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?
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?
Seriously, how the hell do you define play?
Let’s Attempt to Define Play
And so here is one of the biggest stumbling blocks to the study of play: trying to figure out what it really is. It feels pretty safe to assume that the behavior you see in the vine I posted above counts as play behavior. Everything about it screams “I’m having fun!” But we can’t rely on such a subjective definition if we want to study it.
Still, there’s a part of the definition. Play usually seems to be fun. If we want to make that scientific, we can say that play behavior is associated with pleasure, or is at the very least self-reinforcing. But this can’t be our only criteria, because many other behaviors are pleasurable. Eating, for example, is pleasurable, but not necessarily play; the same goes for sex. We need to narrow our definition a little more.
What else can we observe about Chloe’s play behavior? Well, if we break it down very simply, what Chloe is doing in the video is running around, picking up sticks, and shaking them vigorously from side to side. Of course all of those behaviors do not necessarily occur in a playful context: dogs will run, for example, for a variety of reasons, and the motions used in picking up and shaking sticks are very similar to the movements dogs will employ when attacking and killing small animals.
We could say that what differentiates play behaviors that resemble ‘serious’ behaviors is just that- they don’t seem serious. But of course, like fun, seriousness is not exactly a scientific label. We should say, rather, that the behaviors Chloe is performing are different from their serious counterparts in that they are a) exaggerated, such as when she bounces and high-steps towards the camera, as well as b) incomplete, such as when she drops the stick after ‘killing’ it and doesn’t try to eat it.
So to that, we could add that Chloe’s play behaviors are not aimed towards an immediate goal: she’s not aiming to eat, and she’s not running because she needs to get somewhere. This is quite important.
Thus far we have determined that we call Chloe’s behavior ‘play’ because it resembles other functional behaviors but does not have a definite goal, because it is an exaggerated or incomplete version of those other functional behaviors, and because it appears to be pleasurable. In fact, these are three out of the five criteria that preeminent play scientist Gordon Burghardt laid out in his definition of play. The other two- criteria 4 and 5- can’t be so easily observed in the video.
Here are his five criteria:
The performance of the behavior is not fully functional in the form or context in which it is expressed; that is, it includes elements, or is directed towards stimuli, that do not contribute to current survival.
The behavior is spontaneous, voluntary, intentional, pleasurable, rewarding, reinforcing, or autotelic (done for its own sake).
It differs from the “serious” performance of ethotypic behavior structurally or temporally in at least one respect: it is incomplete (generally through inhibited or dropped final element), exaggerated, awkward, or precocious; or it involves behavior patterns with modified form, sequencing, or targeting.
The behavior is performed repeatedly in a similar, but not rigidly stereotyped, form during at least a portion of the animal’s ontogeny.
The behavior is initiated when the animal is adequately fed, healthy, relaxed, and free from stress (e. g. predator threat, harsh microclimate, social instability) or intense competing systems (e. g. feeding, mating, predator avoidance).
For something as theoretically simple as play, that’s one long definition. Burghardt does attempt to summarize it all in a single sentence as well:
Play is repeated, incompletely functional behavior differing from more serious versions structurally, contextually, or ontogenetically, and initiated voluntarily when the animal is in a relaxed or low-stress setting.
It still sounds a bit clunky, but it’s actually about as simple a description as we will ever get. Play itself is not that simple of a behavioral category.
Now, before we move on from what play is to what play does, we should look quickly in closer detail at critera 4 and 5. This is important because play is actually at the center of a spectrum of three behavior types:
Both exploration and stereotypic behaviors can be easily mistaken for play. Exploration refers to an animal’s reaction to a novel environment or stimuli. For example, if you give a child a new toy, they will generally eagerly take it and examine and manipulate it. However, after thoroughly investigating the new toy, the child may toss it aside and play with their favorite beat-up GI Joe doll.
What differentiates exploration from play is that the animal will continue to show interest and attention to objects or places that they have already acclimated to. In other words, the behavior is repetitive. This repetition also takes care of any ‘accidental’ behaviors- i.e., Chloe really does plan to eat that stick after she attacks it. If that was the case, she likely wouldn’t continue attacking it after discovering that it is, in fact, inedible.
In the following video, we see a dog exploring several new toys. Note the differences between the dog’s initial exploration of each toy and the way his behavior grades into repetitive, exaggerated play with his preferred toy after familiarization.
But there is such a thing as over-repetition. Playing video games can be thought of as a form of human play, but when a person becomes addicted to gaming it ceases to be a fun activity. Likewise, animal stereotypies do not necessarily have a component of pleasure to them, and may not appear so voluntary. A ‘stereotypy’ refers to a behavior that is highly repetitive and ritualized. Animals commonly develop stereotypies when housed in understimulating environments.
Superficially, stereotypies can resemble play because they are also “functionless” behaviors. However, while play is defined as repetitive, it is still variable. Stereotypies also frequently occur in situations where the animal is stressed or overstimulated (as coping mechanisms), while play as Burghardt defines it takes place in a relaxed setting.
This brings up some interesting questions about our own behavior. I suggested that humans playing video games (at least as long as those games remain fun) could be considered play behavior. But what about other recreational behaviors such as hiking or swimming or even reading for pleasure- are those considered play? The answer is ‘probably,’ at least under Burghardt’s definition, especially if you do the same activity more than once. (Doing just one hike and deciding you hate it would fall more under exploration than play.)
Any sort of art creation without the intent of making money might then be considered play, though it really is a fine line. Heck, me researching and writing this article is more or less play, if we want to get super meta. I am playing so that I can teach you all about play.
In this respect we should be able to understand that play is not always pleasurable, even if we do it for the overarching reason that it is ‘fun.’ Watching a sad movie is not generally pleasurable in the moment, for example; the same might go for intense forms of animal play. Play may be more of a simulation of many emotions in a safe environment, not just the pleasurable ones.
Okay, so hopefully now we at least have a rough idea of what play is as well as what it isn’t. Now comes the fun part.
Who Plays and How?
Play is a multi-layered thing. Compare, if you will, the playful behavior of a cat and the playful behavior of a dog. Cats show a lot more manipulation of objects via their paws, while dogs show more manipulation with their mouths. Obviously, there is some overlap.
Take a look at these two videos of a cat (Babu) and a dog (Odin) chasing a laser. While both of them employ their paws in the beginning, at the end Babu engages in some furious batting at the dot while Odin makes every attempt to straight-up bite the wall.
It seems fairly obvious that the differences between these play behaviors is because they are modeled after the predation behavior of both species: cats use their claws to strike or knock down prey, while dogs are more likely to stab downward with their paws to pin prey before biting it. We also would not expect to see such predatory play in an herbivorous animal such as a horse- rather, we’d expect to see more play related to escape behaviors. And in fact, we do: horses employ a lot of play behaviors related to running, jumping, kicking, and twisting.
Despite differences between species, there are three main categories that most researchers divide animal play into: locomotor play, object play, and social play. Locomotor play refers to play that involves vigorous physical activity, such as the aforementioned horseplay.
Object play is also straightforward: it occurs when an animal manipulates an object. Horses show object play as well.
The third category, social play, occurs when two or more animals engage one another in play. Like locomotor and object play, social play often mimics ‘serious’ social behaviors, such as mating or fighting behaviors.
Notice how the two horses are inhibiting their biting behavior: despite the laid back ears and snapping teeth, neither actually ever bites the other. Such inhibition is common during social play, even to the extent where an animal will self-handicap if it has an advantage over its smaller play partner in size or strength so that play can continue without someone ‘winning’ too much. (However, this is not always true, particularly in play between separate species. A cat may play with a mouse rather than eat it, but the mouse probably isn’t participating voluntarily.)
It’s important to note that these three categories are not exactly discrete, and often grade into each other. In fact, all three can overlap: a dog playing tug of war with a human is certainly showing active locomotor activity as well as object play and social play at the same time. Most researchers deal with this by making some categories all-encompassing: any play with a social element is always social, while any play using an object without a social element is object play, leaving locomotor play to be categorized only as solitary play without an object.
So I’ve given a few examples of the types of play found in placental mammals like horses, dogs, and cats- who else plays? I think that most people are comfortable attributing play behavior to mammals and some bird species, but it’s harder to say that animals such as reptiles or even fish show play behavior. Do axolotls show play behavior? I’ve cared for them for over three years, and I still can’t say for sure.
It is fairly easy for us to define play behavior in mammals because we are mammals ourselves. The signals that many mammals show when playing, such as the open-mouthed ‘play face’ and the exaggerated leaping and twisting movements, are fairly ubiquitous. In animals that we have fewer behavioral similarities to, it’s much more difficult to discern play behavior with limited function to behaviors we just don’t understand yet. When my axolotl snaps repeatedly at my fingers, for example, is it a form of social or object play, or are axolotls simply incapable of learning that fingers do not equal food after repeated trials? I honestly do not know. Scientists have stumbled across this problem again and again: what looks like play may actually be an anthropomorphism. But the opposite is true: we might not see play in certain animals because we don’t expect to.
Sadly, the literature for many groups of animals is fragmented and inconclusive. A comprehensive review of the literature by Burghardt in 2005 yielded this extensive phylogeny of play in animal species, though unfortunately much of it is based on anecdotal evidence. Also note: just because there isn’t evidence of play in some animal groups doesn’t mean that none will be found in the future.
Seeing any more surprises in that chart? I think by anyone’s ordinary thinking, the only group that should be highlighted in green is the chordates- i.e., the vertebrates. Us. And maaaaaybe we could even accept that octopuses (which are mollusks) might play, but… insects? Crustaceans? Spiders?
By the way, I am not going to put up a phylogeny of recorded play behaviors in every major vertebrate group- fish, amphibians, reptiles, birds, and mammals- because with the possible exception of amphibians, behaviors that look like play have been observed in each and every one of them. Yes, even in fish!
This doesn’t mean that every species plays, mind you; certainly not every mammal species. Even closely related groups can be vastly different- rats play mountains more than mice do, for example, and some species like aardvarks don’t appear to play at all. Still, almost every major group of mammals has some representatives that show play behavior.
The jury’s still out on monotremes, though. Is this platypus playing, or just showing foraging behaviors? (Platypuses wiggle their beaks from side to side in order to detect prey using electroreception.)
But let’s move beyond mammals. Play in birds should not be too surprising for anyone who’s spent time with some of our feathered friends. According to Burghardt, play has been recorded in about half of the avian orders, including in parrots, chickens, ducks, owls, doves, raptors, and songbirds. It was quite surprising to me that he found no recorded evidence of play in ostriches or emus; I think that they do play.
Still, despite how widely play behavior is scattered across the avian family tree, play behavior has only been observed in about 1% of all species. This may be due to the difficulty of identifying play behavior, as I mentioned before, in animals so different from ourselves. Locomotor play, for example, might be harder to identify when the animal in question is flying.
Okay, so reptiles. It feels like reptiles really shouldn’t play. And for the most part… they don’t play all that much. You won’t find much fooling around by your average house gecko or iguana or snake. But there are at least two notable groups of reptiles that do definitely show some play: the turtles/tortoises and the moniters.
Recently there has been a push to include crocodilians in that bunch as well, though again, the evidence is mostly anecdotal. If they play, it’s probably not terribly often.
Amphibians are the sole vertebrate class with no definitive evidence of play behavior. However, just because there’s no definitive evidence doesn’t mean that there is absolutely none. At times the wrestling between frogs, for example, can resemble play behavior, and several aquatic salamander species have been observed “bubble riding.” This peculiar behavior is when a salamander repeatedly swims over top of an air pump in the tank and rides the bubbles upwards. I’ve seen this behavior numerous times in axolotls, and it doesn’t seem to be due to an interest in obtaining more oxygen (the salamanders continue riding the stream if the bubbles are composed of helium).
Axolotl owners will also probably be familiar with their frustrating tendency to repeatedly unearth plants; I had one particularly determined fellow dig a tunnel through the substrate underneath a tank divider and two jars of sand. This same axolotl has a habit of pushing large rocks around and repeatedly biting them, and a fascination for ripping up plants (to the point where he has been banished to a corner of the tank separate from his plant-friendlier companions). Is this play, or is this salamander just programmed to do things that drive me up the wall?
Only careful research will tell, and in the meantime I like to err on the side of fun and provide my axolotls with numerous outlets for activity in their tank. But now we have to go back even further in the vertebrate family tree- let’s discuss play in fish.
First, put your skepti-goggles on and watch this video.
What exactly is going on here? It’s very hard to shake the first notion that the fish is enjoying being stroked, picked up, and thrown by the man, as it returns again and again to his hands. Of course, it doesn’t preclude the possibility of several alternative reasons which I will list here:
- The fish is approaching the man’s hands due to territoriality/aggression (though it does not seem to be attempting to bite them).
- The fish has a skin ailment (such as ich or parasites) and it is gaining relief by rubbing on the man’s hands.
- The fish has been well-trained using food rewards to approach the man’s hands and is expecting a reward for approaching.
All are plausible, and there are probably fish experts who could come up with other reasons as well. However, this isn’t the first time I’ve ever heard of such “play” between man and fish. Famed German ichthyologist Wolfgang Klausewitz reported in one paper that he had observed a friend who had trained fish to come to his hands, and that they appeared to enjoy it when they were tossed into the air, to the point where they competed for who got to be tossed next.
‘Leaping play’ has actually been described in fish a number of times. Fish do leap to escape predators, but those leaps are quite different from ‘playful’ leaps. In most cases the fish will find a stick or other object floating on the water’s surface (in once case, apparently a sleeping turtle) and repeatedly leap over it. The fact that these fish often tend to rub their sides on the object in question has led some researchers to suggest, again, that this is a means of getting rid of parasites. However, fish with skin issues tend to rub themselves on substrate and limit excessively energetic movements like leaping (probably because they are not feeling well).
Beyond leaping behaviors, some fish also may show object play. Elephantnose fish in particular have been cited multiple times in the literature as being fascinated by manipulating objects, from plastic balls to snails. According to Erich Ritter, great white sharks will play with cardboard and other objects floating in the water. Some authors even suggest that ‘food-spitting’ behavior, as when a fish repeatedly sucks in a piece of food and spits it out again, is a form of play because the fish usually do it when they are sated. (By the way, I’ve seen axolotls doing this too!)
Mistaken for food or playful behavior? You decide.
Perhaps you aren’t convinced of play in fish; well, let’s move on to cephalopods. Squid, cuttlefish, and octopi are quite the leap from vertebrates, yet many people are more accepting of the idea that octopi can exhibit play behaviors than fish.
Despite this, there is actually relatively limited evidence that octopi exhibit ‘true’ play behavior. While they are avid explorers of novel objects, it is relatively rare for an octopus to continue interacting with an object past the initial exploratory period, and the fact that they show the same behaviors when food-deprived violates one of Burghardt’s play criteria. Still, on the occasions that they do ‘play,’ it looks very similar to mammalian play- perhaps this is why octopi get so much attention.
Few other invertebrates have been observed doing anything remotely playful. Mantis shrimp and lobsters are two examples of crustaceans that show a lot of interest in manipulating unfamiliar objects, perhaps to the extent where exploration grades into true play. Young cockroaches may play-fight with one another, and honeybees may perform ‘practice’ play flights before reaching foraging age.
Here’s what a serious cockroach fight looks like, by the way.
How would you know whether or not that fight was playful? I guess a cockroach expert would be able to tell.
It is extraordinarily difficult to read intent, motivation, emotion, et cetera into the behavior of any invertebrate, which makes the line between play and instinct remarkably hard to draw. However, if we accept that play could occur in some invertebrates, we have to think about something else: play might not be reserved for highly-intelligent, big-brained animals after all! Is play more primitive and ancestral than we think it is? Furthermore, the fact that it appears in such a scattered way across so many animal groups would suggest that it has evolved multiple times. Perhaps play is that important.
That all said, it is extraordinarily important to take all of this information with a grain of salt. Beyond mammals and birds, much of the play behavior I’ve described comes from anecdotal evidence, not serious scientific study. There is every chance that these behaviors will end up not being play after all- at least by Burghardt’s definition.
We must constantly keep in mind alternative explanations for behavior- this is true of the study of any animal behavior, not just play- but none of the ‘alternatives’ have more evidence in these cases than the theory that the animals are playing. Why treat play more harshly than the rest? We can only ask that more studies be performed.
The possibility that play is a) evident even in ‘lower’ animals, b) may have evolved multiple times, and c) is dissimilar in some closely-related species, suggests that play is something that really matters for some kind of major evolutionary reason.
Why Do Animals Play?
We don’t know.
Yep, that’s it- we just don’t know.
I’d love to wrap this beast of an essay up with some neat little theory, but the fact is that there is honestly no single theory behind why play evolved that has decent evidence backing it up. There’s not even one that kind of fits.
This despite what you commonly hear repeated on television documentaries and by confident teachers: play behavior is practice for adulthood. Play behavior is found in intelligent animals because it exercises creativity. Social animals are more likely to play.
There just ain’t any real evidence for any of that.
Despite the popular conception that play is practice for later life skills, there is almost zero evidence to back it up. Cats who pounced and batted at objects as kittens were no better at hunting than cats with limited object play; the same went for coyotes and grasshopper mice. Rats, meerkats, wolves, and many primate species are no better at winning fights based on how often they play fight as youngsters.
Play may not necessarily be found in larger-brained, social animals either. Our domestic dogs and cats have brains roughly 30% smaller than their wild forebears, yet play a lot more, and many animal species which are solitary as adults play a great deal when they are young. In fact, primate species with more rigidly structured and complex societies tend to play less than primates with loose social organization, and within canids, the ultra-social bush dog shows much less complex play behavior compared to the solitary maned wolf. So play doesn’t seem to offer much social practice.
It also doesn’t seem to reduce aggression or reinforce social bonds between individuals- in one study on meerkats, individuals who played more together weren’t any less likely to get into fights or any more likely to disperse from their home group together.
To all appearances, play doesn’t do jack shit. Except that it does seem to increase an animals’ chances for survival- at least in studies of juvenile horses, bears, and rats. We just don’t know why.
Okay, before we all throw our hands up and leave, let’s look at a few things researchers do know about play:
- Juvenile animals tend to play more often than adults.
- Animals that show greater behavioral complexity are more likely to play (this relates to the fact that animals with extended juvenile periods are more likely to show more behavioral complexity).
- Species of animals cared for by their parents are more likely to play than species without parental care.
- Play behaviors tend to appear early in life as mimics of ‘adult’ behaviors the animal cannot use yet, i.e., pouncing play observed in a kitten before it is old enough to hunt by itself.
- Animals with excess energy not used in growth or metabolism are more likely to play than animals without excess energy.
- Play can be costly (predation, injury, etc.) but often isn’t, especially when a juvenile has parents looking after it.
- Animals are more likely to play when understimulated or kept in sterile environments.
- There is a genetic component to play- different strains of laboratory rats play at different rates! You can actually breed rats to be super-playful!
- And finally, play is a low-priority behavior. Animals that are hungry, frightened, aroused, or stressed are unlikely to play.
The last point is a rather important one, because it has ethical considerations for animals kept in captivity. If we see an animal playing, we can feel safe assuming that the animal is, for the moment, in a calm, satisfied state. However, point seven- that animals are more likely to play when kept in sterile environments- also raises the suggestion that animals may play when they are calm yet bored. This is likely why researchers tend to see more play in captive than by wild animals.
Calling play ‘low priority’ would signify that it is not that important. However, in several studies, animals that had undergone a brief period of nutrient-deprivation played more than either animals undergoing chronic malnutrition and animals that had adequate nutrition their entire lives. The authors thought that the animals that had been nutrient-deprived for a short period were compensating for the lack of play during their time of malnutrition later in life. This suggests that there was a certain threshold of play that the animals needed to reach.
Point five, that animals tend to play when they have more energy to spare, makes a great deal of intuitive sense when we consider play a behavior that is done after all other needs are met. This can partially explain why some species play more than others. The platypus, which spends almost all of its waking hours foraging for tiny amounts of food, probably has little time or energy to spare on play, and most cold-blooded reptiles would suffer from excessive antics. This could be why, of all the reptiles, turtles are one of the most playful groups: it takes less energy to move in water than it does on land.
However, that still doesn’t adequately explain why the animals that do play, play.
Our friend Burghardt came up with the surplus resource theory (SRT) of play. He suggests that when a species evolves parental care, there is much less selection pressure on their infants. For example, kittens do not have to be competent hunters since they are cared for by their parents, in contrast to, say, many reptile species which are left on their own to hatch and go. The lack of selection pressure for competent hunting behavior led to a looser, functionless version of the behavior we’d term ‘play.’ Burghardt suggests that this play was not very costly for the kittens and in actuality incurred a mild benefit- that is, it gave them the opportunity to make their behavioral repertoire more flexible through making ‘mistakes.’
It’s an interesting theory with little actual support yet. But Burghardt does posit that many of the other theories for why play behavior has evolved- fight or predation practice, social competence, etc.- would be secondary benefits to having play behavior and not the initial reason for its evolution. That is why they have only scattered support at best.
Confused? It’s okay, so are most scientists.
There is a bit of evidence that play helps the brain develop more neural connections and learning ability. Studies on rats in highly enriched environments show that if you allow the rat to have a highly varied sensory experience (i.e., touch, taste, smell, etc.) but no play, these benefits are markedly reduced. So the play definitely helps. And primates that show more play behavior are more likely to manipulate objects in complex ways. It seems that play behavior does have something to do with behavioral diversity.
There is also increasing evidence that play helps reduce stress, at least in the short term. This is somewhat ironic because play itself is supposed to occur only in nonstressed animals; but according to research on rats and marmosets, animals show fewer stress responses following play bouts. Play may help relieve social tension when the players anticipate that a stressful event is soon to occur.
But let’s go all the way back to the octopi, mantis shrimp, and cockroaches. None of these animals are cared for by their parents, and all of their play behaviors were observed in adults. In order for the surplus resource theory to apply to these animals, we’d have to assume they were living remarkably stress-free lives with little fear of predation. And the answer to that is maaaaaaybe. Certainly not so much during the harrowing juvenile stages of octopi and mantis shrimp (cockroaches are delicious protein no matter the age).
Well, the theory isn’t perfect, and I haven’t found any research in the way of testing it since Burghardt came up with it in 2005. Unfortunately, play behavior remains mystifying and difficult to study. Imagine trying to get a grant to do an experiment on play behavior on cockroaches! They’d laugh you out of the office.
I still think that if you can manage it, you ought to apply for that grant, though. We see all kinds of behavior in all kinds of animals, from parental care to complex construction to incredible memory and so on and so on. The more I study, the more I suspect that an animal’s position on the phylogenetic tree has less influence on behavior than does an animal’s immediate needs. Necessity is the mother of invention, right?
Maybe they just needed to play.
To see a list of all animal articles I’ve written, click here.
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