Animals' fancies: why members of some species prefer their own sex - homosexuality among animals; includes related article - Cover Story

Science News, Jan 4, 1997 by Tina Adler

Courtship in the barnyard usually puts a smile on farmers' faces and dollar signs in their eyes. That good cheer quickly sours, however, when the the two lovebirds happen to be of the same sex. The problem isn't a moral one, of course. Strictly financial.

Many domestic and wild animals engage in sexual activity with members of both the same and the opposite sex; a smaller number have eyes only for their own sex. Some of these homosexual activities appear to boost reproduction. Female cows often mount each other, thereby signaling any bulls in sight that they are ready to reproduce. In other cases, same-sex affairs may help reproduction indirectly, by promoting the general fitness of a group or individual. For example, in some species, animals are more willing to share food with a member of their own sex after sexual activity with him or her.

Indeed, researchers interested in animal behavior and sexual selection have long held that the main function of homosexual endeavors is to ensure, in a roundabout way, that one's genes get passed along.

The sheep farmer who paid big bucks for a ram's mating abilities and finds the animal ignoring his ewes would certainly question this theory. Besides failing at their jobs, high-libido homosexual rams cause havoc in the sheep pens by disrupting other males mating with females.

A few scientists are now siding with the farmers. Recent studies indicate that homosexual behavior in some species may have much more to do with sexual gratification than with reproduction. Studies are also revealing biological differences between straight and gay animals. These findings may lead to screening tests to help prevent the wrong animals from getting hired for mating jobs. They may also shed light on the possible roots of human homosexuality, some researchers argue.

For the most part, homosexual behaviors in domestic animals are considered normal and helpful for the development of reproduction. "This is not always the case," animal behaviorist Anne Perkins of Carroll College in Helena, Mont., and James A. Fitzgerald of Oregon State University in Corvallis assert in Sexual Orientation (Westport, Conn.: Praeger, in press). Perkins' research on sheep supports this argument.

Scientists have studied the sex drive and sexual orientation of domesticated and wild rams. A heterosexual ram with a strong sex drive will mount either other males or ewes. However, a significant proportion of domesticated males-up to 16 percent-never mate with females during a breeding season, Perkins says.

About 6 percent seem uninterested in any sexual activity. Another 10 percent are homosexual, choosing males even when females are available. Domesticated rams resemble their wild relatives, which scientists have also observed participating in homosexual relations. Ewes rarely engage in such activities.

To try to determine whether rams will service their ewes, farmers simply watch the animals' behavior. They'd like a more foolproof, efficient system, however. Encouraged by new findings on the brain chemistry of homosexual and low-libido sheep, Perkins and her colleagues hope that in the future they'll be able to offer farmers a blood or genetic test.

Estradiol, a form of estrogen, is the bewitching compound that piques a heterosexual ram's interest in females. Ewes and homosexual rams can store a similar amount of estradiol in a brain structure called the amygdala; heterosexual rams accumulate significantly more, Perkins and her colleagues reported in the March 1995 Hormones and Behavior.

Compared to their heterosexual counterparts, homosexual rams have a low concentration of testosterone in their blood, the result of testes that fail to synthesize the hormone as efficiently, Perkins, John A. Resko of Oregon Health Sciences University in Portland, and their colleagues report in the July Biology of Reproduction. Homosexual rams also have lower aromatase activity in a part of the brain, the preoptic area, that helps control sexual behavior in many species. Aromatase, an enzyme, converts testosterone to estrogen.

Though these differences may arise in part during fetal development, a ram's sexual activities may help maintain regions of the amygdala and the preoptic area that mediate sexual behavior, Perkins speculates. Abstinence may cause them to atrophy. "It's a use-it-or-lose-it kind of thing," she explains.

The idea that animals may have sex just because it feels good proves difficult for some people to accept, says primatologist Paul L. Vasey of the University of Montreal. The Japanese macaques he studies, which are hardly anomalies in the animal kingdom, might change the minds of some of these reluctant scientists.

Both wild and captive males occasionally mount each other, but they almost always pick their mates from the pool of available feminine companions. The females frequently engage in same-sex consortships but aren't exclusively homosexual. Females in a captive group that Vasey studies average seven partners each during their breeding season, about half of them male. Among the monkeys' wild relatives, female homosexuality is most common in troops with a relatively low proportion of males.