My wife has amazing breasts.
I should explain that I’m talking about her mammary glands, in particular. (All mammals have mammary glands, though not all mammals have breasts. Some of them don’t even have nipples. But we’ll get to that.)
A few weeks ago, my wife and I welcomed our second child into the world—a wee little squeaker named June. After nearly nine months of sipping her meals through a straw in her belly, this bitty baby popped out, threw some shade at the doctor, and immediately began to suckle from my wife’s breast.
And I guess that’s when it struck me—not only did my wife’s body just funnel all of its resources into the creation of this animal, rearranging organs and increasing its blood supply by 50 percent in the process, but once it had expelled her, my wife’s body shifted modes like a Transformer to further accommodate its mewling creation.
In the hours and days to come, my daughter’s near-constant stimulation would trigger hormones to wash over my wife’s body, initiating lactation. Her mammary glands would produce colostrum, a concentrated proto-milk full of immune cells, antibodies, and protein that kick-starts the infant’s digestion and growth. The colostrum would give way to a more regular supply of milk composed of thousands of bioactive molecules that ward off infection, prevent inflammation, promote immunity, spur organ development, and cultivate a healthy microbiome.
My wife’s body would produce just as much milk as my daughter could drink, replenishing the supply in between feedings, and tweaking the recipe as the baby grew. Human milk is made to order depending on the time of day, the length of the feed, and the diet of the mother.
Like I said: absolutely sucking amazing.
So in honor of my daughter, my wife, and amazing mammary glands, I did some digging into the wonder that is mammalian lactation.
More than 200 million years before the “Got Milk?” campaign, the first mammals crawled onto the scene. Mammary glands are the main unifier of all us mammals—from moles, dogs, and koalas to tigers, lemurs, and platypuses.
“There are over 5,600 species of mammal, probably closer to 6,000 once they all get described, and they all start life on a diet of milk,” says George Feldhamer, professor emeritus at Southern Illinois University and author of the widely used textbook Mammalogy. But as Feldhamer points out, lactation can differ greatly among those of us in the milk-maker tribe.
Human milk, for instance, is a watery brew with just 3.8 percent fat and 1.2 percent protein. Compare that with the milk of a blue whale, which is 38.1 percent fat and 12.8 percent protein. Blue whale milk has the consistency of “loose, runny cheese” and smells like it was made in a Bass-O-Matic.
As the largest creatures to ever live, blue whales also hold the record for the biggest bazookas—each mammary gland is nearly 5 feet long and weighs almost 250 pounds. Like humans, blue whales have two such glands, which means each adult female is carrying around a quarter of a ton of milk-producing machinery. Of course, all this milk-making power is what helps the blue whale calf pack on 37,500 pounds during its suckling period.
But the leviathans aren’t even the fattiest milk producers in the animal kingdom. That prize goes to hooded seals. Their milk can reach 61 percent fat, a richness even Paula Deen would find excessive.
But decadence in nature is never needless—hooded seal mothers nurse their pups for just four days on pack ice before returning to the sea to find food. This is the shortest lactation period of any mammal. That means the pups must pack on as much weight as possible, as quickly as possible, or die alone on the ice. Drinking nearly 45,000 calories of liquid butter each day is the only way such an arrangement can work.
While we’re talking about marine mammals, I should probably clarify that neither seals nor whales are swimming around with anything resembling cleavage. All marine mammals lack breasts as we know them and keep their working parts inside the body proper.
The pinnipeds (seals, sea lions, and walruses) have retractable nipples, which sounds like a burn straight out of the junior-high locker room but in truth just means the nipples tuck inside when they’re not in use. This likely cuts down on drag in the water and protects the tidbits from the cold.
The cetaceans (whales, dolphins, porpoises, narwhals, and the like) take this trick a step further by burying their nursing parts below a fold of skin called the “mammary slit.” These nipple-housing envelopes are found on either side of another, longer slit, which contains the genitals.
Because dolphins and whales don’t really have lips like most mammals, it’s been posited that the mother’s abdominal muscles actually squeeze the mammary glands and eject milk into the calf’s mouth. There’s some evidence that the babies can assist the process by curling their tongues into troughs, the better to create suction and gulp down their liquid lunches.
Of all the sucklers though, opossums are the most committed. Following a gestation period of just 12 days, baby opossums emerge from their mother’s vaginas blind, deaf, naked, and with brains that are only 9 percent developed, which isn’t much of a brain even for an opossum. About the only thing these marsupials have going for them is a massive set of claws, which they use to climb up the mother’s body and into her pouch. Once inside, those who find a teat latch on and don’t let go for two straight months.
The nipples swell inside the babies’ mouths, creating a lock so tight that scientists have found that trying to forcefully separate a baby from its mother can result in torn lips and nips. (I think I just heard my wife shudder from the next room.)
Over the course of 60 days, the opossum’s nipple will stretch and grow up to 35 times its original length. This creates a tether between mother and child not unlike an umbilical cord and every bit as vital. A baby opossum that does not find a nipple after birth will be dead in minutes. And the same goes for any offspring that become detached before they’re ready to wean.
Nipples—what strange little bio-valves they are. For all the time we spend talking about nip slips and Instagram’s nipple censorship policies, we pay scarcely little attention to the wild nipples all around us.
According to the 2015 edition of Guinness World Records, the animal with the most nipples is the female shrewish short-tailed opossum, which can boast a 27-gun salute. When scientists dart and collar female polar bears, they can determine an approximate age by nipple size, since the teats get longer and wider as the animals become teenagers. Kangaroos nurse two different generations of joeys at a time, which means one nipple is devoted to carbohydrate-rich milk for the neonate in its pouch and another nipple delivers fat-rich milk for the yearling living outside. (Kangaroos are baby-making machines.)
In porcupines, bleached fur around the nipples is proof that the female has reared young in the past. Uldis Roze, porcupine expert and author of The North American Porcupine tells me that porcupettes (really, that’s what they’re called) have also been documented to play a little game of Simon with their mother’s nipples. They first tug at one nipple, then another, then another, suckling milk from each gland in an order that they will repeat throughout their nursing months.
Anyone who has owned a cat or dog knows that nipple placement also varies throughout Mammalia. Ours are near our armpits, as are elephants’. But many other animals—cows, squirrels, giraffes—keep theirs toward the hindquarters.
And then there are bats. Bats are already pretty special when it comes to mammals, because they’re the only ones that said, “Eff it, we’re conquering the sky!” Bats also have fascinating sex lives. So perhaps it’s not so surprising that they have awesome nipples.
Like us, bats nurse from teats on the upper body. But reports kept coming in over the years from scientists seeing another set of nipples located down on the pelvis of some species. In many cases, the “pubic teats” did not seem to be attached to functioning mammary glands. So what possible purpose could these extra nipples serve? Oh, an awesome one.
Pubic teats “are used as devices for the young to hold onto when the mother is flying,” says Nancy Simmons, curator-in-charge of mammals at the American Museum of Natural History.
The pup simply latches onto the mom’s pubic teat, wraps its legs around her neck like it’s doing a reverse-hurricanrana, and then the two fly off into the night.
All of this is described in Simmons’ 1993 paper in which she examined the nipples on 1,723 bat specimens representing 206 species. Her work remains the definitive word on bat nipples—which one can only hope affords her plenty of free drinks in every bar on Earth.
While we’re talking about lactating Chiropterans, I should at least mention that the only male mammal that has been shown to produce milk in the wild is also a bat.
Dayak fruit bats captured in Malaysia have been shown to express a relatively small amount of what appears to be milk. However, before you start lauding these guys for being some sort of male milk nurses, I should also say that there’s no evidence that they help nurse the pups. And their drippy nips may be more of an effect of their diet than some sort of evolutionary advance.
“It is likely that Dayak fruit bats ate leaves or fruits containing plant estrogens, which stimulated their mammary tissue, which then produced some secretion,” says Paul Racey, professor emeritus at the University of Aberdeen and author of a dissenting opinion on whether male bats truly lactate.
Whatever the case, there’s a yet more impressive feat than males making use of their nipples—and that’s nursing young without any nipples at all.
I refer of course to the monotremes, the only mammals left alive that lay eggs.
“The platypus and echidnas don’t actually have nipples,” says Feldhamer. “They have mammary glands, they secrete milk, but it just drips out onto tufts of fur.”
Still, there’s a lot to be said in support of nipples. In addition to guiding the milk directly into the baby’s yawning maw, nipples minimize the milk’s exposure to microbes. Perhaps this is why scientists have found the presence of an antibacterial agent that occurs exclusively in monotreme milk.
This is far from an exhaustive list of everything interesting about mammalian lactation. The Stejneger’s beaked whale, for instance, has milk that’s blue-green in color. Rodents and many other mammals (including humans) tend to have half as many offspring per brood as they do nipples. The dwarf mongooses of Tanzania are one of the only wild mammals in which females that have never been pregnant will spontaneously lactate to cooperatively care for other young in the pack.
Every way you look at it, lactation is biological alchemy. That jug of cow’s milk in your fridge or breast milk in the freezer? It’s evolution in a bottle.
But now if you’ll excuse me, I have a baby to burp.