Here are excerpts of an article I found quite interesting.

 “When an animal eats, it acts like a computer; that is, the most
sophisticated kind of computer, that could choose the best quality foods in
the right amounts, better than an expert dietitian ever could. Conversely,
man is like a broken-down computer, which compels him to eat anything,
anyhow, and which sometimes leads him to obesity or alcoholism (flaws that
never occur with animals in a natural condition).... “Food, according to its
chemical composition, is broken down into fats (glycerides), sugars
(saccharides), and proteins (nitrogenous food such as eggs, grains, meat,
and fish).... “When it makes its choice, an animal is able to pick the foods
it needs to balance input against output accurately from the relevant
nutrients. An American, Professor Richter, was the first to demonstrate that
rats were remarkably able when it came to selecting from a range of foods
the appropriate amounts of protein, vitamins, and mineral salts necessary
for their continued health. Even better, rats can change their minds, when
their internal balance is experimentally tampered with. In this way, rats
automatically increase their salt intake after removal of their adrenal
glands; they will eat fats over sugars once they have been turned into
diabetics; they select whatever vitamin they happen to be deficient in....
“This is most striking in chickens.... Chickens pick the amount and kind of
food they require solely on the basis of the needs of the egg they lay
daily.... “When producing egg white, the chicken only eats whole,
high-protein food. When the egg is taking up water, the hen drinks
plentifully. Finally, when the shell is forming, the hen goes for calcium.
One might imagine that that was due to circadian rhythm. Not so at all,
since when chickens are raised from birth in constant light_that is, when
they don’t experience nighttime_their eating cycles remain unchanged.
Further proof would be contributed from chickens that do not or no longer
lay, or even from roosters. In the foregoing, there is no staggered intake
of protein, calcium, or water. What’s more, given that fowl can make up for
the loss (incurred through laying its eggs, a case in point) by relevantly
adjusting the quality of their food, they can also balance their diet_which
man can’t do.”

“Marching orders straight from the organs,” Science et Vie (Science and
Life) n° 729, June 1978.