical forest, where the trees look just like cotton candy, and when a strong enough wind blows, a rain of spiders falls upon your head.
The itsy-bitsy spider crawled up the water spout, down came the rain…
Before we talk about huge colonies of spiders, let’s talk about the more modest social behaviors found within Araneae. The most basic of these, of course, is the social behavior required to communicate intentions to do the do. For even the most antisocial, aggressive animal needs to to be able to survive this particular encounter with their own kind. (Given that they reproduce sexually, anyhow. Spiders do.) I discussed spider sex and the behavior that leads up to it quite extensively in my last article, so I won’t rehash it here.
But the opposite sex isn’t the only sex you have to worry about, especially if chances to mate are rare and precious. Remember those super flamboyant peacock spiders who dance erotically enough to rival Channing Tatum? Pretend you’re one of them, a male, trying to woo a vaguely interested lady. If a same-sexed competitor starts edging towards the object of your desire, how are you gonna tell him to hop off your dance floor?
You might say, “Just eat him,” and that’s probably what first comes to most spiders’ minds too, but things, alas, cannot always go so simply. I mean, on the one side, there’s this jerk trying to edge in on your one-man show, but on the other side, there’s a lady spider who is bigger and stronger and hungrier than you with a very short attention span. It pays to be delicate here.
So what do the male spiders do? They keep dancing, but turn it into aggressive dancing.
(Skip to 1:46 to see the male-male competition.)
In fact, male spiders respond to the presence of rival males in a number of different ways. Some simply increase the intensity of their courtship displays aimed towards the female- the DANCE LIKE YOU’VE NEVER DANCED BEFORE approach. Others may aim signals already present in their courtship repertoire towards the other male- the male Saitis barbipes, for example, performed leg-stretches, an ordinary component of their mating dance, towards rivals as well. Spiders of all kinds just really love waving their legs.
Competition for mates isn’t the only time that spiders of the same sex clash with one another. For example, brush-legged wolf spiders are rather territorial over their hunting grounds. Rather than engage in costly fights during every encounter, the spiders will use a variety of escalating warning signals such as leg-waving, tapping, and mock-charges to intimidate others.
The reason I bring up these types of behaviors, as simple as they are, is that their existence suggests that there are costs to being too aggressive for spiders. Perhaps a large male might simply leap upon and cannibalize a smaller one without any trouble, but if the male is close to him in size he risks becoming dinner himself. Agonistic displays help stop the struggle before it gets too dangerous. In fact, displaying works so well that some insects have evolved displays of their own to mimic their spider predators and make them back off. You see the dangers of becoming too social? Might lose your lunch!
Remember this from the last article? A moth (B. hexaselena) mimics a jumping spider predator (P. formosa). From Rota & Wagner, 2006.
By the way, male spiders are not the only ones to display agonistic behaviors; females do as well. Often it’s a warning to a male that she doesn’t want to mate with him.
(Skip to 3:00 to see the female rejecting the male; surprisingly, it involves a lot of wiggling her butt in his face.)
In fact, because of these behavioral displays, far less cannibalism- even with food-deprived animals- occurs among spiders than might be expected. (Species in which sexual cannibalism is a reproductive strategy notwithstanding.)
That’s all well and good for communication between adult spiders. But what about females and their young? Is there such a thing as a protective mama spider?
You probably won’t be too surprised to hear me say there absolutely is. Most female spiders weave protective egg sacs made of especially stiff silk to guard their eggs. These also provide young spiderlings a place to hatch and grow safely for their first few days, whether or not mom is there. But occasionally, female spiders will continue to guard their egg sacs after they construct them by chasing off potential predators.
In species such as Stegodyphus lineatus, the main threat to the egg sac comes from a surprising source: other spiders, specifically males. The males don’t eat the eggs once they get to them- rather, they detach them by dragging the egg sac to the entrance of the female’s nest and dropping it on the ground far below. The female, with no eggs, will be ready to mate again: just what the male wants. A good 8% of a S. lineatus female’s offspring are killed by opportunistic males. Unsurprisingly, females of this species can be especially aggressive towards their potential lovers.
Other spider species, like wolf spiders and nursery web spiders, circumvent this little issue by carrying their egg sacs with them wherever they go.
Wolf spider carrying her egg sac with her spinnerettes. Other species carry them in their mouths. (Source.)
Some mama spiders continue to carry their spiderlings once they emerge, until they’re strong enough to make their own way.
Baby wolf spiders climb up on mom’s butt the moment they hatch. Aww! (Source.)
The Hawaiian happy face spider (yes, that is actually what it’s called) not only guards her egg sac and carries her spiderlings, but allows them to feed from her kills until they are able to fend for themselves. Individuals of this species have even been observed adopting orphaned spiderlings into their own broods!
A happy face spider (Theridion grallator) with a couple spiderlings in tow. Her butt is so happy to see you! (Source.)
Other female spiders use unfertilized eggs to feed their newborns, and others regurgitate their latest meals, allowing their babies to swarm all over their faces to suck it all up. And some even sacrifice themselves, allowing their young to make them into a nourishing snack. This endearing behavior is known as “matriphagy.”
By the way, this doesn’t occur quickly- the young feed on the bodily fluids of their dying mother for a number of hours. Delightful!
“Come, children, feast on my vomit!” (Image by Mor Salomon.)
“Now who’s going to be mommy’s mercy angel?” (Image by Jorge Almeida.)
Not terribly much is known about spiderling-to-spiderling social behavior, despite the fact that in all spider species, the young spend one instar (i.e., they molt once) together in the egg sac before they emerge. So all newborn spiders have minimal-to-no aggression to their own kind: in fact, they can’t even hunt other species for a while, which is why mom might help them at the start. But generally, once they disperse from one another, they grow into solitary, aggressive hunters.
Dispersal itself may involve either skittering away through the undergrowth or the charming practice known as ‘ballooning.’ Perhaps you know it if you’ve ever read Charlotte’s Web: the spider spins a kind of reverse parachute, lets the wind catch it, and… whee!
By the way, adult spiders also balloon at times, particularly when heavy flooding drives them to migrate from their home webs. This can lead to a mass exodus of spiders to higher ground, producing some… interesting topography.
Oh, Australia. (Photo by Lukas Coch.)
It may ease your mind a little to know that the spider species known to gather together like this are completely harmless to humans. But once you get that thought out of the way, you may wonder how adult spiders, which are- as I established- generally solitary and territorial creatures can tolerate living together in such close quarters. Certainly the insect population in the area would have a sizable dent put in it.
Well, the short answer is that most don’t, at least not for very long. Sexual, territorial, and maternal behaviors are, after all, the bare minimum as far as social behaviors are concerned, and most spiders are perfectly capable with just those in their repertoire. But some do go further, and they are split into two categories: the subsocial and cooperatively social spiders. Subsocial spiders are kind of a loosely defined group, since the term subsocial itself is only loosely defined: basically, any spider species that spends part of its life in a group is considered subsocial. Under this definition, many of the protective mama spiders I spoke about above fall under that banner. But even those are at the far left of the subsocial continuum. Other subsocial spider species will hang out for long periods of time with grown-up members of their own kind, whether they be offspring or siblings or even potential mates.
One subsocial species, the orb-weaver Parawixia bistriata, has a unique system where individuals built separate, adjacent webs during the night. I do mean separate- the spiders will tussle over prime web spots and defend them. Yet as the sun rises, the growling and snapping calms down, and spiders who haven’t gotten a chance to build a web are usually allowed to snack on the remnants of other spiders’ meals. As the day gets warm, everybody huddles together a big ol’ spider love ball until the sun goes down and it’s time to spin webs again.
I feel the love crawling all over my body. (Photo source.) (By the way, the photo at the very top of this article is a P. bistriata colony in their spread-out form hanging from some Brazilian power lines.)
The flat huntsman spider (Delena cancerides) is a subsocial species that is even more cuddly than P. bistriata. Like the name implies, they are not web-spinners but rather active night hunters who retreat to a den at dawn. While young huntsman spiders are still growing, they share the nest with mom. Interestingly enough, within the nest there may be siblings from multiple broods sharing the space. Not only that, but mom isn’t the only one bringing food back for the babies: the older siblings are, too!
Flat huntsman spider siblings from different broods sharing some tasty crickets. (Photo by Linda Rayor.)
The flat huntsman spider may also be the only spider to have evolved a form of kin-recognition. I mentioned before that spiders like the Hawaiian happy face spider willingly accept foreign offspring into their broods- this is true of nearly all spiders that show a degree of maternal care. But not the flat huntsman spider: despite their gentle manners within the nest, these spiders will attack and eat any spider they encounter that isn’t related to them on the outside. In fact, if an unrelated spiderling is placed within the nest, it has a very high chance of getting killed (especially if it’s more than a few days old).
Yet these same cannibalistic spiders are quite considerate towards their siblings. In one rather horrifying study, huntsman spiders were placed with either a smaller sibling or a smaller non-relation without any food. The non-relations were eaten within a day, but the huntsman spiders paired with their siblings literally starved to death over six weeks rather that hurt their baby brothers or sisters. Even then, the younger ones wouldn’t feed on the bodies of the older ones!
That is pretty dang nice for a spider. A couple other subsocial spider species have been put to this test as well- and failed it.
Still, the flat huntsman spiders only live in colonies of up to three hundred individuals. Is three hundred spiders a lot of spiders? Compared to one or two, yes it is, but it is peanuts compared to the size of some other social spider colonies. We are talking tens of THOUSANDS of spiders here.
Apparently, the brownish coloration in the web is from the bodies of millions of dead mosquitoes.
These, my friends, are the the truly social spiders- the cooperatively social spiders.
Colonies this complex only occur a handful of spider species, and the fascinating thing is that they don’t appear to have a common ancestor. Cooperative social behavior evolved at least twelve different times in the spider lineage. That means that there is a very rare but very compelling set of circumstances, environmental and internal, that cause spiders to go social.
As for just what these circumstances are, the answer is naturally quite complex, because nothing tends to be simple when it comes to evolution. But let’s start with what traits most social spiders have in common.
The big one to start with is webs. All cooperatively social spiders weave them, and almost all subsocial spiders do, too- the flat huntsman spider is a big exception in many ways. In other cases, spiders from families that generally don’t spin webs to hunt have regained that ability to go social- one such example is the social lynx spider (Tapinillus sp.).
But they can’t spin just any kind of web, especially if they’re going to be cooperatively social. Remember P. bistriata, the spider that likes to form a leggy love ball? They may be cuddly when they sleep during the day, but at night they spin the flat, spiraling webs characteristic of orb weavers, and it’s a one-spider-to-a-web deal. The webs simply won’t work with multiple spiders using them to catch prey. A bunch of them would just end up triggering trap-lines all over the place and causing a great deal of confusion. Remember, web-spinning spider vision is absolutely awful, so vibrations are kind of important for them.
The webs utilized by cooperatively social spiders that hunt in groups are going to be the messy-looking, three dimensional type that are known as sheet webs or cobwebs.
Examples of web types from nine different social spider families. From Aviles, 1997.
To help differentiate the struggle vibrations of prey from the vibrations of their friends- because wouldn’t that be an unfortunate mix-up- many social spiders not only utilize three-dimensional webs but even synchronize their movements with one another in a living spider-wave. The regular vibrations from their pals are easy to differentiate from prey vibrations.
Another factor common to nearly all social spider species is their relatively small size compared to other members of their families. This may be due to a selection for paedomorphosis, or juvenile characteristics- in other words, social spiders get their friendliness by extending that tolerant “baby phase” where everybody hangs out in the egg sac and nobody tries to eat anybody else. This is the same way that the loves-everybody-he-meets dog evolved from the I-really-don’t-trust-you-and-might-bite-your-face-off wolf. Yes, I am saying that social spiders are to dogs what regular spiders are to wolves.
Indeed, most social and even subsocial spiders (aside from the flat huntsman spider I discussed above) are extraordinarily tolerant of members of their own species, related or not, and will happily fuse colonies with complete strangers. This trait is quite different from any social insect- but we’ll get to that in a second.
Paedomorphosis and three-dimensional webs may both be factors that facilitated social evolution for these spiders, but there are plenty of spiders that have shrunk in size and spun tangled webs out there that are solitary: far more than are social, in fact. We have yet to really touch on the reasons why social spiders became social.
Surprisingly, the main theory lies in where most of them live: the tropics. Those of you who know a little bit about the ecology of tropical rainforests will know that there is an extraordinary amount of niche partitioning taking place there: in other words, there is a LOT of competition from other species, so much so that everybody needs to get really specialized. Spiders, being rather generalist predators, might struggle with this, particularly if their size limits them to a certain subset of prey items. In fact, social spiders tend to be found in environments where there are much larger prey items than small ones, available year-round. But there are also a lot of different predators year-round as well. (Sometimes prey and predator are one and the same, too.)
With this particular set of pressures and opportunities, you can see how cooperative social behavior might be selected for: more individuals can work together to take down larger prey, defend young against predators, and repair webs after it rains. Actually, tropical environments facilitated cooperative social behavior in more than just spiders: it’s the proposed place of evolution for many eusocial members of the wasp, bee, and ant order, including the famous honeybee!
But enough about why social behavior evolved. I haven’t talked nearly enough about what cooperatively social spiders even do, and what sets them apart from subsocial spiders. It isn’t just colony size- some cooperatively social spiders do have colonies on the small side, with only a few hundred individuals. What sets them apart, rather, is that they do absolutely everything together, all the time. Even that cuddly, family-friendly flat huntsman spider eventually disperses from his or her nest to strike out alone. But the colony-living spiders don’t do this. Ever. They don’t disperse.
What? you may ask. How can they not ever leave home? Other colony-living invertebrates have ways to disperse- the flying ant queens and their mates, swarming and fission in honeybees, et cetera- so how do social spiders find mates that aren’t related to them?
Oh, you poor innocent thing. Maybe you’ll understand when I tell you that on average, members of a social spider colony have polymorphism levels (a measure of genetic variation) of 5-8% between them. They are inbred as all hell.
Inbreeding is actually an issue for most spiders that have a modicum of social behavior, even subsocial ones: as I said, there’s only one species known to show any sign of kin recognition, the flat huntsman spider. And they’re the one exception to this rule, with rather healthier 32-68% levels of polymorphism. Most subsocial spiders are well below that, and they do disperse.
Now, it’s not as if dispersal never ever happens among the cooperatively social spiders. For all sorts of reasons, colonies can get split up or fail, leaving a few individuals on the lam, and sometimes a few female spiders do strike out on their own for no discernible reason. As I mentioned before, other colonies will take these foundlings in with relatively little fuss, though in most cases separatists tend to succumb to the elements soon after leaving the main colony. But the difference between social spiders and pretty much any other social invertebrate- nay, social animal– is that they have no behaviors that actually trigger dispersal. If it happens, it happens more or less by accident. Otherwise, they happily mate with their cousins.
I’m putting an image of a naked mole rat here because INBREEDING. (Photo by Rochelle Buffenstein.)
There is another major difference in the way spider colonies organize themselves compared to insect colonies. Hymenopteran (ants, bees, wasps) colonies have a single breeding female known as the queen, while others like termites (usually) have a single breeding pair: the king and queen. In fact, all eusocial or pre-eusocial organisms rely on the fact that only a tiny fraction of the population breeds: this means that nonbreeders are all the offspring of a single mother, and have a genetic incentive to help care for their younger siblings.
In social spider colonies, nearly all females breed, and everybody takes care of everybody’s offspring. No discrimination involved whatsoever. This may seem rather antithetical to the very concept of kin selection- the idea that animals prefer to help others they’re closely related to- but, in fact, it is not. There is so little genetic differentiation within spider colonies that you’re probably just as related to your sister as you are your second cousin’s uncle’s mother’s grandfather’s niece by adoption.
In other words, spider colonies have so little genetic diversity that they might as well be one giant organism composed of thousands of spiders. A SUPERSPIDER.
Anelosimus eximus: WE ARE ONE BEING.
In this sense, the inbreeding ain’t so bad after all. Everybody is equally motivated to help everybody else with all the chores. Except for the males, who are tiny, but they only make up a fraction of the population and who cares about them.
And if you thought a solitary hunting spider spelled doom for an insect, well…
Imagine having your innards sucked out through a hundred tiny straws. 😉
With the power of sheer numbers, these spiders- which are generally about 5 millimeters long- have reportedly taken down prey as large as rats. People have found small mammal skulls tangled in their webs.
…Luckily, they aren’t dangerous to humans at all. YET.
(Ok, but seriously, they never will be, no need for more Bad Spider Press.)
The point is that their numbers allow them to take on nearly any insect that stumbles into their web- which is good, because if they were only dealing with tiny ones, they wouldn’t be able to keep feeding their numbers. They need that very specific tropical environment where there are lots of big bugs all the time to keep themselves going.
And it isn’t just a free-for-all on the prey items, either. Even if prey is caught in a web, they may still be able to struggle their way out of it. This means that hunting spiders are going to have to be fast and efficient at subduing the insect if they don’t want to be left with empty stomachs and a gaping hole in the web.
Studies on both social and subsocial spiders have found a unique degree of coordination in their responses to struggling prey. Young Amaurobius ferox spiders from the same brood stay together for a while after they eat their mother (it’s ok, it’s how she wanted to go) and build sticky little webs of silk stretched over stones or crevices. When an insect gets caught, the spiderlings rush them in organized waves. The first to arrive grab the prey by its antennae and legs, pulling them in opposite directions a la that one medieval torture device. With the prey thus immobilized, the next wave can inject venom into its tender abdomen. Using this strategy, teeny tiny spiders can take down prey over ten times their size.
Black ovals represent spiders on this figure, and each also corresponds to a degree of cricket panic. From Kim et al., 2005.
So now we know social spiders are pretty good at hunting together. But this brings up another question: how do they divide up their labor? How do the spiders decide who gets to do the hunting, who gets to do the web-spinning, and who gets to do the child-rearing?
You might think, as with breeding, everybody does everything. But there is a reason that other colony species divide themselves up into castes like workers and soldiers: it’s more efficient. If Jenny Bee is nursing a grub and then hears the call to go out and get nectar, it’s not super great if she immediately drops the grub on the floor and flies outta there. You gotta… you gotta prioritize, Jenny Bee.
Social insects have several ways of dividing themselves into castes- it can be by birth phenotype, as in ants, or by age, as in honeybees, for example. How about social spiders- do they divide themselves into castes? Well… as with everything else, social spiders are a little weird in this respect.
They do sort of divide up the labor, though it’s arguable whether or not you could claim they have castes. But there are spiders that usually hunt, spiders that usually nurse the young, spiders that repair the webs, and so on. As for the great determiner of who does what- there really isn’t one.
Not one as ironclad as birth or age, anyway. Indeed, a social spider may actually switch jobs during her life several times. The real determining factor of who does what actually seems to be… wait for it… personality.
Yes, spider personality: that is the official theory.
I imagine you have your head in your hands right now, wondering what the hell a spider personality would even look like. How does one determine where a spider falls on the Briggs-Meyers personality test? Do they have extremely tiny spider questionnaires? What differentiates an ISTJ spider from an ENFP spider?
Alright, kidding aside, this is actually one of the most fascinating things about social spiders. Researchers have identified multiple personality types in social spiders so far, specifically in Anelosimus species. In one experiment, researchers classified the spiders as either “bold” or “shy.” They determined this by blowing on spiders so that they retracted their legs and pretended to be pebbles. The spiders who un-pebbled the quickest were the bold ones, while the ones that remained curled up were the shy ones.
This is serious science, kids.
And it seriously involved putting brightly-colored paint on tiny spider butts. (Photo by Lena Grinsted.)
A separate experiment used a different test to determine two other spider personality traits: they put two spiders in a box, and considered ones that stayed close together “docile” and ones that moved further apart “aggressive.” As you might imagine, the spiders that had higher docile scores tended to be the ones taking care of the babies, while the aggressive ones tended to be the hunters and colony defenders.
What about bold and shy spiders from the previous experiment? Well, the bolder ones were definitely more involved in defense and hunting, but really sucked at keeping babies alive. They didn’t test the shy ones for comparison, though.
One more fun fact about that particular experiment- the researchers used a pink vibrator- yes, THAT kind of pink vibrator- to shake the spiderwebs. Because it had reproducible vibrations. Apparently they bought it just for the study.
This whole personality bit is especially interesting because of how low the genetic diversity is within the spider colonies; furthermore, there aren’t any obvious physical traits associated with each personality type: for instance bold, aggressive spiders that like to hunt may be smaller than shy, docile ones that like to nurse babies. More importantly, having a particular personality trait didn’t mean a spider was ‘locked in’ to a specific task: it just predicted what they were most likely to do. Could spider personality be a factor of the environment, rather than just an encoded genetic trait? In other words, is it possible for spider personality to be shaped and changed over time?
Some research says yes, at least partially. While the traits do appear to be somewhat heritable, the amount that they are expressed actually depends on the social group that the spider is with.
This was demonstrated in yet another study that involved disturbing poor defenseless spiders. In this case, spiders were first assessed for bold/shy personality traits via the pebble test. Then the researchers ruined each colony’s nests like the jerks that they are. In addition, they shuffled around the members of some colonies so that they were hanging out with a completely new bunch of gals.
The result? When spiders were with others they were familiar with, their personalities were strong and consistent: bold spiders were BOLD, and shy spiders were SHY. But in an unfamiliar group, the differences in personality seemed to fade away, and spiders behaved pretty uniformly. It took them a long time to regain a degree of personality difference, and when they did, not everyone showed the same traits that they had before.
You may have experienced this phenomenon yourself, when figuring out how to interact with a brand new group of people- your personality is somewhat ‘muted’ until you figure out just where you fit in. It’s called social niche specialization, and it’s been found in other animals, not just humans and spiders. As the theory goes, each member of a social group gradually takes on a specific social “role,” whether it’s being the boisterous confident one or the quiet thoughtful one- and the roles may be different for the same individual depending on which people they’re around. Strange as it seems, spiders too take on different roles based on group composition.
Nobody EVER wants to work with Sheila here (Cebrennus rechenbergi, and yes they really do this).
It is a bit ironic to have individual personality be a defining factor in species that depends on its genetic similarity for existence, isn’t it? I suppose that just goes to show you that genes don’t make (all of) the man. Er, the spider. The spider-man.
But coming back to that whole inbreeding thing: it may have occurred to you earlier than this that inbreeding can be, ehm, problematic for the survival of the species. While it is true that many species seem to be protected from the deleterious effects of inbreeding depression- and social spiders are- there are other big issues with letting everybody get too genetically similar to one another. Namely, if everybody’s too similar, it might only take one bad disease, parasite, predator, or natural disaster to take them out. Things get more dangerous when you consider the fact that dispersal in social spiders is rather limited, AND they only thrive in very specific environmental conditions, as I discussed earlier.
Even more damning is the fact that though cooperative social behavior has evolved multiple times, separately, in spiders, these speciations all occurred relatively recently, leaving the social spiders at the very tips of the spider family tree. What this means is that there’s no evidence for a social spider species that lasted for more than a few million years; it could be that many evolved and died out that we simply don’t know about, leaving no evidence for their earlier existence, and no descendents. And that’s another problem with limited genetic diversity- it is very hard to split into new species when everybody ends up with pretty much the same genes.
So cooperatively social spiders, despite their seemingly egalitarian and efficient societies, may all be one nasty accident away from going extinct entirely. They may be at what is known as “evolutionary dead ends.”
But there is a little hope for social spiders. The fact that they have no regular means of dispersal is actually a boon in this sense. The populations of social spiders generally rise and fall in chaotic manners, and fission and dispersing events seem to happen by chance. Irregular population patterns are actually protective against catastrophic events like disease or environmental disturbance because the spiders don’t depend on specific cues to expand or split their colonies.
I myself would love to have these amazing creatures around for a few million years more, because they are crazy weird and I can’t learn enough about them. But even if they are functionally dead branches of the evolutionary tree, the rest of spider-kind isn’t going anywhere, and history suggests that when conditions are right, lineages of social spiders will evolve again.
I hope you’ve enjoyed Spider Behavior Month(s), and the next time you see a spider, maybe you’ll have a better appreciation for the truly amazing critters that they are. Happy web-spinning, Spiderfriends!
This is a large and beautiful wolf spider friend I made today. Completely harmless, and happy to go back outside where there were more things to eat! Be good to spiders, folks.
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Note: some of the videos embedded in this article show predators eating animal carcasses.
To describe what kleptoparasitism is, I’m going to use a Pixar film.
I don’t remember this poster looking so ominous.
Yes, Pixar’s 1998 film A Bug’s Life, while incorrectly making worker ant Flik a male*, provides a very excellent example of kleptoparasitsm. Poor Flik’s ant colony is forced to gather extra food every year in order to offset the losses from the larger and more aggressive grasshoppers, who regularly come and steal the ants’ food.
While real grasshoppers aren’t known for pilfering from ant food stores (they would find themselves very quickly dismembered and tossed on the pile), this type of interaction is very common between types of species that share the same food source, or need the same types of resources, such as nest-building material. Generally, it is the larger, more powerful species that nabs the resource from the smaller one. Hence the term ‘kleptoparasitism’: parasitic theft.
Most kleptoparasitism takes place opportunistically- few species have evolved to be thieves one hundred percent of the time because it’s simply not practical. The only exceptions to this occur if you consider brood parasitism a kind of kleptoparasitism. For example, common cuckoos exclusively make use of the nests built by other birds and have lost the ability to care for their own young. Conversely, slave-making ant species actually steal the young of other ants to do all the work in their colony. Et tu, Flik?
Kleptoparasitisitic relationships can also occur between members of the same species- one could consider the nabbing of prime territory or mates a form of kleptoparasitism. In fact, many species that have kleptoparasitic relationships with one another are very closely related, particularly in insects. This makes sense- the more similar you are, the more you compete for the same resources.
The relationships I want to talk about the most today, however, are the ones between species that are not closely related, but share the same ecological niches. And I want to discuss an interesting hypothesis, one that was also explored in A Bug’s Life. If you’ve seen the film, you’ll remember the ending:
Yep, Flik saves the day and helps his colony defeat the grasshoppers by reminding them that they vastly outnumber their parasites and, additionally, are equipped with limb-severing pincers. The ants are then able to overwhelm the grasshoppers with sheer numbers and presumably feast upon their remains offscreen.
So: how effective are numbers as a defense against kleptoparasitism?
Kleptoparasitic Relationships Between Large African Predators
Kleptoparasitism is a major problem for mammal species of a particular kind: small, lightweight, and specialized predators. On the African savannah, two species are notable targets for kleptoparasitism: the cheetah and the African wild dog.
Of the large mammalian predators on the savannah- the lion, leopard, cheetah, hyena, and wild dog- it is the cheetah and the dog who have the highest hunting success rates, of about 50 and 80 percent, respectively. Comparatively, hyenas and lions have closer to 30% success rates when they attempt to make a kill.**
Hyenas and lions, while not as speedy or efficient as the dogs and cheetahs, certainly know how to use their greater weight to their advantage- hyenas weigh about twice as much as both species, while lions have nearly four times their bulk. It is estimated that about 50% of all cheetah kills are parasitized by lions or hyenas, a rather grim statistic for an endangered species.
Unfortunately, there is very little a single cheetah can do against a hyena. Aside from the differences in weight and power, the specialized hunting style of the cheetah means that any injury could be fatal. Cheetahs (and wild dogs) are cursorial hunters, which means that they chase their prey over long distances. This requires a lightweight body and excellent stamina. A lion might be able to stalk and ambush its prey with minor injuries, and even with major ones it still has a chance of using its weight to steal prey from smaller creatures. But cheetahs and wild dogs hunt nearly exclusively for their own food.
The lion is the heaviest predator on the savannah, and they know it: as many as half of the kills a lion eats might be stolen from other predators. They kleptoparasitize even more than the much-maligned hyena, which in some studies hunts for as much as 95% of its own food.
However, hyenas can also kleptoparasitize the larger and more powerful lions. Their secret? Just as with the ants, it lies in numbers. If a group of hyenas outnumbers the lions feeding at a kill by a factor of four, they may challenge them. It’s a toss-up to whether or not they will succeed, though the presence of much larger male lions can make things much more dangerous. One study found that 71% of all hyena mortality was due to lions.
In fact, lions are a major cause of death for all of their competitors. African wild dogs and cheetahs also have much to fear from lions: lions will seek out and kill their cubs without even bothering to eat them afterwards. For this reason, while these species can forge a rough coexistence with hyenas, they actively avoid spaces that lions frequent.
You’ve been lied to, folks: Lions ain’t majestic beasts with flowing hair. Lions are gigantic assholes that make up for their crappy hunting skills by stealing meat from smaller animals. You say lion king, I say lion tyrant.
Remember, son, hyenas are inherently evil and there’s nothing wrong with stealing their food and murdering their young. Now go eat your rotting carcass.
So on the savannah, killing your prey may be the simple part: the harder part is keeping it. Leopards, at least, have solved this issue by dragging their prey up into trees to eat at their leisure, though this doesn’t always work. Other species with less of an inclination for tree-climbing will make their kills in tall grass or dense foliage to avoid detection, though inevitably small scavengers like jackals and vultures will give them away. In fact, at times the scavengers can oust the killers themselves.
Much of the behavior of these smaller hunters, in fact, is heavily influenced by kleptoparasitism- more than you might think. After all, evolution doesn’t work in a vacuum, and if there’s someone out there making it, there’s always someone else ready to try and steal that success.
This raises a rather interesting question: are some of our assumptions about the social nature of these hunters wrong? Recall that if they greatly outnumber lions, hyenas can steal their kills. Conversely, this means that even the lions need to be in large groups to resist losing their spoils. Do these predators group up specifically to reduce kleptoparasitism?
But wait, I hear you say. The reason why animals hunt in groups hunt in groups is because they need to work together as a team to kill much larger animals!
Well… yes and no. Animals that hunt together in groups are actually restricted to larger prey***, because they need to have enough food to feed everybody when they’re done. What feeds six lions more effectively: one impala or one Cape buffalo?
This buffalo looks like it’d eat a few impala itself on an off day. (Photo source.)
If you think about it this way, hunting larger prey is a handicap and not at all an advantage- especially when you consider the increased danger from things such as injuries. That cute little impala isn’t going to kick your head in with the same force that a Cape buffalo will. (Also, the impala is less likely to turn around and stomp on your remains to be sure the job is done.)
It’s a fact that most predatory animals- estimates range from 85-90%- hunt by themselves. So what led some animals to seek out big, dangerous prey that they couldn’t finish alone? Probably not the thrill of it all. Remember how dangerous injuries are for some predators? There has to be a big payoff for all that risk.
But perhaps we’re looking at it the wrong way- maybe the tendency to group up evolved before predators started attacking oversized prey. In that case, why did the hunters start working together?
Resource distribution may be a large factor: if prey animals are spread out evenly through the environment, predators will probably have the most success hunting alone. But if prey animals are clumped all together and tend to try to defend one another, a group may have a better chance of splitting somebody away from the herd than a solo hunter.
However, the distribution of resources doesn’t neatly tie up every loose end. While some species have a higher likelihood of making a kill in larger groups, many do not. In fact, most mammalian predators are quite capable of hunting smaller prey by themselves- single wild dogs can kill impala, and it is actually more common for hyenas to kill prey alone than when in a group. In a large study of species that either hunted in groups or alone, ecologists Packer and Rutton actually found that there tended to be no overall benefit in terms of hunting success when animals hunted in groups.
Additionally, most group hunters live in larger groups than the ones they hunt in- hunting parties tend to be cliques that split away from the main group. And the phrase ‘group hunting’ itself may be misleading. While African wild dogs statistically have increased hunting success in larger groups, individual dogs within a hunting party may all initially be pursuing different prey animals at first, then converge upon the first animal that becomes vulnerable. In this sense, they are hunting “alone” until the final moments.
I mentioned earlier that hyenas usually make their kills by themselves: however, they may be trailed by other members of their clan that attempt to take the meat for themselves once the hard work is done. So not only do group-living animals have to share their meat, they might have to share it with individuals that didn’t even help get it. How’s that for a raw deal!
So exactly what advantages do hunting in groups confer? Resistance to kleptoparasitism may actually be a big factor. A hyena may be able to steal from a lone wild dog. But what about ten?
(They may not be able to kill the hyena, but they sure can annoy the shit out of her.)
In this video, finally, the smaller hunters are able to drive off the larger ones and finish their meal. Just like Disney taught us in A Bug’s Life!
Now, I’m not going to suggest that this resistance to kleptoparasitism is the only reason some predators hunt in groups- there’s a whole host of other factors that I haven’t even touched on, like the ones that have nothing to do with eating- group living confers other benefits, especially for a little fellow who might become prey as easily as predator. But I do challenge you to reconsider what you thought you knew about why animals hunt in groups.
Let me close with cheetahs, since I discussed them a great deal earlier. Most cheetahs hunt alone. However, many male cheetahs hunt in groups of 2-3 related individuals known as ‘coalitions,’ some females hunt with their cubs, and some adolescents of either sex will hunt together.
Most studies of cheetahs conclude that while coalitions enable cheetahs to seek out larger prey, the amount each animal eats is no higher than it would if it hunted alone (and may even be less). Resistance to kleptoparasitism may provide benefits- though, surprisingly, one hyena is still capable of scaring three fully grown male cheetahs off a kill.
The answer this time, in fact, probably lies with social behavior and has nothing to do with hunting at all. Female cheetahs tend not to be territorial, and live in vast home ranges overlapping those of many other females. This makes it more difficult for male cheetahs to defend access to females- unless they have wingmen to back them up.
So in the end, cheetahs still kinda get screwed no matter what. Good luck, guys.
Or just return to the Nonfiction section to see a list of all articles I’ve written. Thanks for reading!
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*No, I will never get over this.
**Note that these percentages are extremely rough, and vary tremendously based on factors such as group composition, environment, and the experience of the animals involved.
***There are some circumstances where this does not hold true. Chimpanzees, for instance, hunt small prey in groups so that each hunter hopefully catches his own prey, and dolphins and other group-hunting marine predators usually hunt schools of smaller fish.
My good friend Chloe is a mutt without an off button. 8 am? Time to play. 12 pm? Time to play. 7 pm? Time to play. 4 am? Guess what time it is!
Of course such behavior is a pure delight to watch, but as scientists we must ask ourselves the question: why? Why so much play, Chloe? What’s the point?
We appear to have lost Chloe’s attention.
It may seem silly to ask why play behavior occurs, because after all it’s play. Play! Play is all about fun; what’s there to study? And in fact, that attitude led to play being neglected as a study topic for much of scientific history. But play behavior is a category of behavior just like foraging behavior or sexual behavior or social behavior. If it occurs a lot- and it does- it must be evolutionarily important.
Okay, so we want to study play. So what counts as play behavior?
*Note: I use the word “suicide” a lot in this article, not to refer to the symptom of depression but to certain animal behaviors.
Animals aren’t like humans, because unlike humans, animals aren’t selfish. Animals work together for the good of their species, even to the point of sacrificing their very lives so that others may live. Take the humble lemming, which, as shown in this Disney animal documentary from the 1950s, will literally jump off cliffs to prevent others from suffering from overpopulation.
By the way? All of the above is bullshit. (Some of you were starting to get worried for a moment there, I know!)
Lemmings do not jump off of cliffs to prevent overpopulation. The footage above, as is now widely known, was faked- the lemmings were actually pushed off the cliffs by the filmmakers. Yes, it’s true.
It may seem logical to assume that animals work for the good of the species- after all, isn’t survival of the species what evolution is all about? Wouldn’t it make sense that animals would evolve mechanisms to make sure that their species is as successful as it possibly can be?
In fact, this ties in to a popular evolutionary theory called “group selection” that had traction (off and on) up until the 1980s, and has even had a resurgence recently. According to group selection theory, natural selection does not work with individuals, but rather on groups. That is to say, a gene that causes a disadvantage to an individual may persist in a population because it provides an advantage to the entire group. Hence our suicidal lemmings.
The problem with the theory group selection is that the basis itself is flawed. We know now that evolution works at the genetic level, and is dependent on whether or not individual animals get to pass on their genes. The word “individual” is key here.
To explain using an example: say that there was a gene that caused lemmings to commit suicide when they noticed that population levels were getting too high for the environment to sustain. Seems grim but logical, right? Well, in order for this gene to actually work, there would have to be some members of the population that didn’t have the suicidal allele. Because otherwise, all of the lemmings would jump off cliffs and that’d be it for the species.
Multiple alleles for the same gene, like the one that determines eye color for humans, can coexist in a species just fine, so that’s not a problem. Here’s the problem with this:
See those dots? Let’s pretend that the blue dots are animals with a suicidal allele and that the orange dots are the animals in the population without it. I’ve even given the suicidal allele an edge by having it be more common in the population, increasing the odds that it will be passed on.
Ok, let’s wait a few months and check back in on our population.
Wait a minute, what happened? Only orange dots are left!
It really doesn’t matter how widespread the suicidal allele was in the population- it was a suicidal allele. When animals kill themselves, they don’t pass on their genes- so no blue dots lived long enough to give their offspring their suicidal tendencies. Even if they had, the orange dots will always have an infinite advantage over the blue: they have a much better chance of living longer and passing on their genes. A trait which reduces an individual’s chance of successfully rearing offspring would never evolve- because that’s the exact opposite of how evolution works.
Everything is orange.
This is where the phrase “selfish gene” comes from. It’s not as though genes are actually capable of being selfish- it’s just that the ones that exist are the ones that were the most successful at propagating themselves in the past. The ones that weren’t successful- even if they were super nice, friendly genes- no longer exist.
Longtime readers of this blog might be quick to point out that I’ve discussed how other supposedly nonreproductive behavioral strategies can be maintained in a population, such as asexual and homosexual behavior. The key difference between these and our theoretical “suicidal” behavior is that asexual and homosexual individuals can still gain indirect fitness benefits by helping their relatives. Committing suicide in order to reduce the population to an environment’s carrying capacity may indeed benefit an entire species, but how is the now-dead animal sure that his sacrifice is benefiting his relatives in particular? A lot of individuals have to die all at once for this strategy to work.
The idea that evolution benefits individual genomes, not entire species, is evident when you study species that have, as I like to call it, evolved themselves into a corner. These are usually hyperspecialized species that have adapted to a single, very unusual habitat or have a very tight symbiosis with another organism. The popular giant panda, for example, is in trouble because its very specific mountain bamboo habitat is disappearing. Likewise, if the special fungi that leafcutter ants farm were to go extinct, so would they (and vice versa) because each provide food for the other.
Which would be a shame, because throwing leaves down on leafcutter ant trails is one of life’s great pleasures. (Photo by me.)
Compare the delicate nature of these species with the adaptability and damn-near-impossible-to-eliminate-ablity of a species like, say, a brown rat or cockroach. In terms of success and numbers, those species are certainly winning over the likes of the giant panda, and will still be winning whether or not we turn the coveted bamboo forests of China into parking lots.
If group selection were a viable theory, one might assume that it would guard species against overspecialization. But species overspecialize because evolution doesn’t look ahead: in the short term, specializing can make a species wildly successful. It’s just not a good strategy for the long haul.
I’ll bet that many of us don’t even realize how many of our assumptions about animals end up sounding a lot like group selection. For example: when a prairie dog squeaks at a hawk, it’s to signal the others to run. When birds of a feather flock together, it’s because they have more eyes and ears to watch out for predators for each other. Heck, when any prey species does anything to signal that a predator is near, it’s to signal all its friends and save their lives, right?
I’m sorry to have to say this, but by human standards, many animals are just terrible people.
I’m not saying animals can’t behave altruistically- i.e., for the sake of others to the detriment of themselves- they do, all the time. However, animals generally behave altruistically because, in the end, there is something in it for them. Or at least their genetic material.
Take those famous prairie dogs. Much has been made of their complex predator alarm system- different calls for different types of predators, and even the distance, looks, and behavior of said predators. Upon hearing these hyper-specific calls, the other prairie dogs in the colony know which escape strategy to use.
Super cool, right? And given that the prairie dogs who first give the alarm are likely putting themselves at risk by making themselves more noticeable by predators, it’s a pretty selfless act. With one major caveat. Those selfless rodents are far less likely to call out if none of their close relatives are in danger.
It comes down to protecting your own genetic material once again, whether or not it’s housed in your own body. And this is true of practically all species that use alarm calls- and more often than not, these alarm calls can be used for even more selfish reasons. Great tits and other birds will frequently give alarm calls not but because a predator is coming- but because they want to scare their larger companions away from the food. Some species of antelope even fake alarm calls to keep females nearby. If a female starts losing interest in a male topi’s advances, he snorts as though a lion is nearby to scare her into moving close to him again.
Some animal signals that we’re used to thinking of as alarm calls or warnings for others in the group are actually nothing of the sort. Those of us in the eastern U.S. are acutely familiar with the white-tailed deer, and of the bright white tail that gives them their name. Well, the tail raising behavior is not, as is commonly assumed, a means of warning others in the herd. It is instead a communication to the predator– a way of saying “I see you, so pick on the guy without his tail up.” The fact that other deer may run at the sight of this is a side effect. The signal’s not meant for them.
There’s only one cougar this signal’s aimed at, if you know what I’m saying. (Photo source.)
A more dramatic version of this is found in gazelles, who perform a behavior known as stotting (or pronking, or pronging). A stot is a vertical leap, which the deer perform when they spot a predator. Stotting actually can slow down gazelles when they’re fleeing, since it’s literally just leaping straight up, but that’s kind of the point: not only are the gazelles showing the predators that they’ve seen them, but they’re proving that they’re fit enough to risk idiotic vertical leaps while they’re running away.
And I do mean idiotic.
Why are we so sure that stotting is a signal meant for the predator and not the other gazelles? Well, for one thing, gazelles stot more when they see coursing predators- your wild dogs and your cheetahs- than they do when they see ambush predators- your lions and your leopards. It makes sense, because in a flat out chase, a signal of how much endurance you have might get the predator to go after a weaker-looking guy. But in an ambush, you’d better dispense with the hopping and get the hell out of the strike zone. Companions be damned.
And as a matter of fact, whether or not an animal stots is a pretty good predictor of whether or not it’s going to survive an encounter with a predator. If you’re too tired to stot, you are pretty much doomed. So you can actually think of it as a kind of favor to the predator, when it comes right down to it. It shows them which individuals they’re going to have a shot at catching, and which they aren’t.
If we’re going to be honest with ourselves, most prey animals would very much prefer that one of their friends gets eaten than the predator go hungry anyway. Because a hungry predator will attack again- while a well-fed predator grants them a reprieve.
So, let’s come back to a fundamental principle here: why do so many prey animals like to live together in large groups? Most ungulates, for sure, but also flocks of birds, schools of fish, swarms of midges… you get the idea.
To understand why this happens, first you have to understand the very real risks this generates for the animals in question. Ever tried to spot a lone sardine in the open ocean? Well, how about a school of thousands? One is a lot easier to find than the other. And predators find them more easily too- so if grouping up is an anti-predator defense, there has to be a damn good reason for it.
The “many eyes” theory is a popular one: it suggests that animals in large groups can trade off the job of looking for predators with others as the day goes on, so that at some point everybody has a chance to relax and feed. The problem with this theory is that a lot of animals are jerks and don’t pull their weight when they don’t have to. In bighorn sheep, ewes with calves spend a lot more time looking for danger than ewes without calves do. In fact, if there are a lot of lambs in a group, the lamb-less ewes spend even less time looking out for predators, because the predators are more likely to go after the lambs than them. Isn’t that sweet.
In general, the rule seems not to be so much “more individuals, more vigilance,” but rather “at-risk individuals spend a lot of time looking for predators and the others mooch off of their efforts.” So while big males and females without young benefit from having more eyes in a herd, in the end, mothers and calves might not.
Let’s look at a couple facts. As predator risk increases, herd size tends to grow larger, and the space between individuals gets smaller- and interestingly enough, individuals that look like each other tend to segregate from those that look different. There’s a moral lesson in that somewhere, but we ain’t about morality here on Koryos Writes.
The most crucial fact of this is the spacing between individuals. If you’ve never seen oceanic predators all teaming up on a “bait ball” of small fish, it’s pretty amazing.
By watching this video, you might feel that something’s off about the way those herring are moving. Namely, why do they all stay in one place, so close together, so that at the end a whale pops up and managed to scoop a huge proportion of them into its gullet? It doesn’t seem like the, er, smartest strategy.
Guess what: it’s not. Not for the group, anyway. These animals probably would be better off, on the whole, if they moved the heck away from each other and all swam in different direction. So why don’t they?
The answer, as you may suspect, is that while this behavior is bad for the whole group, it can have extremely positive effects for a number of lucky individuals. That is why the reigning theory behind this behavior is called selfish herd theory.
Selfish herd theory states this: that when a hungry predator sees one prey animal, that’s the one he’s going to attack. 100%. But when a predator sees two animals side by side, each animal’s chance of being attacked decreases (in a perfect world) to 50%. Three animals side by side? Hoo boy, the risk goes down to 33% each. I like those odds a lot better.
Of course, not all spots in a herd are created equal. The very safest places in a herd are right in the center, surrounded by other delicious-looking targets on the outside. This is why the center of a herd is quite a coveted spot to be, and why you will actually often find the most socially dominant members of a prey group not out in the lead, but in the center.
In the video below, you’ll see the results of a study where researchers attached GPS trackers to sheep (red) and a herding dog (blue). Observe the way the sheep immediately bunch up when the dog gets close.
Those on the outside of the herdrun far more risk of being picked off, and the danger only increases the farther they lag behind their conspecifics. So when somebody spots a predator, they don’t separate- they bunch up, all attempting get to the center. It doesn’t matter how bad this can get for the group as a whole, since the benefits for those lucky few are astronomical. So long as their behavior of sticking to the center means they get more genes out- which it does- they’re going to keep producing babies with a stick-to-the-center mentality. And that’s why they call it a selfish herd.
Now, I left a few things out of this very complex topic: while there are a very significant number of downsides for prey to live in large groups, there are some upsides: less time is devoted to finding food individually (though you’ll then have to compete for it) and it’s a lot easier to find a mate.
Similarly, other theories have decent support as a reason for why animals gang up. The predator confusion hypothesis suggests that predators get more confused as the number of targets they have to look at increases (at least in small-brained predators like sticklebacks). Also, some herds do use their numbers as a means to gang up on predators- cape buffalo are one famous example. However, this depends on how big you are compared to your predator.
The point is that selfish herd theory doesn’t explain EVERYTHING about why prey animals group together. But it’s a pretty big factor.
A factor called “please eat my friends instead of me.”
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Wiley, R. H. (1994). Errors, exaggeration, and deception in animal communication. Behavioral mechanisms in evolutionary ecology, 157-189.
We’re not talking bubonic plague here- we’re talking biblical.
With 2,227 individual species identified, members of the order Rodentia make up about 40% of all mammal species. The rat and mouse family, Muridae, makes up 700 of those species by itself. The small-bodied, fast-breeding, gnawing omnivore is clearly a highly successful body plan in almost any environment.
But there’s “successful” and then there’s “too successful.”
Murids have evolved to be opportunistic, taking advantage of resources whenever they find them: if they come across a steady food source, their population can swell exponentially.
In nature, there are gradual patterns of feast and famine, especially in seasonal areas, and animal populations naturally go up and down with the availability of food, abundance of predators, etc. In stable environments, this will result in cyclic population patterns.
Graph of the cyclic, sometimes dramatic, nature of lemming populations. (Contrary to popular belief, the decreases in population do not occur due to mass lemming suicide.) Source.
In especially good years, of course, the populations will go up more than usual. And when conditions are really, really good- well, that’s when things get interesting. Did I say interesting? I meant terrifying.
When humans started farming, way back in the day, they started the concept of surplus crop. Not surplus like the buried acorns of a squirrel or an Arctic fox’s dead duckling hoard, no. We’re talking silos of grain. Silos! Hundreds of pounds of grain! Can you imagine the first little mouse seeing those silos and sending a grateful little mouse prayer to mouse god in mouse heaven?
Now, for most animals, even a massive surplus of food wouldn’t make the population get too out of hand if it was all clustered in one place. This is because many animals are territorial and intolerant of having too many unfamiliar individuals near them at one time. Indeed, this is even true for most rat and mouse species, as social as they tend to be.
It may be, however, that the increase in available food is not the only factor driving the population explosions in rodents; in fact, several studies have found that there is a natural upper limit in population size no matter how much food they provided. So things shouldn’t get too bad…
…Except they do.
If you live in Australia, especially if you come from a farming family, the words “mouse plague” probably mean something to you. Probably something that is not entirely positive.
Flash back to 1994. It was a good year for crops; there was an unusually high amount of rainfall, and yields were wonderful. But it wasn’t just wonderful for the farmers- the humble house mouse was celebrating too.
Now, these mice are not native to Australia; they were imported along with people. But like many rodent species, they settled in to the relatively placental-free environment, exploiting niches that no native species had yet explored. Farmers were certainly already familiar with the frustrating mouse booms that frequently came hand-in-hand with successful yields. But in 1994, it got ridiculous: the field mouse population swelled to an estimated 500 million.
Here is a video about that particular plague, though I will warn that it contains scenes of mild gore, dead animals, and of course an uncomfortable amount of mice.
This isn’t a case of “oh, ew, a few mice,” this is “we can’t drive on the roads because our cars skid over mice,” this is “we can’t put on our shoes without having to dump out ten mice first,” and this is, most terrifyingly, “we go into our barns and see our farm animals being literally eaten alive by mice.”
That year mice destroyed 500,000 tons of wheat, or 30% of the country’s crop, costing billions of dollars. Thankfully, it didn’t last long- after a few months, the population eventually burned through the available resources and crashed.
(Say, human populations sure have been increasing pretty sharply for the past few years, haven’t they?)
While the 1994 Australian mouse plague is one of the most extreme plagues ever recorded, it’s certainly not the only one. As long as humans have been farming, there have been rodent plagues- records of rat and mouse infestations span as far back as ancient Egypt (no doubt spurring the domestication of the housecat!)
Australian farmers pose behind a massive pile of poisoned mice in 1917.
Today, such plagues are rare in places such as the US and most of Europe, but they are common in places such as China and India; anywhere there is a temperate climate with a good chance of heavy seasonal rainfall runs the risk of a rodent plague.
In the video above you can clearly see mice living shoulder-to-shoulder, having to crawl over top of each other, and literally flowing like waves because they are packed so tightly together. How could any animal tolerate these living conditions? As I mentioned before, there is a limit to how much contact even highly social animals will tolerate.
There are a couple of theories floating around as to why these rodent plagues occur. First, they occur most often under specific circumstances: a drought period spanning 1-2 years followed by a significant rainfall. But plagues don’t always occur even when these conditions are met, though their frequency of occurrence is- at least in Australia- actually starting to increase.
One of the most interesting theories for why rodent plagues only occur sometimes in seemingly ideal conditions is that rodents have “increase” phenotypes- that is, some mice have a specific set of behavioral traits that can lead to massive population increases, while others have a set of traits that leads to more gradual population increase. If not enough mice with “increase” phenotypes are present in the population, a mouse plague can’t occur even in the best of situations.
Interestingly enough, these “increase” phenotypes bear striking similarities to behaviors we select for in our own domesticated animals: increased social tolerance and docility, high reproduction rates, low infanticide rates, low dispersal rates, and low rates of territorial aggression. In other words, a surplus of more “tame” mice in the population might be what triggers these plagues. A population crash occurs when a combination of factors like increased mortality via disease, starvation, predation, etc. and/or an increase of the alternative, more aggressive and territorial phenotype arises.
The most terrifying thing about it all is that there is almost nothing we can do if a rodent plague happens. Short of raining poison down on the animals (which the Australian government has done), no amount of traps will get rid of such a horde once it appears. All the farmers can do is wait for the population to crash and hope that they won’t literally be eaten out of house and home.
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Ylönen, H., Jacob, J., Runcie, M. J., & Singleton, G. R. (2003). Is reproduction of the Australian house mouse (Mus domesticus) constrained by food? A large-scale field experiment. Oecologia, 135(3), 372-377.
Why Members of Sexual Species May Choose to Stay Chaste
Sometimes I hear people making derisive comments towards people who prefer not to have sex, something along the lines of how it goes “against nature” to never have sex, therefore something is horribly wrong with them, etc., etc.
The specific plague I wish upon those unpleasant people is an infestation of termites. Why termites? I’ll talk about that in a bit.
At one point highly social behavior presented kind of a paradox to the traditional, selfish-gene style evolutionary theory. Charles Darwin famously admitted that it was the social behavior of the bee that was going to bring down his entire construction, because most bees- nay, the vast majority of all individual bees spend all their times leading pious, sexless lives centered around helping one other bee reproduce. At the peak of the season, honeybee colonies can have 60,000 nonbreeding individuals- and just one sexually active queen.
Darwin, of course, did not yet know about genes, but he had an inkling that heredity was a clue- that by helping their relatives, the bees were actually helping themselves. Later scientists have filled in more of the gaps using modern molecular science, and yes, from a genetic standpoint, helping a relative is something like helping a piece of yourself.
But at what point does the value of helping close relatives outweigh the value of actually reproducing? That is a question biologists have been grappling with for quite a while. Because in the game of evolution, what matters isn’t how big your species’ population is- what matters is how many of those individuals share your genes.
I took a lot of pictures of dogs peeing on things for this article.
Exhibit A, Razzle.
If you own a dog, have walked a dog, or just have seen a dog on TV, you have probably seen a dog peeing. Particularly that stereotyped male raised-leg posture that Razzle is demonstrating above. (In this case, stereotyped refers to a fixed and repetitive set of movements, not a form of doggie-profiling.)
Dogs have a better sense of smell than we do. Heck, most mammals do; we just happen to be in a group- the simians- that ended up using vision a lot more than scent. At some point we more or less lost a means of communication that is absolutely fundamental to the lives of our hairy, warm-blooded cousins.
I’ve talked a bit before about how basic biological behaviors- such as sex or grooming or eating- can be co-opted by evolution to have a social meaning. For canids, urination has become a huge part of how they exchange information with one another.
We have a hard time studying this behavior because of our own limited sense of smell, and I think we are only beginning to grasp just how complex this scent-based communication can be.
I am about to tell you more than you ever wanted to know about dog pee.
I didn’t really even WANT to make a post about this.
The alpha-beta-omega model of wolf packs is dead in scientific literature, hammered into the ground, so to speak, and it’s been dead for over ten years. So why am I still hearing about it on TV and reading about it in articles? Why are popular dog trainers that encourage you to “be the alpha” still taken seriously?
I think the unfortunate truth is that the idea that there are strong and ferocious leaders in wolf packs and that you, too, can take on that role with your dog is just somehow appealing to people. Almost romantic, in the older sense of the word. And because of this, it makes money. It sells werewolf media. It sells dog training classes. Educational science channels that have no business promoting this false ideology keep it on board because it gets people watching.
If you couldn’t tell, I’m pretty fed up with the whole thing.
Okay, let’s talk about dominance, particularly what the word even means, because popular media does a terrible job of explaining it.
Juvenile rhesus macaque at the Beijing zoo. Source.
I have had my leg humped by a very small monkey named Peanut.
Peanut herself did not think that this was anything out of the ordinary; in fact, as she enthusiastically gyrated on my thigh, she kept glancing backwards to meet my eyes, her little face arranged in what seemed to be a contented expression.
At the time, Peanut and her companions at the rhesus macaque nursery I volunteered at were all less than three months old. They were just beginning to develop more complex behaviors than the scream-eat-poop trifecta (not that those three were any less present). The males were beginning to manipulate their own penises, and both sexes would frequently engage in enthusiastic humping sessions with objects and other monkeys. The other monkeys weren’t always thrilled about this.
I suppose this would make you imagine that the nursery just became a masturbatory frenzy at this point, but that really wasn’t the case. The vast majority of the time wasn’t spent in sexual exploration but in actual exploration, of their home spaces, of toys, of social behaviors like grooming, playing, and fighting.
I guess where I’m going with this is that when many humans are confronted with the idea of animal sex, they tend to fly to extremes: either animals are sex purists and only have it with the intent of reproduction (and NEVER use contraception!) or animals are hedonistic, lustful creatures that will spend every moment that they can touching themselves.
I think that this is because admitting that sexual behaviors- reproductive or not- make up a small but important part of an animal’s life is somehow more uncomfortable than either of those options. Perhaps it just hits a little too close to home.
But, as the title states, I am about to argue about why the study of animal masturbation (and other aspects of animal sex) is important. It matters. It matters for the ethical treatment of animals, it matters for conservation, and it matters because the study of something shouldn’t be limited by the fact that many people want to pretend that it never happens.