All animals must eat to obtain energy for resting metabolism and to cover the
energetic expenses associated with reproduction and behavioral activities.
Utilization of the ingested meals requires digestion and subsequent absorp￾tion of nutrients. Given the extreme diversity in foods selection and food
abundance between different environments and the large differences in
metabolic needs among different animals, it is not surprising that feeding
habits and strategies differ enormously among vertebrates. Some species
which inhabit environments where food abundance is either scarce or fluc￾tuates on a seasonal basis exhibit extraordinary adaptations to long-term
fasting. Other species show extraordinary specialization for procuring,
subduing and digesting particular foods items. Such morphological and
physiological specializations have evolved in response to environmental
conditions, but may be physiologically or functionally constrained.
Apart from the need to acquire energy and absorb nutrients, the form and
functions of the gastrointestinal organs have been shaped to reject a plethora
of antigens, bacteria and viruses that attempt to invade the body through
this open barrier.