This article has been updated.

OMG they're so tiny! That's the universal human reaction to the sight of the panda cubs at the National Zoo. Somehow these tiny, hairless, squeaky, utterly helpless, butter-stick-sized creatures are supposed to live outside mama's womb and eventually grow into a adorable giant pandas. You have to wonder: What's the biological explanation for the tiny babies? Why wouldn't natural selection favor panda cubs that were more robust? Who came up with this cockamamie system?

First, the terminology. The key word to remember is "altricial."

“It’s a fancy word that means pretty much helpless," said Don Moore, a senior scientist at the National Zoo and an expert on mammals.

[Famous giant panda fakes pregnancy, gets nice things]

The opposite of altricial is precocial. A chick freshly hatched from the egg is precocial, walking around, eating and peep-peeping up a storm. A human baby, however, emerges on the altricial side of the spectrum.

A giant panda is a bear. Bear cubs are extremely altricial in general. Of the placental mammals, Moore said, the panda has the tiniest babies in comparison to the size of the mother. The size ratio can be as small as 1 to 900.

So why so small? The answer has a lot to do with bamboo.

A giant panda is a creature that has evolved to eat a diet primarily of bamboo, a woody grass that is hard to digest. To adapt to this rather spartan diet, giant pandas have evolved a low metabolism. They sit around a lot. They take the concept of "low key" to an extreme.

[Pandas literally evolved to be lazy]

This low metabolism means the female's blood-oxygen level is relatively low. That tiny panda cub actually has a better shot at survival if it can breathe fully oxygenated air — and so being outside in the world is better than being inside the mother. Moreover, the kind of fatty acids that the cub needs can't be passed from the mother to the cub through the placental barrier, Moore said.

“In order to get the fatty acids to the baby, to make the baby grow faster, they can't go across the placenta, but they can go out in the milk," Moore said.

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So again, there's an advantage in being on the outside.

Now then, what about those rambunctious brown bears that are constantly roaming around and fishing for salmon in wild Alaskan rivers and don't just sit around all day looking cute? Why do they also have small cubs? Moore points out that those bears have their own low-metabolism state — called hibernation. They give birth while hibernating in winter. The cubs are born and feed off the mother's milk exclusively for months before they emerge from the den in springtime.

Bears are at the top of the food chain. This means they don't get chased by other animals. And it means they don't have to worry a lot about other animals attacking their babies.  That's another factor, Moore tells us in an email, in the altricial nature of bear cubs:

Carnivores can “afford” to “go short” with their gestation, and maybe for them it is more efficient to grow a baby outside the womb. Because they live near the top of the food chain, carnivores can “afford” to give birth to more altricial babies in a cave or den, and to raise their babies there. Bears especially can use their body reserves to nourish a growing fetus during a short gestation, and then can use different body reserves to nourish an altricial newborn with their fatty milk.

The altricial nature of panda cubs poses a challenge for scientists at the National Zoo who are trying to keep the two cubs alive and healthy. They were born Saturday, one at 138 grams and the other at just 86 grams. The scientists for now call them "the larger cub" and "the smaller cub."

[Panda twin cubs are special to the National Zoo, but they're not rare worldwide]

They have had mixed results in their efforts to swap out the two cubs so that the mother, Mei Xiang, spends time with, and bonds with, both of them. Late Sunday night, the panda team tried and failed for hours to get Mei Xiang to leave the larger cub and move to the smaller cub that had been placed in the den. The team finally succeeded about 7 a.m. Monday. As of late Monday the cubs appeared to be healthy, said Zoo spokesperson Pamela Baker-Masson.

She said of Mei Xiang, “Just as she is learning how to manage cubs, so are we.”

Update Tuesday 12:30 p.m.:

The Zoo has issued the following bulletin about the panda cubs:

Mei Xiang has not been a willing participant in the panda team’s efforts to switch the cubs since 2 p.m. yesterday afternoon. She has the larger cub in her possession. The panda team is caring for the smaller cub and will continue efforts to swap the cubs about every four hours. However, because the smaller cub has been away from Mei, the panda team is now managing it more intensely. The little cub’s behaviors are good. The team is concerned about its fluctuating weight since the cub is now more than 48 hours old. The most important thing for the panda team is to help the cub get enough fluids and nutrients. To accomplish this, they are bottle and tube feeding the cub. The cub has shown some signs of regurgitation which can lead to aspiration in such a tiny creature. To be prudent, the veterinarians are administering antibiotics to prevent possible infection. It’s very important to keep the cub hydrated so they are alternating an infant electrolyte solution with formula and administering fluids under the skin. The cub is urinating and defecating well. The veterinarians have not seen any sign of respiratory distress.

Our observations of the larger cub from yesterday indicate it is doing well and we’re confident Mei Xiang is taking very good care of it. We remain in a high-risk period.

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