The birth of tiny twin pandas to the National Zoo’s Mei Xiang was cause for national celebration this weekend (we are, after all, are the country that practically went into a state of panic when the zoos’s panda cam went offline during the shutdown of 2013).

But the mood quickly soured as the smaller of the cubs started to regurgitate its food and struggled to keep on weight. Since Monday, Mei has refused to care for the tinier twin, despite zookeepers’ efforts to foster maternal affection for both her babies during the most precarious period of their infancy, and on Wednesday the zoo said the cub had died.

Twin pandas are rarely born at American zoos (this is only the third in U.S. history) but in the wild, behavior like Mei’s is actually quite common. Giving birth to multiple offspring is an evolutionary advantage, because it increases the likelihood that one will survive to adulthood. But when it becomes evident that one cub is developing faster than the other, panda mothers will often focus their attention on the stronger infant.

“This is the dark side of pandas, that they have two and throw one away,” Scott Forbes, a professor of biology at the University of Winnipeg, told the New York Times in 2006.

For obvious reasons, zoos rarely acknowledge the fact: “They consider it bad P.R. for the pandas,” Forbes said.

Biologists used to think this kind of behavior was pathological, an indicator that the mother was stressed or upset in some way. But it’s now believed that giant panda mothers like Mei Xiang are more Sophie Zawistowska, the William Styron character who was forced to choose which child to send to a Nazi gas chamber, than Margaret White, the crazed mom of Stephen King’s Carrie. Abandoning the weaker infant doesn’t make Mei a bad mother. Indeed, by the rules of the animal world, where the need to ensure the future of one’s DNA reigns supreme, it might even make her a better one.

This Sophie’s Choice-style calculus is by no means limited to pandas — it’s common throughout the animal world, where the whims of nature can be as cruel and unforgiving as those of Styron’s fictional Nazi. The fact is that it is very difficult to be a mother in the wild, far more than we coddled humans can conceive.

Panda cubs are extremely altricial — “a fancy word that means pretty much helpless,” National Zoo scientist Don Moore told The Washington Post on Tuesday. They are born exceptionally small (at just 3 ounces, the smaller cub was born weighing 0.07 percent of its mother’s bulk) and totally incapable of fending for themselves.

For the first three months after birth, panda mothers do almost nothing other than care for their cubs; an Atlanta Zoo study found that mother pandas spent 80 percent of their time holding their babies. The newborns are so small that they can’t move or hold themselves up; a mother must constantly clutch her cub to her chest in order to allow it to feed. It’s difficult to care so intensively for one helpless infant, let alone two. And just imagine the logistics of constantly holding two cubs at once without the benefit of opposable thumbs and baby slings.

We went to China and found pandas in all stages of life from birth to cuddling with mom (Lee Powell/The Washington Post)

Meanwhile, mom has to take care of herself. Giant pandas have evolved to subsist on a diet of extremely low-nutrient bamboo, which they are poorly equipped to digest. Normally, they mitigate this problem by barely moving and snacking almost constantly (an adult can consume roughly 30 pounds of bamboo a day). But for the first few weeks of their cubs’ lives, mother pandas will go days and even weeks without eating or drinking. While cubs fatten up, their mothers are basically starving

All of this means that mother pandas in the wild are faced with a choice: risk their own lives and those of their offspring to care for both cubs, or choose the one most likely to survive and leave the other to die. First-time mothers are particularly likely to reject one of their newborns, according to the Atlanta Zoo — they’re just not skilled enough at parenting to know how to nurture both.

In captivity, zookeepers like those at the National Zoo try to counteract that impulse by alternating the cubs every few hours, so that mothers only ever have to care for one at a time. But as Mei Xiang demonstrated, those efforts don’t always work.

“Immediately a mother panda knows it’s a tough world out there and goes for the survival of the fittest,” biologist Marc Brody, who helps breed pandas in captivity at China’s Wolong Nature Reserve, told National Geographic in 2013, the last time Mei gave birth.

Among bears, cats, dogs, primates and rodents, it’s common for mothers to eat a deformed or dying infant. Most of these animals are unable to hunt or forage while caring for their newborns, and like panda moms, are close to starving while their offspring nurse. A baby that is likely to die is an important source of protein and nutrients, one that can help her produce milk to feed her other young.

“They become a resource, one she can’t afford to waste,” said Tony Barthel, a mammal curator at the National Zoo’s Asia Trail. He was interviewed by National Geographic last March, after a mother sloth bear consumed two of her babies and turned her back on a third. It turned out that the cubs were sick with a flu that eventually spread to the rest of the bears in the exhibit — it’s possible that the mother somehow knew, and that her behavior was protective.

Mother sandtiger sharks literally feed their children to one another. These huge (6-10 feet long!) pointy-snouted fish store fertilized embryos within their body, then give birth to live young. During gestation, the first offspring to reach a certain size will eat all of their smaller siblings — a practice bluntly termed “adelphophagy,” or, “eating one’s brother.”

Sometimes abandoning her children is the best thing an animal mother can do for them. Take, for example, mother rabbits. These floppy-eared creatures are known to abandon their newborns in their burrows and hop away, returning for only a few minutes a day. This isn’t because they don’t care for their young — rather, it’s believed to be a measure to prevent predators from discovering their nests. Adult rabbits, unlike their offspring, give off a strong smell that predators can track. If a mother visits her nest too often, she might inadvertently lure a hungry owl or fox right to her front door.

Veterinarians at the Smithsonian's National Zoo examine the first cub Mei Xiang gave birth to on Aug. 22. (National Zoo)

Still, despite the oft-repeated claim that adult animals will abandon their babies if they smell or sense that a human has touched them, biologists say that it’s actually very difficult to get a mother to give up a child when she doesn’t want to.

“In general, wild animals bond with their young and do not quickly abandon them,” Laura Simon, field director for the Urban Wildlife Program at the Humane Society of the United States, told Scientific American in 2007.

Nature’s most impressive mother of all is the giant Pacific octopus, who sacrifices her own life to give her 50,000 or so offspring a chance at survival. For months, she’ll carry her thousands of tiny eggs inside her body, then, when the water temperature is right, expel them into the water and knit them into an elaborate curtain, which she hangs from the roof of the underwater cave she has made into a den.

For more than half a year, writes NPR’s Robert Krulwich, the mother maintains a constant vigil beneath her dangling young. She gently blows water onto them to keep the eggs aerated and waves her arm across the curtain to keep grit from settling on it. Not once does she leave the cave. Without food and exercise, she grows smaller and grayer; her skin starts to decay.

Her last act is to use her siphon to blow the eggs free. Her offspring are a perfect miniature of their mother — 6 millimeters long with tiny tentacles and an instinctive sense of what to do next, according to Krulwich. But even with all that effort and all those thousands of young, only two of them are likely to make it to adulthood.

The mother octopus will never see that happen. Once the last of the eggs has hatched, the mother slowly, achingly puffs out of the cave, sometimes make it only as far as a few feet before she stops breathing and dies. Her offspring will have to make it on their own — she’s already given them all she has.