One Thing They Aren’t: Maternal
Oh, mothers! Dear noble, selfless, tender and ferocious defenders of progeny all across nature’s phylogeny: How well you deserve our admiration as Mother’s Day draws near, and how photogenically you grace the greeting cards that we thrifty offspring will send in lieu of a proper gift.
Here is a mother guinea hen, trailed by a dozen cotton-ball chicks. Here a mother panda and a baby panda share a stalk of bamboo, while over there, a great black eagle dam carries food to her waiting young. We love you, Mom, you’re our port in the storm. You alone help clip Mother Nature’s bloodstained claws.
But wait. That guinea hen is walking awfully fast. In fact, her brood cannot quite keep up with her, and by the end of the day, whoops, only two chicks still straggle behind. And the mama panda, did she not give birth to twins? So why did just one little panda emerge from her den? As for the African black eagle, her nest is less a Hallmark poem than an Edgar Allan Poe. The mother has gathered prey in abundance, and has hyrax carcasses to spare. Yet she feeds only one of her two eaglets, then stands by looking bored as the fattened bird repeatedly pecks its starving sibling to death.
What is wrong with these coldhearted mothers, to give life then carelessly toss it away? Are they freaks or diseased or unnatural? Cackling mad like Piper Laurie in “Carrie”?
In a word — ha. As much as we may like to believe that mother animals are designed to nurture and protect their young, to fight to the death, if need be, to keep their offspring alive, in fact, nature abounds with mothers that defy the standard maternal script in a raft of macabre ways. There are mothers that zestily eat their young and mothers that drink their young’s blood. Mothers that pit one young against the other in a fight to the death and mothers that raise one set of their babies on the flesh of their siblings.
Among several mammals, including lions, mice and monkeys, females will either spontaneously abort their fetuses or abandon their newborns when times prove rocky or a new male swaggers into town.
Other mothers, like pandas, practice a postnatal form of family planning, giving birth to what may be thought of as an heir and a spare, and then, when the heir fares well, walking away from the spare with nary a fare-thee-well.
“Pandas frequently give birth to twins, but they virtually never raise two babies,” said Scott Forbes, a professor of biology at the University of Winnipeg. “This is the dark side of pandas, that they have two and throw one away.”
It is also something that zoos with ever-popular panda displays rarely discuss.
“They consider it bad P.R. for the pandas,” Dr. Forbes said.
Researchers long viewed infanticide and similar acts of maternal skulduggery as pathological, a result of the mother’s being under extreme stress. A farmer’s child pokes around in a rabbit’s nest, for example, and the mother rabbit responds by methodically consuming every one of her eight baby bunnies. By standard reckoning, it made little genetic sense for a mother to destroy her young, and maternal nurturing was assumed to be a hard-wired affair.
More recently, scientists have accrued abundant evidence that “bad” mothering is common in nature and that it is often a centerpiece of the reproductive game plan.
In the blockbuster movie “The March of the Penguins,” the emperor penguins were portrayed as fairy parents, loving every egg they laid and mourning every egg that cracked before its time. Among the less storied royal penguins, a mother lays two eggs each breeding season, the second 60 percent larger than the first. Just before the second egg is laid, the mother unsentimentally rolls the first egg right out of the nest.
In Magellanic penguins, the mother also lays two eggs and allows both to hatch; only then does she begin to discriminate. Of the fish she brings to the nest, she gives 90 percent to the larger chick, even as the smaller one howls for food. In the pitiless cold of Antarctica, the underfed bird invariably dies.
Like penguins, many species that habitually jettison a portion of their progeny live in harsh or uncertain environments, where young are easily lost and it pays to have a backup. At the same time, the harshness and uncertainty make it virtually impossible for a mother to raise multiples, so if the primary survives, the backup must go. Sometimes the mother does the dirty work herself. More often, she leaves it to her preferred young to dispatch of its understudy.
When Douglas W. Mock of the University of Oklahoma began studying egrets in Texas three decades ago, he knew that the bigger babies in a clutch would peck the smaller ones to death. Still, Dr. Mock was caught off guard by what he saw — or failed to see. He had assumed that the murderous attacks would surely take place while Mom and Dad egret were out fishing.
“I figured that, if the parents were around, they’d try to block these things,” he said. “I have three older brothers, and I never would have made it if my parents hadn’t interceded.”
Instead, Dr. Mock witnessed utter parental indifference. The mother or father would stand by the side of the nest, doing nothing as one chick battered its sibling bloody. “The parent would yawn or groom itself and look completely blasé,” said Dr. Mock, author of “More Than Kin and Less Than Kind: The Evolution of Family Conflict.” “In the 3,000 attacks that I witnessed, I never saw a parent try to stop one. It’s as though they expect it to happen.”
Since then, siblicide under parental supervision has been observed in many bird species, including pelicans, cranes and blue-footed boobies.
One researcher watched a nest of African black eagles for three days as the larger eaglet alternated between tirelessly stabbing at its sibling and taking food from its solicitous mother’s mouth. There was prey to spare, but the mother did not bother feeding the second, abused baby. When the eaglet’s poor, tattered body was finally tossed to the ground, the researcher calculated that it had been pecked 1,569 times.
Pigs, too, have their own version of litter culling by sibling rivalry. Piglets are born with little eyeteeth that stick out sideways from their lower jaw, Dr. Mock said, and they use these teeth to slice at the faces of one another as they jockey for the best teats. The runt of the litter is so often sliced and bullied that it cannot get enough milk. It must spend every spare moment fighting to nurse and may get crushed by its mother.
In other cases, mothers turn infanticidal because they are born optimists, ever tuned to the sunny expectation that good times lie ahead. Each year they breed for a banquet, producing a maximum of begging bairns as the season starts; and when there is plenty of food, they will provision every young.
If the feast does not materialize, however, they cut their losses. Kangaroos have an elaborate method for child rearing through fat and lean years. In a good season, a mother may care for three offspring simultaneously, each at a different stage of development: the eldest, already hopping around on its own but still nursing; the second, a joey, which lives in her pouch and breast-feeds; and the youngest, an embryo stashed internally in a state of suspended animation.
During a severe drought, the mother will first refuse her breast to the autonomous juvenile, leaving it to forage as best it can. If the drought continues, her milk dries up and the joey dies and falls from her pouch. At that point, the embryo kept in cold storage begins to develop toward joeyhood. Tomorrow will surely be a better, wetter day.
Some mother hawks and owls are practical optimists, not only halving their brood when necessary but also eating them.
“Cannibalizing the victim serves the dual function of providing a timely meal and ensuring that there is one less mouth to feed,” Dr. Forbes, the University of Winnipeg biologist, writes in his new book, “A Natural History of Families.”
A hungry mother can be the stuff of nightmares — especially if it is the mother next door. Chimpanzees are exemplary mothers when it comes to caring for their own, said Sarah Blaffer Hrdy, a primatologist and the author of “Mother Nature: A History of Mothers, Infants and Natural Selection.”
Unlike humans, Dr. Hrdy said, the apes never abandon or reject their young, no matter how diseased or crippled a baby may be. Yet because female chimpanzees live in troops with other nonrelated females, a ravenous, lactating mother feels little compunction about killing and eating the child of a group mate. “It’s a good way to get lipids,” Dr. Hrdy said.
As meal plans go, cannibalism can be no-muss, no-fuss. A mother nurse shark has two uteri in which her babies develop, safe from the ocean’s predators. But the nurse shark is not a mammal, and she has no placenta. How to feed her fetal fish? On the fins and flesh of fellow fetal fish.
The mother incubates as many as 20 eggs per womb. The eggs hatch and start to grow, and when their jaws are sufficiently mature, they commence feeding on one another. By gestation’s end, just one sharklet emerges from each uterine chamber.
Extracting nutrients from one’s offspring need not be fatal, though. Among ants of the rare genus Adetomyrma, Dr. Forbes writes, “queens chew holes in their larvae and then consume the oozing fluid,” a practice that explains why the insects, found in Madagascar, are known as Dracula ants. The sampled larvae recover and mature into ants, but they bear lifelong scars of their early bloodletting.
There are voracious mothers and vampiric mothers, and then there are phantom mothers. In the annals of mammaldom, the maximal minimalist of a mother must surely be the rabbit. Only recently have scientists studied rabbit behavior closely enough to appreciate what a marvel of efficiency a breeding rabbit is, said Robyn Hudson of the National University of Mexico.
Rabbits live together in complex burrows, where an expecting female will build a little nest and line it with grass and fur that she plucks from her flank. When she is ready to give birth, she enters the chamber and in less than eight minutes plops out 10 pups, “like peas in a pod,” Dr. Hudson said.
Without bestowing on the litter so much as a single welcoming lick, the mother hops back out, closes up the entrance and leaves the helpless, furless newborns to huddle among themselves in the dark. Over the next 25 days, the mother will return to the nest for a mere two minutes a day, during which she crouches over the pups and they frantically nurse.
“Her milk is under high pressure, and it’s almost squirted into their mouths,” Dr. Hudson said. “You can see them visibly expand, like little grapes.”
Two minutes are up, and she’s out of there. On Day 26, she abandons them completely, and the bunnies must crawl from the nest and make their way in the world on their own.
The mother rabbit may seem awfully cold for a warmblood, but her aloofness makes sense. Rabbits are a highly popular prey, and many predators will pursue them into their burrows. To keep the fox from the nursery door, the mother rabbit shuns the room. Her absence may not make her pups’ hearts grow fonder, but it may keep those hearts thumping a little longer. Autor: Natalie Angier