Vertebrates

All vertebrates (animals with backbones) have a well-developed and highly
organized endocrine system. The system consists of the following glands:
the pituitary, the pineal, the thyroid, the thymus, the pancreas, a pair of adrenals
(each adrenal actually acts as two glands—the adrenal cortex produces
unique hormones and functions independently of the adrenal medulla),
a pair of parathyroids, and a pair of ovaries or testes. Endocrine
tissue in the gastrointestinal tract readies the system for the digestive process.
During a pregnancy, the placental tissue assumes an endocrine function.
Although the kidneys do not produce a hormone directly, they release
an enzyme which converts a blood protein into a hormone that stimulates
red blood cell production.
All vertebrates have a pituitary. The pituitary is a small, round organ
found at the base of the brain. This major endocrine gland interacts with the
hypothalamus of the nervous system. Together, they control behavior. The
hypothalamus receives information about physiological events in the body
by monitoring the composition of the blood. In turn, the hypothalamus signals
the pituitary by either a nerve impulse or a chemical messenger. The pituitary
responds by releasing or ceasing to release hormones that will have a
direct effect on physiology or will stimulate other endocrines to release their
hormones in order to alter the physiological event and influence behavior.
The endocrine system exerts its effects on a biochemical level.
The human endocrine system is typical of vertebrate endocrine systems and
their effect on behavior, although certain hormones may have a more pronounced
and obvious effect in other vertebrates. For example, melanocytestimulating
hormone, which is generated by the anterior lobe of the pituitary,
greatly increases skin pigmentation in amphibians. This creates a protective
coloration. In humans, the darkening effect is not achieved unless excessive
hormone is administered. The protective function is not apparent. There
are enough similarities among human and animal endocrine functions and effects, however, to warrant the use of data from both ethology and human
behavioral studies in determining the biological bases for behavior.