In many animal species, juveniles aren't just smaller versions of adults. In mammals, the contrast is not as spectacular as in species that undergo metamorphosis, but still quite noticeable.
For example, puppies have shorter limbs and flatter faces than adult dogs. Fawns are coloured differently than adult deer, and males do not start growing antlers until they reach sexual maturity. There are behavioural differences as well: habituation is much stronger and faster in juveniles. In humans, children have different proportions than adults and lack secondary sexual characteristics. But why? Wouldn't it be simpler to form a small version of the adult and grow linearly?
- Why delay sexual maturity in the first place?
- Why tie sexual maturity to a host of other changes? For example, why aren't babies born with adult hair patterns? (Yes, because hair growth responds to sexual hormones, but the response could be different.)
- Why the different body proportions?
- Why lose play behaviour and learning ability as adults?
I'm interested in mammals in general and humans in particular.