My 8 year old son asked me a question I could not answer (and Google, at least in so far as we tried, was no help either).
We were watching a documentary about Salt Water Crocodiles in Northern Australia.
The crocodiles seem very energy efficient: they barely move if they don't absolutely need to (and when they do, it is mostly to eat). Particularly, they hardly ever run, but when they do they are very fast and powerful.
So the question he asked me was: how can crocodiles remain so fit if they don't do any exercise?
If humans just sat around and moved only to eat, ran only when they absolutely had to, our muscles would atrophy. Other mammals seem to be in a similar boat to us humans.
Why not crocs? What distinguishes animals that need to exercise to be strong from animals that don't?
This is the closest question I could find here, but I don't think it assists in answering this question: Do insects' muscles become stronger with exercise?