CORVALLIS, Ore. - Scientists studying causes of "male-oriented" mating behavior in some rams have found links between the male's sexual preference and chemical activity in the animal's brain, according to a recent study.

The research report was co-authored by Fred Stormshak, an Oregon State University animal scientist. Results of the research were published in the scientific journal Biology of Reproduction.

Stormshak and a team of scientists investigated the sexual behavior of "male-oriented" rams - adult male sheep that prefer to copulate with males rather than females.

"This is a costly problem for sheep producers because breeding rams are worth $300 to $500 each," said Stormshak. "Outwardly, there is no way to tell whether a ram is male-oriented so the producer runs the costly risk of buying an animal that will never produce any offspring."

Sheep producers lose thousands of dollars each year from the purchase of breeding rams that turn out to be male-oriented, Stormshak added. About eight to 10 percent of total ram numbers are male-oriented, he said.

The research team also included John Resko, Charles Roselli and Jerome Choate of Oregon Health Sciences University; and Anne Perkins of Carroll College in Helena, Mont.

The researchers focused their efforts on the part of the brain called the hypothalamus, a center for the formation of reproductive behaviors in most mammals.

"We concentrated on aromatase activity in the hypothalamus," said Stormshak. "Aromatase is an enzyme that converts testosterone into estrogen in the male."

In some species, he added, the hormone estrogen brings about mating behavior in which males seek females for copulation.

These data reflect the biology of sheep and are "not construed to be applicable across species," Stormshak pointed out.

Stormshak and his associates conducted their study on a large group of rams from the U.S. Sheep Experimental range flock in Dubois, Idaho. Research on male-oriented behavior in rams has been conducted at the Idaho sheep facility for several years.

After reaching 16-18 months of age, the rams in Stormshak's study were given sexual behavior tests to determine their preference for either females or males during mating opportunities.

"We found that in regard to mating behavior, the rams fell into three categories," Stormshak said. "One group mated with females again and again, a second group mated with females occasionally and the third group did not mate with females at all."

Following further mating preference tests, tissue and blood samples were collected from selected rams in the study. Analysis of those materials led to the general conclusions that male-oriented rams convert less testosterone to estrogen in the hypothalamus component of their brains, and that these rams have reduced capacity for production of testosterone, compared to female-oriented rams.

"These results raise the question of why the testes of some rams produce less testosterone than others," Stormshak said. "Unfortunately, there is no answer for that question now."

Stormshak and his colleagues are following up the study with another project that will analyze the possibility of altering the male-oriented behavior of rams by placing an estrogen implant in the body of the animal.

"Because it is the estrogen level in the hypothalamus region of the brain that may determine the sexual behavior of the ram, it may be possible to restore tissue levels of estrogen comparable to those of heterosexual rams and affect sexual behavior accordingly," said Stormshak.

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Fred Stormshak, 541-737-2325