Can We Talk About How Weird Baby Mammals Are?

Kangaroos with giant arms. Bats with clown feet. Sometimes, young animals’ proportions are shockingly different from their parents’.

a human baby with a clear face, a baby cow with clear legs, a baby bat with clear legs, a kangaroo with clear arms
Getty; The Atlantic

As adults, bats—the only mammals in the world capable of bona fide flight—are all about their wings. The trademark appendages can span up to 66 inches; they help bats snag insects, climb trees, attract mates, even fan their bodies in the summer heat. But as babies, bats are all about their giant clown feet.

Most mammals exit the womb with hind limbs that measure only about 20 to 60 percent of their maximum size. But compare a newborn little Japanese horseshoe bat’s foot with its mother’s, and they’re “almost identical,” says Daisuke Koyabu, an evolutionary embryologist at the University of Tsukuba, in Japan—even while the newborn’s wings remain fragile and small.

The comically flipped proportions of newborn and adult go beyond bats. Other creatures, too, weather some major anatomical transitions as they pass through puberty—a reminder that young animals sometimes live entirely distinct lives than their elders do. The changes are aesthetic, but they’re also functional. The body parts that matter most to animals later in life aren’t necessarily the ones that help them survive when they first slide out of the womb.

In a world of weird-looking babies, infant bats may reign supreme. Koyabu and his colleagues have found that the newborn animals’ hind limbs already clock in at 70 to 95 percent of their adult length. And the babies’ honking feet aren’t just for kicks. For many days after a still-developing pup is born, its mother must haul it around more or less full-time. But bat moms can’t carry their babies in the traditional sense, because “their forelimbs are wings” that are busy flapping and soaring, says Nicole Grunstra, an evolutionary anthropologist at the University of Vienna. So the infants use their ginormous feet to cling to their mother’s fur. It’s an impressive operation for both parties, considering that the winged newborns can weigh up to 45 percent as much as their parents do, the rough equivalent of a 140-pound human delivering a 63-pound infant … who also happens to have size 8 or 9 feet. (As if that weren’t bad enough, some bats are born feet first.)

Other species’ early-life anatomy is similarly practical, if not quite so strange. Newborn whales, dolphins, and other mammals have über-developed tails, so they don’t drown in the open ocean; cows, wildebeests, and other hoofed creatures are born with highly developed legs so they can sprint about, sometimes within minutes of birth, to keep up with their herd. And many nonhuman primates have sturdy, dexterous forelimbs at birth so they can hitch a ride on their mother’s front or back. (The newborns of a few monkey species have arms and hands so strong that they can hoist themselves out of their mother’s vaginal tract, then clamber up her front for milk.)

Then there’s outsize anatomy that doesn’t make babies that much more independent—but can still help them stay safe until they can hold their own. Adult animals of many species, humans among them, go gaga over the big eyes, large foreheads, and pudgy, kissable cheeks of their young. The reaction begets more caregiving, which protects the infants while they remain in their fragile, freshly birthed state. People are so into cute that they’ve bred some of these exaggerated traits into certain companion animals, such as dogs—though puppies’ sometimes hilariously large paws aren’t all that disproportionate at birth, especially compared with the feet of their distant bat kin.

Wonky dimensions can come with costs. After a gestation that lasts only a month or so, red kangaroos emerge from the birth canal as little more than a jelly-bean-size nub, pink and hairless and blind. At this stage, they are about as developed as an 8-to-12-week-old human fetus; the brain and skull are shrunken, the backside tapers into near nothingness, and the lower legs—so important in adulthood—are “stubby and nonfunctional,” says Kathleen Smith, an evolutionary biologist at Duke University. These still-fetal structures leave the joey in a pretty perilous state. “It’s really extreme—they could die suddenly,” says Ingmar Werneburg, an evolutionary morphologist at the University of Tübingen, in Germany, who’s been collaborating with Koyabu on bat work. But that’s the cost the little creature must pay to ensure that its short gestation prioritizes the development of its front half: a strong, sucking mouth, flanked by some seriously jacked forepaws—the bits of anatomy it requires to crawl out of its mother’s vagina and into her pouch, where it can fuse itself to a life-sustaining teat.

Part of the kangaroo’s problem is its super-short stretch in the womb. But even bats, which spend an average of three to four months in utero—surprisingly long for mammals of their size—pay a tax for their adult-size feet. They’re also born hairless, with ultra-fragile wings just a third or so of their adult size, says Taro Nojiri, a biologist at Juntendo University, in Japan, who has been studying bats with Koyabu. And they, too, have to change their development strategies when they fully enter the world.

Eventually, the Picasso-esque extremes of childhood give way to a different set of proportions—and, sometimes, a very different way of living. Once bats have graduated out of toddlerhood, their feet take a functional backseat to their echolocating mouths, sensitive ears, and powerful wings; joeys, too, execute a switch-up, as their legs and tail undergo a massive growth spurt during their months suckling in their mother’s pouch. It’s a near reversal of the animal’s entire architecture—proof positive, Koyabu told me, that “the morphology of the newborn is not just a miniature morphology of the adult.” Infant animals are not just preludes to their elders, but their own entities, with unique needs, vulnerabilities, and experiences of the world—even if that uniqueness can saddle them with bodily proportions that feel comedically out of whack.

Perhaps bats’ first days would be simpler if they came out with their wings already raring to go; maybe kangaroo joeys would have an easier journey from vagina to pouch if they could just use their hind legs. But gestation is a race to funnel resources to the organs the newborn will need most, and the sooner it’s over, the better for Mom. “Pregnancy in mammals is horribly dangerous,” Smith told me. The longer it stretches on, the greater the risk to the mother, and the more time she spends with her body not entirely her own. The good news, Smith said, is that limbs and organs are quite good at playing catch-up. The important thing is to build up the bits of the infant body that will give the kid the best start possible—no matter how odd it might look.