So what’s the deal with boobs, anyway? I mean… boobs! Wow, what a great invention. We wouldn’t even have lips without boobs, man. And what is the deep connection between the boob and the butt?
All right let’s talk about boobs and non-boob things on other animals and how it happened and where it went and yes, why men have nipples.
First, what is a boob? Or breast, if you want to get technical. Essentially, it is a housing container for the mammary gland- hey, that’s where we get the term ‘mammal’ from!- and a whole lot of fatty tissue.
The mammary gland is composed of little lobules containing cuboidal cells that secrete milk, which is pushed up through ducts out the nipple by the act of suckling. Milk is obviously meant as a means of nourishing mammalian young, which compared to the young of other animal groups are pretty ineffective at feeding themselves and generally getting around on their own.
Milk seems like a pretty good adaptation to feed warm-blooded young, when you consider the fact that they’ve got very high energy needs and they (usually) aren’t developed in eggs (so they end up coming out of mom a little squidgy and under-formed). Rather than having to constantly find food for your hungry squidgy little baby, you can just pop it on a nipple and keep it quiet for a while. Of course, the drawback is that you’ve got to consume enough food to keep producing milk.
The scientific community is not quite in agreement as to the when and why of the earliest mammary glands, but it is thought that they showed up actually quite a bit before mammals even evolved, in mammal-like reptiles or therapsids in the Triassic era.
That would make lactation (in one form or another) at least 200 million years old, which is kind of impressive, so please think about that next time you lay your hand on a breast. It is an ancient and holy object.
The first milk-type substance was probably produced by modified apocrine sweat glands, of the type that occur (on humans) in the armpits, genitals, inner nose, eyelids, and ear canals; kind of the extra-sweaty places. (The remaining sweat is produced from the less extreme eccrine sweat glands).
The hypothesis goes that this first milk was actually meant to keep eggs nice and moist in arid climates (remember, we were still laying eggs at the time, but not BIRD eggs- soft reptile-like eggs with very permeable exteriors). This milk wouldn’t have had much nutritional value, but it was probably antimicrobial in nature to help protect those developing babies.
So from that starting point, eventually milk did begin to have a nutritional value, probably as eggs began hatching earlier and earlier with babies less and less formed. Now understand that this first milk did not come out of an udder or even a nipple. Remember, sweat glands. This milk was just sweated right out of the underbelly. (Learn more about the evolution of lactation here.)
If we want to talk about sweaty, egg-laying mammals, we’ve got great examples of that today in the monotremes, a class of mammals with a resounding five species, the platypus and four types of echidna. They’ve got no nipples, but they do have two parallel little milk grooves along their belly, and they actually often lie on their backs when nursing their young.
Rather than beginning to lactate when their young are born, monotremes begin lactating as soon as they lay their eggs, lending support to the egg-moisturizer origin hypothesis.
Another thing to note is that monotremes have beaks, not soft lips, which is another reason why they wouldn’t have nipples- you can’t nurse without lips to form a seal around the nipple. Instead their young lap up milk from the milk grooves.
Past monotremes we’ve got marsupials (your kangaroos and your wombats), which do have nipples and do bear live young, albeit in a horrifyingly premature fashion.
And then, finally, we’ve got placental mammals, like us humans. Although I have to admit that it would probably be considerably less painful and stressful if we got to give birth to tiny squidges like kangaroos instead of watermelon-sized screamers. The newborn baby kangaroo even has semi-developed front limbs that it uses to crawl from mom’s vaginal opening into her pouch with. (Unlike kangaroos, most marsupials have downward-facing pouches to help facilitate this.)
Now, in placental mammals the exterior of the mammary gland has evolved a variety of different forms and locations. Thanks to our ancestral therapsids, we have ‘milk lines’- parallel lines along the belly and chest where milk can be produced with the right encouragement. So modern mammals can have nipples present anywhere along these lines. Up top, down below, in the middle… anywhere it’s convenient.
Unlike human breasts, most animal mammaries only swell when producing milk (we’ll get to why human boobs are so flashy in a minute). Which is why, even though bats also have two nipples up on the chest like humans do, you don’t see any bats flying around with hangy-outy tits. Rouge the bat in Sonic the Hedgehog is drawn entirely incorrectly.
Mammals can have anywhere from 2-18 nipples (pigs are the lucky winners of the most, which makes sense if you’ve ever seen what a lactating pig looks like). Dogs have 8-10, cows have four, rats have twelve, a cat has eight.
The Virginia opossum has 14 nipples, not 13 as is commonly reported. They are arranged in an oval with one in the middle. They are weirdly uneven.
It makes sense that animals like primates and bats would have nipples at the chest level, as primates would nurse their young by sitting up and holding them and bats would nurse them hanging upside-down (they can even fly with their young still nursing, which is pretty badass). Some species of bats actually have nipples directly under their armpits, the better to tuck their young under their wings.
However I’m not entirely sure why elephants evolved to nurse their young at the chest. Maybe it was so mom could make trunk contact with the baby while it nurses, though I have no idea if the front-boob in Proboscidea evolved before trunks did. A side note, another large factor in nursing is the ability for mom and baby to bond, which is VERY important in mammalian development, involving a lot of oxytocin and warm fuzzy feelings. Mammals raised without skin-skin contact of some sort usually have emotional difficulties later in life.
Someone on tumblr brought up the question as to whether brood size relates to nipple number. The answer is generally yes. There’s the one-half rule, in which nipple number is usually about twice the average litter size. The theory goes that this is to ensure that not only average-sized litters but large litters all have a good chance of surviving.
The one-half rule seems to work fairly well across most species, not just rodents. It seems pretty intuitive that primates, bats, and elephants which all usually have one and occasionally two offspring at a time, would have two nipples.
Now, female mammals are not the only ones with nipples! Though not all male animals have nipples- male horses and male rats and mice do not, for example. But it’s much more unusual for males not to have nipples than to have them.
The reason males have nipples (since it’s such a burning question for many people) is simple: our bodies start out with a ‘feminine’ template before sex differentiation in the womb. This includes nipples and an unfused uro-genital area. There was really no need for most males to evolve to lose nipples- because it would be a loss, not a gain, of a trait. (Not sure why some rodents and horses don’t have them, but there’s probably a good reason.)
To put it another way, there was no pressing evolutionary need for males not to have nipples.
(As a side note, I find it funny how hard it is for some people to accept the idea of a female body template, but they can readily accept the concept that a clitoris is a vestigial penis- which it isn’t.)
As far as I know, there is only one species in which male lactation has been recorded in the wild- the Dayak fruit bat. Yeah, bats again! This hasn’t been studied much, unfortunately, but it seems like the males produce less milk than females, possibly on a seasonal basis, because they aren’t always lactating. It would seem like this would mean that the species is monogamous, so that the male bat could help raise his own offspring, but there hasn’t been much evidence of that in other bat species, nor much evidence of the young suckling from male Dayak fruit bats, so I’m a bit skeptical on that front. There’s another theory that the bats consume plants with a lot of estrogen-mimicking compounds, which causes the males to lactate.
Males of other species can lactate, of course, but it’s generally under unusual circumstances, hormonally or genetically. Infants can lactate too, by the way, simply because of the high amount of prolactin they receive in their mothers’ milk. This is seen in maybe 5% of newborn babies.
Let’s talk a bit more specifically about human breasts, since I mentioned how unusual they are earlier. If you’ve ever observed a female great ape who is not currently nursing young, you will notice that her breasts are not so… out there.
The swollen breasts of the sexually mature female human are hypothesized to have evolved around the same time as bipedalism, when genital swelling would be a less obvious indicator of fertility because the female genitals would be tucked in between the legs when walking.
Some ethologists (notably Desmond Morris) even propose that boobs evolved to look like butts to encourage human males to want to mate face-to-face, which would be easier for bipedal apes. (This sounds like a recipe for disaster and hilarity during initial mating trials.)
Regardless of whether you buy that or not, human breasts are pretty biologically linked to sex and not just baby-feeding. When the nipples are stimulated in females, it activates the same parts of the brain as when the clitoris, vaginal opening, or cervix are stimulated. This also occurs in some males, stimulating their respective genital are of the brain.
By the way, while nipples don’t seem to be a sexual characteristic for most other mammals, female elephants are quite fond of stimulating each others’ nipples with their trunks. (Trunks frequently find themselves elsewhere as well, but you didn’t hear that from me.)
I think I’ve milked* this topic enough for now, so I will leave you with a few closing notes. Respect the boob. Respect the nip. Appreciate the evolution that went into these physical structures.
*YOU HAD TO KNOW THAT WAS COMING