But ranchers have occasionally found that their prized rams just didn't perform. So, for more than a decade, scientists at a remote sheep experiment station in Dubois, Idaho, have been trying to figure out why some rams are "duds."
At first, the researchers examined sperm counts and hormone levels, but they found nothing unusual. However, when they applied a little animal psychology, they concluded the non-performing rams were, to put it in human terms, gay.
"You can have the best sperm in the world, but if you are not interested in inseminating females, it is not going to get delivered," said Anne Perkins, who worked at the sheep station in the early 1990s. She found that 8 to 10 per cent of the rams would shun willing females and try to mount other males.
The gay rams aren't alone. "There is homosexual behaviour throughout the animal kingdom, documented all over the place," ranging from lesbian macaque monkeys in the forests of Japan to gay penguins at the Central Park Zoo in New York, noted Prof. Perkins, who is now chairwoman of the psychology department at Carroll College in Montana.
As animal researchers delve deeper into the gay wild kingdom, their findings are bound to spill into the emotionally charged debate about what drives human sexuality.
And the sheep at the experiment station in Idaho are continuing to provide evidence that sexual preference is biologically determined, possibly before birth.
By studying brain samples from the sheep, researchers at the Oregon Health and Science University in Portland recently discovered distinct difference in the brains of "gay" and "straight" sheep.
In particular, they found that a densely packed cluster of cells in a region of the hypothalamus that plays a role in sexual behaviour was "significantly larger" in rams that preferred females compared with the male-oriented rams.
More than a decade ago, Simon LeVay, a neuroscientist based in San Francisco, made headlines when he published a study that purported to show differences in the hypothalamus region of the brains of homosexual and heterosexual men.
Skeptics initially challenged Prof. LeVay's work. They noted that some of the brain samples had come from men who had died of AIDS and they suggested that the disease, or other factors, might have been responsible for the brain differences.
Now, the study of disease-free sheep brains appears to lend credence to Prof. LeVay's finding in humans. "Our research, for that reason, is important," said Charles Roselli, a professor of physiology and pharmacology at Oregon Health & Science University.
Indeed, his study of sheep brains has provided even more clues about sexual orientation. The brains of homosexual rams contained less aromatase than the brains of the heterosexual ones. Aromatase is an enzyme required for testosterone to work -- and testosterone plays a critical role in male sexual arousal.
Prof. Roselli thinks that the brain differences in the sheep, and possibly in humans, arise during fetal development. Various studies indicated that the testes of the male fetus release testosterone at a critical period of brain development when sexual preference is being established.
"The testosterone exposure helps to maintain more cells in the area [governing sexual preference], so that later in life they can function in their adult capacity," Prof. Roselli said.
But in rams that prefer other males, this hormonal exposure was somehow disrupted and "they were incompletely masculinized," he speculates.
Some researchers have been able to change the sexual orientation of animals by altering the hormonal milieu during fetal growth. Elizabeth Adkins-Regan, a professor of psychology and neurobiology at Cornell University in Ithaca, N.Y, has focused her attention on the sexual development of zebra finches.
She has injected certain hormones into incubating eggs and produced female zebra finches that preferred other females. The females even grew "substantial quantities" of testicular tissue.
