Fertile faster, if herd under roar.

Fertile faster, if herd under the roar

Breeding season seems to bring out the most vociferous behavior in many male animals, with their loud mating noises of various kinds. Clamorous contests of bellows or whistles often precede battles between males competing for female attention. A few studies have shown that this vocal display may go beyond challenging a male competitor to a fight: In several bird species, for example, those females serenaded by lusty male songs ovulate earlier. Apparently the strident red deer of New Zealand and Great Britain have the same biological effect on their females, according to a study by Cambridge University's Karen McComb.

Reported in the Dec. 17 NATURE, the study "provides the first evidence that male vocalizations can affect the timing of ovulation in female mammals," says McComb. It was already known that stags "holding" harems of females during the breeding season roar an average of twice per minute, even in the absence of threatening males. To test the hypothesis that roaring can advance the ovulation dates in female red deer, McComb studied a herd kept on a New Zealand farm. Female deer were divided into three groups: one exposed to roaring recordings, one that included a male that was vasectomized to prevent impregnation of the female, and a third isolated from both males and roaring. A potential fourth group, that of females exposed to a surgically silenced male, was not included for humanitarian reasons, says McComb. After two weeks of "treatment," females in each group were further subdivided and bred with one of three stags, to minimize any effects on fertility by individual breeding males.

Conception dates -- indicative of ovulation timing -- were determined by the dates on which calves were born. Calving among red deer occurs an average of 233 days after conception. McComb found that, as expected, females exposed to vasectomized males had the earliest ovulation dates. (Other animal studies had shown that male behavior and male pheromones affect ovulation.) But those exposed to recorded roars also became fertile earlier -- a phenomenon that enhances calf survival, says McComb, since calves born later in the season have a higher mortality rate. Stag roaring also increases the probability that the most vocal males will mate with a greater number of fertile females before a competitor replaces them.