Why Some Animals Have Really Big Babies

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Evolutionarily speaking, every human is a bit of a preemie. The nine months most babies spend in the womb are enough for them to be born with open eyes, functional ears, and a few useful reflexes—but not the ability to stand, sprint, climb, or grasp onto their parents’ limbs. Compared with other primates, our offspring are wobbly and inept; they’d probably get their butts kicked by infant lemurs, gorillas, and even tiny tarsiers, which all come out more fully formed. Think of it this way: Researchers have estimated that, for a newborn human to be birthed with a brain as well developed as that of a newborn chimp, they would have to gestate for at least an extra seven months—at which point they might run 27 inches from head to toe, and weigh a good 17 or 18 pounds, more than the heftiest bowling ball on the rack.

The technical jargon that scientists use for underdone newborns like ours is altricial, though when experts are on the phone with journalists, they’re sometimes more apt to toss around words like useless or pathetic. “It’s almost embarrassing how helpless we are, when we compare ourselves to the wild world,” says Jared Stabach, a research ecologist at the Smithsonian. “It makes me wonder how we got to 7 to 8 billion people on this planet.”

Among animals, humans aren’t alone in emerging in such a state. Most songbirds are altricial, hatching in large clutches of chicks that come out pink, naked, blind, and struggling to keep warm, making them super dependent on their parents; much of the same is true for bears such as pandas, which deliver young that are just one-900th of the mother’s size and can’t pee without outside help. The pressures wild animals face—intense predation, food scarcity, environmental stress—can sometimes push certain species to emerge from eggs or wombs earlier. But experts don’t fully understand the reason people are born so vulnerable. One hypothesis, called the “obstetric dilemma,” holds that human hips, which shrank and slimmed as our species evolved to walk upright, are now too narrow to accommodate baby heads of any larger size. Another posits that birth is the termination of an unsustainable lease: Human parents may evict their fetal tenant around nine months to unburden themselves of its thirst for nutrients, or perhaps the baby gladly vacates the premises, having hit the point of diminishing returns.

However you look at it, pregnancy is marked by intergenerational strife, if not an all-out war between an offspring and its parent. Birth, then, in addition to welcoming new life, can bring about an end to the harshest hostilities—and its timing partly reflects the terms of a tightly negotiated truce.

That idea might be tough to square with the common conception of pregnancy as “this joyous, wonderful, great time, with the fetus and the mom working toward the same shared interest,” Jessica Ayers, an evolutionary social psychologist at Boise State University, told me. The goals of child and parent, however, don’t always align, even when one is growing inside the other. Fetuses may maximize their chances of surviving after birth by extracting as many resources from their parent as they can. Their tool for mooching is the placenta—technically, the very first organ that any human produces, Ayers said—which allows a fetus to access its mother’s blood vessels and siphon out nutrients. The human placenta actually entrenches itself so aggressively into the uterine wall that it sometimes leads to severe hemorrhaging at birth, Ayers told me, when the tissue starts to rip away.

For the mother to prioritize her own well-being—and her chances of bearing more offspring—her body must hoard at least a few nutrients for herself. “So there’s a conflict over allocation,” says Ava Mainieri, an evolutionary biologist who studied maternal-fetal conflict at Harvard. The fetus repeatedly attempts to remodel its parent’s interior—her circulation, her blood sugar, even her immune system—in an effort to drain a few more drops of the reserves that she is vying to save.

Human mothers, then, may have forged a delicate détente with their offspring by delivering them in a feeble, naked, clingy state. But that’s not the only resolution that can be reached.

Plenty of other creatures give birth even sooner, yielding offspring in a more altricial state. Marsupials, in particular, take this strategy “to the extreme,” David Haig, an evolutionary biologist at Harvard, told me. A red kangaroo, for instance, will gestate her fetus for just four or five weeks before plopping a pink, hairless, jelly-bean-size joey into her pouch, where it will remain for another eight or so months. When little roos are born, their legs “aren’t even properly developed,” Haig said. Roughly 100,000 times smaller than their mother, the joeys are basically a mouth with giant forepaws, built to crawl into the pouch, grab onto teats, and suckle—and do little else. “The rest of the body catches up later,” Haig said.

Lactation is a massive chore for moms, often requiring many more calories than even gestation does—and some animals are born so underdeveloped that they have to nurse for an exhaustingly long time. But an early birth can still quell some of the roughest aspects of the maternal-fetal tussle. Offspring “have less negotiation power once they’re born,” Amy Boddy, a biologist and evolutionary theorist at UC Santa Barbara, told me. A pregnant animal can only do so much to keep a fetus from stealing nutrients from within; a new mother, meanwhile, can choose to pause feeding or coddling its newborn at any time. Chicks, pups, and infants may beg, cry, or whine to extort food from their parents—but these efforts aren’t as direct as what occurs in utero. “The solution to being completely manipulated by the baby is, kind of, to give birth,” said Boddy, who has two children of her own. (In that way, birds have a good gig: They package all the nutrients that their developing offspring need into an egg, then shuttle out the whole shebang … though this does mean that certain species, such as the kiwi, must first haul around eggs that can take up 20 percent of the space in their body.)

Yet other animals have struck an armistice with compromises of a different kind. Their offspring are precocial, born so well formed that they’re able to leapfrog nearly all the feeble travails of infancy. That self-sufficiency comes in handy for creatures that can’t afford to dote on their infants for long, or that are constantly on the move, like antelope, cattle, and horses. But it exacts a parental tax: The babies of precocial species tend to gestate longer—vacuuming up more of their parent’s internal stores—and emerge much larger. “I think of it as front-loading the investment, versus delaying it,” Boddy told me.

Wildebeest, one of the most precocial mammals known to scientists, must weather an eight- or nine-month pregnancy while migrating hundreds of miles across the savanna, often as they nurse an older calf—all before popping out a fresh kiddo that might weigh more than 40 pounds at birth, more than 15 percent of the mass of typical females. To carry such gargantuan offspring, an animal must be in tip-top shape—and constantly seek out resources to nourish her growing load. Still, the prenatal sacrifices really pay off: Just a few minutes after a wildebeest calf is born, even before the amniotic fluid on its flanks has dried, the animal is standing up and galloping around its parents; by the next day, it will be sprinting at almost full tilt, fast enough to keep up with the rest of the herd. It’s almost like giving birth to a “much more coordinated toddler” with the gams of a track-ready teen, says Anna Estes, a wildebeest expert at Carleton College,. All of that certainly behooves (sorry) the calves in question: “If they’re not moving, they’re dying,” Stabach, the Smithsonian ecologist, told me.

Some superprecocial mammals have evolved ways to mitigate the toll of a heavy, extended pregnancy. The placentas of wildebeest and other hoofed creatures, Haig told me, aren’t as invasive as the ones that human fetuses produce—which makes birthing long-gestating, big-bodied calves less risky. When the little ones are delivered, “the placenta comes away very cleanly,” Haig said. Just as her newborn does, “the mother can also get up and walk off very quickly.”

That’s certainly not the tack that humans take. Our “childhood” tends to last for almost two decades in many parts of the world—far longer than the measly stretch a fetus spends camped out in the womb. Giving birth may offer a cessation of certain parent-offspring clashes. But the peacetime ends up being only temporary: When generations stay this tightly knit, more disputes are sure to follow.

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