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.
But Prof. Adkins-Regan didn't have to go so far as to inject hormones into incubating eggs to shake up the normal sexual order of things. Just removing all the adult males from the nesting colony did the trick. She noted that zebra finches tend to form lasting pair bonds with both males and females, sharing nearly equally in parental care, including the incubation and feeding of the chicks.
When Prof. Adkins-Regan removed all the adult male finches after the chicks were hatched, the birds grew up without a defined sexual preference. "They were content with either sex as a partner. So social experience is playing a role," she noted.
Can the social lessons gleaned from the study of zebra finches be applied to humans? In other words, could raising children in the absence of male or female role models shape their sexual preferences? Prof. Adkins-Regan doubts it, simply because zebra finches and humans are very different creatures. "The zebra finch is a species in which the birds pair for life," she noted. The same can't be said for humans. "We don't have any deep evolutionary history of having pair bonds."
What all the new research does suggest is that sexual orientation is established very early in development -- and, once it is established, it is set for life. "I don't think you could find a scientist who would say it's a learned behaviour -- we are so past that," Prof. Perkins said.
The hot new fields of research involve factors that might influence the developing fetus - including genetics, diet, chemical exposures and even maternal stress during pregnancy.
Yet researchers readily admit they have a hard time figuring out how homosexual behaviour fits into the classic Darwinian theory of evolution. "It's difficult for some people to accept that animals would have sex with the same sex, if it doesn't result in procreation," because the behaviour essentially represents an evolutionary dead end, Prof. Perkins said.
And it raises the possibility that animals might be doing it just because it feels good.
Paul Vasey, a professor of psychology and neuroscience at the University of Lethbridge in Alberta, has been trying to solve the riddle by studying Japanese macaques.
Virtually all the females of this primate species are bisexual, regularly switching between male and female sexual partners.
He speculates that way back in the evolutionary history of the species, the females learned to mount the males in order to attract or maintain their attention.
The females will still hop on top of the males, often stimulating themselves by rubbing their clitoris with their tails or against the backs of the males, Prof. Vasey said. More often than not, the male will then swivel around and mount the female.
But even if it doesn't, "the female is still capable of deriving immediate sexual gratification from the mounting," Prof. Vasey said. "And if you can mount a male, you can mount a female just as easily."
Sometimes the females may prefer each other sexually because the males are being overly aggressive or unco-operative.
Prof. Vasey said the female-male mounting practice could be considered an adaptive behavioural trait for increasing access to the dominant males. And the female-female mounting practice is simply a "functionless byproduct of an adaptation."
"The Japanese macaques represent one possible evolutionary pathway by which homosexuality can occur, but I am sure that there are multiple ways that it might evolve," he added.
Prof. Vasey has just completed a study on the brains of Japanese macaques to see if they contain structural differences similar to those found in the brains of homosexual sheep and humans. It turns out that they don't, but Prof. Vasey is not really surprised. His study examined the brains of females, while the other studies investigated male brains.
Prof. Vasey and the other researchers caution that there are limits to how much we can learn about ourselves by studying the animal world. After all, human relationships involve love, trust, commitment and shared values.
But when it comes to pure raw sex, the basic biological process of "what turns you on is probably very similar to what turns on rats and sheep," Dr. Perkins said.
Paul Taylor is a Globe and Mail assistant national editor, responsible for health and science coverage.