Endocrinology is the study of hormones, chemical messengers that target organs and influence many processes, from growth to reproduction. Here we will consider only those hormones that influence behavior. Hormones are secreted by ductless glands and are carried through the body by the circulatory system. Hormones called neurosecretions are produced by neurosecretory cells in the nervous system and are carried through the body by both nerve axons and in the blood. Both the nervous system and the endocrine system act as feedback systems; the nervous system produces much faster results, whereas the endocrine system is slower acting, longer lasting, and produces more general responses. The endocrine system has been mostly evolutionarily static, meaning it is often highly similar between species.
The control center of vertebrate endocrine systems is the pituitary gland, located in the brain. The hypothalamus funnels information into the pituitary gland mostly via the osmotic balance in several key ganglion nuclei-- the optichiasm, superoptic, medial, lateral, dorsal, and ventral nuclei. The pituitary gland is really two glands formed from two separate embryonic structures. The anterior pituitary secretes hormones, including growth, thyroid stimulating, and follicle stimulating hormones, as well as prolactin. The posterior pituitary acts as a storage bag for hormones. It is responsible for oxytocin and antidiuretics, which control water balance. Hormones secreted by the pituitary are intended for target organs, which either produce other hormones in turn, or function in response to the pituitary signal.
There are three basic types of hormones: steroids, small peptides, and catecholamines. Steroids are large, 17 carbon, 4 ring molecules that bind to the nucleus after entering it. Examples of steroids are estrogens, which includes estradiol (commonly called estrogen) and progesterone; androgens, which include testosterone and dehydroxytestosterone (DHT); and corticosteroids, such as cortisone. Small peptides bind to the cell surface and include some hormones and neurotransmitters that perform analogous functions. Important peptide hormones are endorphines, which includes natural forms of morphine; oxytocin and prolactin which are involved in parental care, and vasopression, which is an antidiuretic that causes urination after you drink. Catecholamines also bind to the cell surface and include epinephrine (commonly called adrenaline) and noradrendaline, which are involved in the "fight or flight" response caused by extreme duress.
One way in which such hormones can function as a feedback system is in reproduction. A good example of a reproductive feedback loop can be seen in ringdoves, studied by Daniel Lehrman. Gonadotropins stimulate the gonads of both males and females to produces steroids. Courtship and nest building follows this increase in gonadal steroids. Rising levels of female progesterone stimulate egg laying and incubation. At this stage, gonadotropins decrease, as do female courtship displays. Males, however, remain responsive to females at this point. After approximately one week of incubation, levels of prolactin increase in both sexes, allowing both males and females to produce a milk-like substance; gonadotropin and steroid levels decrease. When the eggs hatch and the chicks mature, prolactin levels decrease, gonadotropins increase, and the cycle begins again.
Hormones are involved in a large array of animal behavior, from sexual tactics to "tamability." Below are a number of examples.
Sexual Tactics Among Midshipmen Fish