ClareStar

See Which Animals Have the Most Enormous—and Tiniest—Babies

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. But their small size relative to mom puts them at risk of being crushed while their mother is caring for them. 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.  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. 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.