News Release

The researchers, studying a small mouse-like mammal called a prairie vole, have discovered that in the spring, compared with winter, the animal's brain produces much more of a protein that indicates that nerve cells essential for mating have been activated.

The findings were detailed in a scientific paper published April 17 in the journal Brain Research.

Scientists already knew that voles produce the protein when they are exposed to a specific chemical scent called a pheromone, which is contained in the male's urine and other secretions. Female voles show no interest in mating with males unless they are exposed to the appropriate chemical signal from the males' urine. The chemical stimulus is converted in the brain into a signal that triggers the release of hormones in the female, leading to mating behavior, said Randy Nelson, an associate professor in the Johns Hopkins Department of Psychology.

"It sounds rather unpalatable but remember, most non-human animals go around sniffing each other," said Gregory Ball, another associate professor of psychology who is involved in the research. "We are kind of a strange mammal in that we don't use olfaction in the way that most of our relatives do."

Hopkins researchers wanted to test their hypothesis that the animals might actually be more sensitive in springtime to the chemical signals behind reproduction.

"So we presented the female with urine at different times of the year, and then we looked at the effects on cells in the brain," Dr. Ball said.

The hypothesis turned out to be correct: The female voles' brains produced more of the protein, called fos, in lab-simulated springtime conditions than during simulated winter.

The absence in winter of springtime levels of the protein, even after exposure to the urine, is critical. It indicates that the urine smell in winter did not trigger the cascade of events in the nervous and endocrine systems that leads to mating.

Scientists believe that an explanation for this seasonal difference in protein production, and thus for the lack of wintertime sexual motivation, may lie in changes in brain physiology as the number of daylight hours increases and decreases. Additional research is testing that idea.

In humans, although women have major changes in hormone production during ovulation, they do not experience estrus, or cyclic willingness to mate. Human sexual behavior is not so profoundly regulated by hormones.

"That is a very important thing that affects our social system, that we can engage in mating at any time in the cycle," Dr. Ball noted.

Also working on the research was former doctoral student Christopher Moffatt, who has since received his doctorate in psychology and is currently a research assistant professor at the University of Massachusetts at Amherst. The three researchers specialize in behavioral neuroendocrinology, which deals with how the brain responds to hormones.