When the National Zoo’s giant panda Mei Xiang gave birth to a miniscule baby on August 23rd, it looked like the tiny, hairless infant just tumbled right out of her. A couple hours later, a second baby popped out.

Weighing in at just 86 grams and 138 grams, the two blind little cubs were dwarfed by their 238-pound mother—who weighed over a thousand times more than her smaller baby. (Learn why the smaller panda cub ultimately did not survive.)

The extreme tininess of these babies might seem unusual—panda mothers typically weigh 900 times more than their newborns, while human mothers are only around 20 times heavier than their babies. But in fact, mammals' infants come in a huge range of sizes.

Mammalian babies big and small Emily M. Eng and Kelsey Nowakowski, NG StaffSOURCES: San Diego Zoo; Animal Diversity Web, University of Michigan; NOAA

Why So Little?

Whether an animal has big or little babies depends on how self-sufficient the babies need to be at birth. Babies that need a lot of care are called altricial babies, and animals that are born more developed are called precocial. Both pandas and people fall firmly on the altricial end of the spectrum.

Panda cubs rely on their mothers for everything—warmth, food, and even help urinating and defecating. One reason that so much of their development happens outside the uterus could be that breast milk is better at transmitting nutrients to the panda cub than the placenta is, says Megan Owen, associate director of applied animal ecology at the San Diego Zoo. But their small size relative to mom puts them at risk of being crushed while their mother is caring for them. (See “These Newborn Pandas Face 4 Big Threats to Survival”).

In contrast, precocial young like giraffes can walk (clumsily) soon after birth. These more self-sufficient babies tend to be relatively large compared with their mothers, Owen says. When giraffes are born, for example, they’re already 10 percent the size of their mothers.

Born Yesterday

At the very tiniest end are marsupials like the red kangaroo, born 100,000 times smaller than its mother. Marsupials don’t have placentas, and their young are so altricial when they come out that they’re practically fetal.

“They can be born jellybean-sized—and by born we mean they exit the uterus the same as a human baby would,” says Alistair Evans, a professor of evolutionary morphology at Monash University in Australia. “They are so small that they cannot survive at all for themselves, and they need a really, really long time to be attached to the teat to get milk.”

Baby red kangaroos exit the uterus, crawl into the pouch, and stay there, attached to the teat, for much of their early development.

“They’re able to make a little embryo and sort of put it in suspended animation for a year or more while they wait for a good environment, wait for a good rain or a lot of grass,” he says. “We couldn’t do that very easily.”

Born to Be Wild

Whether a species gives birth to tiny, helpless babies or larger, more developed infants depends in part on the environment the species is adapted to, says Evans.

Carnivores at the top of the food chain, for example, don’t face much predation as they care for defenseless young, says Barbara Finlay, a professor of psychology at Cornell University who studies animal development. Plus, many have dens they can raise their young in. But prey animals, in general, have to be able to run from predators early on.

As with all rules, these have exceptions: mice are not top predators but have helpless young, and Finlay says that guinea pig young are actually pretty precocial.