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!).