One of the ironies of life is the
circular nature of its logic. Animals eat to provide themselves with the
materials and energy for life, then use most of their
bodily traits and energy to find more food.
Central to understanding this
dilemma is to grasp that running a body is work and requires an investment by
the animal.
(1) Some foods are harder work to
digest than others. Grass and leaves are harder to digest than fruits, seeds or
meat. That is why grass- and leaf-eating animals often have extra digestive
plumbing (four stomachs in ruminants, caecums in some
species) and/or put their food through extra digestive processes (cud-chewing,
re-eating of feces in rabbits and gorillas, etc.). Another strategy is simply
to eat prodigious quantities of food, extract what the animal can as quickly as
possible, and discard the remainder as quickly as possible. Elephants, for
example, effectively digest and utilize only about half the vegetation they eat
before eliminating it as feces.
(2) Easier-to-digest foods may
support richer life processes. One scientist has suggested that leaf-eating
monkeys are less intelligent than fruit-eating ones because it is harder for
them to supply their brains with sugars for mental energy.
(3) Even easy-to-digest food like
meat may be a burden. Cheetahs are said to have extra-short intestines, to get
rid of food as fast as possible, so that their sprint speed is not slowed.
Losing some of the nutrition in the food already swallowed is worth it to
ensure that more food can be caught.
(4) Digestion is closely related to
an animal's general metabolic rate, i.e., whether it is ectothermic
of endothermic. An ectothermic animal needs little
food because it doesn't burn any for body heat, but at the same time may find
it harder to catch food due to its relative lack of activity (i.e., less energy
to hunt with). It may even have trouble digesting it: zoos often leave
alligators unfed during the cold season for fear they won't get enough heat
from the sun to digest their food and that it will rot in their stomachs. Even
a mammal, the slow-moving sloth, has evolved to live a low-metabolism life, and
must bask in the sun to get its body warmth up high enough for effective
digestion of its mostly-green-vegetation diet.
(5) Needing less food means that ectothermic predators can be much more numerous relative to
prey than endothermic predators. (If a snake eats a rat once a month and a hawk
must eat one every other day, there can be 15 times as many snakes as hawks for
a given population of rats.) This was one reason some scientists decided
dinosaurs were warm-blooded: the predatory-prey ratio was low by ectothermic standards. More recently, similar arguments
have pointed the other way: dinosaurs don't have the right kind of noses for endothermia (mammals and birds need lots of oxygen to burn
lots of food to make heat and also have complex nasal bone structures called turbinates to recycle water: dinosaurs, like other
reptiles, lack both these features).
(6) Hibernation and aestivation also
reflect metabolic needs. An endothermic animal facing a long spell without food
might well envy an ectotherm's ability to do without.
Some found a better way: instead of envying cold-blooded animals, they BECOME
cold-blooded animals for the duration of their privation. For that is
essentially what true hibernation/aestivation amounts to...a slowing of
metabolism very similar to becoming cold-blooded.
These are just a few examples of the
multitude of physical and behavioral adaptations that nature has given animals,
all to effect the most efficient use of food for a given type of animal in a
given ecological niche -- and all related to balancing the equation between
food sources and food needs.
Nature's equation-solving
calculations are incredibly complex and a fantastic variety of creatures and
adaptations come out of them. All can be traced back to a few basic concepts,
such as the need to get food in order to get food.