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!).