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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.

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  • $\begingroup$ Narrowed down to mammals only. $\endgroup$
    – Leopold says Reinstate Monica
    Aug 6, 2016 at 17:16
  • $\begingroup$ 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). $\endgroup$
    – tsttst
    Aug 6, 2016 at 18:50
  • $\begingroup$ 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 $\endgroup$
    – Adam Davis
    Jun 14, 2017 at 14:02
  • $\begingroup$ 4) 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. $\endgroup$
    – Adam Davis
    Jun 14, 2017 at 14:04

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GREAT QUESTIONS!!! Why delay sexual maturity in the first place? -I believe it is also related to "the best-used" of resources. Firstly, it is unnecessary and impossible for children to have sex (biologically and evolutionarily), they don't have the physicality to sustain sexual intercourse or the ability to bear a children inside their body. Secondly, it is to focus resources on growth. Children uses lots of energy on growth. Human body does amazing jobs on allocating body resources or material so that we are in the best shape possible. If they don't do sex, they can focus more on growth.

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.) - This one is a hard one. I believe it is just how biologists categorize human growth. We can study how human bodies develop in each stage, which is I believe it's really important on morphology.

Why the different body proportions? -Evolutionary advantages!!! It is believed that baby has a greater head portion than adult. That's why people say children learn better than adults. Human values their brains more than other organs of the body, developing intellectual capacity as early as possible allows them to win in natural selection.

Why lose play behaviour and learning ability as adults? If I understand you correctly, you are asking why adults lose their play behaviour and learning ability as they grow up. For learning ability, I believe it is caused by degeneration, just like the other organs of the heart. We have a mortal body and so once the body hits its maximum growth capacity, it will start going downhill unfortunately. For play behaviour, I believe it is a human behavioural changes due to life experience, social expectation and peer interaction. We lose interest of 'games' as we grow up because they are considered to be childish, silly and not useful.

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  • $\begingroup$ Can you provide references? What you believe isn't really broadly useful. $\endgroup$
    – kmm
    Aug 7, 2016 at 0:36

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