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
Midshipmen fish actually have two morphologically and behaviorally distinct types of males. Type I males are large, build and occupy nests, and make a droning sound that attracts females to come and lay eggs. The type I male then fertilizes and cares for the eggs. Type II males are smaller and resemble the females. They cannot sing and are too small to defend a territory. Instead, type II males have enormous testes (approximately 20 % of their body size). These "sneaker" males swim into an occupied territory, attempt to fertilize the eggs with a "sperm bomb," and are aggressively chased away by type I males. These fish actually have three types of sex hormones rather than the normal two, a fact that is responsible for the distinct types of males.
Tamability of Foxes
In addition to reproductive behavior, hormones play a sometimes unexpected role in other areas. Dmitry Balyaev tried to tame wild foxes by artificially breeding them for 40 generations. He was selecting for "tamability," characters such as friendliness and the ability to bond with humans, which would contribute to a good pet. Balyaev encountered a surprising result--the domesticated foxes also showed many traits common to other domesticated animals like floppy ears, a short curly tail, and a piebald coat (patched and spotted). In selecting for "tamability," Balyaev had selected for animals with delayed adrenal cortex development, resulting in decreased levels of corticosteroids. The lack of pigment in the piebald coat was caused by the delayed migration of cells during development, and the animals were easier to tame because they had a longer period of early development, the time when an animal is most fearful. Balyaev's breeding experiment shows an interesting relationship between hormone levels and domestication.
Monogamy in Voles
Monogamy is by no means a dominant mating strategy. While 90% of bird species are monogamous, only 10-15% of mammals are monogamous. Prairie voles (Microtus ochrogaster) are extremely monogamous, while their close relatives, Monatane voles (Microtus mantanus) are polygamous as are most voles. Male prairie voles form a strong pair bond with females, remaining together for life, and showing aggression toward strangers of either sex. Males contribute to parental care and older siblings stay with their parents to help care for new siblings. There is some indication that hormones are primarily responsible for prairie vole monogamy. Oxytocin is a peptide that causes uterine contractions and mother-infant bonding. In prairie voles, oxytocin also causes the female to pair-bond with the male. The vaginal stimulation of birth results in the release of oxytocin, as does mating. Prolactin stimulates parental care in both the male and female. Behavioral differences in males come from different binding sites in the brain, but the hormone is released in both sexes. Vasopression, responsible for fluid balance, also causes aggression, and is needed for males to pair-bond with females. A hotly debated question, and a good one to think about, is whether or not humans are a monogamous species (there is no wrong answer!).