... and even before then, I believe.
> ... birds also have brains as big as mammals ...
I recall from somewhere the suggestion that maybe large brains are a
consequence of the thermodynamics of small endotherms. A small
creature that keeps itself warmer than its surroundings by metabolism
has to burn a lot of calories to do so. It can't save fuel by holding
still -- the resting metabolic rates of chipmunk-size mammals are ten
times those of comparably sized reptiles, thus the food requirements
are also ten times as great. One may as well move around as shiver;
thus there is no real energy cost to engaging in such energy-consuming
behaviors as investigating and manipulating the environment, play,
randomly doing this and that, and so on. If there is any benefit to
these behaviors, evolution may more easily select for them.
Note furthermore that the brain is a great consumer of calories. If
you've got to burn them anyway, just to keep warm, there relatively
little extra cost to using them to support a larger and more capable
brain.
-- Jay Freeman, First Extropian Squirrel