What Human Cheeks, Bat Feet, and Kangaroo Arms Have in Common – DNyuz

What Human Cheeks, Bat Feet, and Kangaroo Arms Have in Common

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. Bats love their clown feet, but as infants they are obsessed with them.

Most mammals exit the womb with hind limbs that measure only about 20 to 60 percent of their maximum size. However, a newborn little Japanese horseshoe bat’s feet and its mother are almost identical, says Daisuke Koyabu (an evolutionary embryologist at University of Tsukuba in Japan). Even though the baby’s wings may be fragile, they still have the ability to fly.

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 full of strange-looking babies infant bats might be the most powerful. 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. The infants cling onto their mothers’ fur with their huge feet. 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 uber-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, dextrous forelimbs at birth so they can hitch a ride on their mother’s front or back. A few species of monkeys have strong arms and hands that enable them to lift themselves from their mothers’ vaginal passage , to climb up the front of her mother 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. This leads to more caring, which helps the babies remain healthy and in their newborn state. Some people are obsessed with cuteness and have bred certain traits to companion animals such as dogs. However, puppies can be quite large at birth and are not all that disproportional compared to their 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. Ingmar Werneburg (German evolutionary morphologist, University of Tubingen) says that the joey could suddenly die. 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. Even bats spend three to four months in the womb, which is unusually long for mammals their size. They also have to pay a tax on their 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. They also have to adjust their developmental strategies once they enter the real 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 could have a simpler start to their days if their wings were already ready to fly. Maybe kangaroos would make it easier to travel from their pouch to their vaginal area if they had to use only 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. Smith stated that the good news is that organs and limbs can catch up. It is important to give your baby the best possible start, no matter how strange it may seem.

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