Animals require an array of nutrients to
stay healthy, including specific amino
acids, vitamins, and elements, as well as
organic compounds that have high
Animals feed on a wide variety of items.
In animals that ingest their food, there is
usually a close correspondence between
the structure of the animal’s mouthparts
and the function of those mouthparts in
capturing and processing food.
Digestion occurs in the digestive tract,
which is compartmentalized into organs
that have specialized functions in the
digestion of food, absorption of nutrients
and water, or excretion of wastes.
Lack of homeostasis with respect to
nutrients such as glucose and cholesterol
can cause disease.
f you’re like most people, you probably give a fair amount
of thought to the types of food that you eat but little
thought to what happens to that food in your digestive
tract. As a meal moves through the digestive system, its chemi-
cal composition and physical characteristics change dramatical-
ly. Structurally and chemically, complex raw materials are
broken down into simple components that can be used to build
and maintain cells. Large packets of food that enter the mouth
must be reduced to monomers that can be absorbed by cells
and enter the bloodstream. The digestive system is analogous to
an oil refinery or a lumber mill, where bulky, complex raw ma-
terials are processed into smaller and more usable products.
The types of food that are available to different animals vary
widely, however, and food is often in dangerously short supply.
Based on these observations, it is logical to predict that animals
have a variety of means for obtaining food and that animals are
under intense natural selection to make efficient use of the food
they have. How do animals get their food, and how do they
process it? Which substances in food are used as nutrients, and
how do humans and other animals maintain appropriate levels
of key nutrients in their bodies?
Research on feeding and digestion is fundamental to under-
standing many of the most basic aspects of animal anatomy and
physiology. But research on animal nutrition has important
practical applications as well. For example, this chapter address-
es questions about whether certain nutrients can be used to
boost athletic performance, and why type II diabetes, heart dis-
ease, and other nutrition-related diseases are on the rise in many
human populations. Just as plant nutrition is fundamental to the
productivity of agricultural and natural ecosystems, nutrition is
a basic component of animal health and welfare. Let’s begin
with a look at what animals must eat in order to live.
A young crocodile has just caught a reed frog. Most animals obtain nutrients by ingesting them.