The Ventromedial Nucleus and Female Sexual Receptivity
According to your text ((p. 194) destruction of the ventromedial nucleus of the hypothalamus reduces a female rat's responsiveness to a male's sexual advances. Results of brain damage experiments can be difficult to interpret (for example, the important effect might be the destruction of pathways passing through the area), and at best the technique does not tell us how the nucleus contributes to female receptivity. Japanese and U.S. researchers working together used a virus to carry genetic material called small hairpin RNAs into the nucleus of female rats, where it blocked development of the alpha type of estrogen receptors. These females were no longer interested in sex and fled or even attacked amorous males. Females given a dummy treatment with neutral RNA continued to show normal responsivness. Proceedings of the National Academy of Sciences, Vol 103, 10456-10460. A video compares two females' behavior.
Homosexuality and the Darwinian Paradox
There is plenty of evidence for a genetic component to male homosexuality; the paradox is that the genes survive in the population even though they reduce the gay man's likelihood of producing offspring. A 2004 study by Andrea Camperio-Ciani and her colleagues (p. 241 in the text) found an increased rate of births among gay men's maternal relatives. Now they have carried that research a step farther by developing a mathematical model of the inheritance of gay behavior. The model that fit the data best assumed two genes, at least one on the X chromosome, which increase fertility in women and decrease it in men. Camperio-Ciani's team suggests that the genes are not so much "gay genes" as genes that increase attraction to men. PLoS ONE, Vol 3, e2282.
Bisexuality in Other Species
Homosexual behavior has been observed in 1500 species. Often it is situational, due to unavailability of adequate numbers of the other sex or in response to the stresses of zoo confinement. In some species, such as the bonobo (a relative of chimps), it provides bonding and conflict resolution. This popularly-written article offers insight into the behavior through several intresting anecdotal stories. You can read the article at Scientific American.
Role of Oxytocin in Humans
Oxytocin is called the "sociability molecule" because it affects bonding in nonhumans (pp 195-6), and there is increasing evidence of a role in human behavior. In one study, volunteers were more likely to recognize pictures of faces they had seen before if they had received a bit of oxytocin in a nasal spray the first time they saw the pictures; there was no effect with pictures of objects, indicating that the effect is social in nature. Journal of Neuroscience, Vol 29, 38-42. An interesting animal parallel is that male mice lacking the gene for oxytocin fail to recognize females they have been paired with before. Nature Genetics, Vol 25, 284-288. Research suggests that a component of oxytocin-induced bonding is an increase in trust (see p 417), and a recent study identified the brain areas involved. In an investment simulation, male volunteers were given money that they could share with another player to "invest". Players who received a placebo nasal spray behaved as expected: When the investor failed to share profits, the player reduced the amount of money he gave the investor later. But players who received a squirt of nasal spray containing oxytocin continued to hand over the same amount of money. Functional MRI indicated that oxytocin reduced activity in the amygdala, which plays a role in fear, and in the dorsal striatum, which contributes to learning from mistakes. Again, the effect appeared to be social; oxytocin affected these brain areas only during play with another human, not when the game was played on a computer. Neuron, Vol 58, 639-650.
The Question of Human Pheromones
In spite of several intriguing positive studies, whether humans retain the ability to respond to pheromones remains controversial. This article examines both sides of the issue while raising questions about the functionality of the human vomeronasal organ and discussing reanalyses of McClintock's menstrual synchrony studies. New Scientist, Dec. 6, 2008, pp. 38-41. Also see Scientific American (online).
