Altricial mammals - large, helpless litters
Precocial mammals - small, able litters
What about us? Humans have small litters that are helpless! Why?
Answer -> Human babies are born as embryos, and they remain so for the next 9 months of their life.
If we measure time according to the rate of metabolism, almost all mammals live for the same amount of "time". For example, all mammals breathe the same number of times. Small, short-lived mammals just breathe (and metabolize) faster. By this relative measure of time, human gestation should take around 18 months.
Then why do births occur after just 9 months? Is it because we need the next 9 months to get familiar with the sights and sounds of the real world? No, the reason is more likely to be "mechanical". The culprit is our large brain. It would be hard for a delivery to happen if the brain became any larger!
Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould
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