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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.

Leopold says Reinstate Monica
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  • Narrowed down to mammals only. – Leopold says Reinstate Monica Aug 06 '16 at 17:16
  • Great question. Note that, while humans are often assumed to be born less developed than four-legged mammals, your question actually asks an even more profound (and general) question (with humans being perhaps the least interesting case). – tsttst Aug 06 '16 at 18:50
  • From evolutionary standpoint (no time to look up resources) 1) delay before reproduction ensures weak DNA that leads to early death isn't passed on 2) there is a peak healthiness that provides stronger outcome for offspring, choosing partners based on age, in addition to other health factors often visible, gives an evolutionary advantage to one's DNA 3) nurturing and care is institutionally provided to the young looking offspring, which increases chances of reproduction, so those offspring that look young before reproduction have an advantage – Adam Davis Jun 14 '17 at 14:02
  • if you've already lived a long life, then losing the desire to explore, be a dare devil, and learn new ways of doing things is an advantage - the patterns developed that brought you to old age will be better than trying new, untested patterns. So the evolutionary pressure would be to establish these patterns firmly and decrease the generation of new patterns of thought and behavior.
    • – Adam Davis Jun 14 '17 at 14:04