'Soft Spoken' Bison Bulls Win More Mates

During the American bison (Bison bison) mating season, bulls produce loud, low-frequency bellows. These bellows are most often used when one male challenges another and may enable rival bulls to determine which is dominant. A team of scientists has now revealed that the way bellows convey information between bison is counterintuitive and perhaps far more complex than they had expected. Their findings indicate that males whose bellows are quieter win more mates and produce more offspring than their louder-bellowing rivals. They had expected to find the opposite—that more dominant males produced the louder bellows.

The research team, led by Megan Wyman, included experts from the University of California, Davis and the Point Loma Nazarene University. They studied bison in the Fort Niobrara National Wildlife Refuge in north-central Nebraska. For two consecutive rutting seasons, which occur during the months of July and August, the scientists kept a daily, 14-hour watch on the bison heard.

From the safety of a pickup trucks (the bison were habituated to these trucks and were not scared off by them), Wyman and her colleagues measured vocalizations, making note of challenges among bulls as well as which bulls successfully mated with females.

They used hand-held sound-level meters to record the amplitude (loudness) of the bison bellows and were careful to note any factors that might influence the sound measurements such as wind direction, orientation of the bison's head relative to the sound-level meter, and the distance of the bellowing bull from the meter. The team also noted each observed mating between bulls and cows and rival challenges among bulls to produce a clear understanding of the social structure of the herd and the relative mating successes of the various bulls.

At the end of the study, Wyman sifted through the sound-level readings to include only those of the highest quality and was left with 408 readings from 44 bulls. When she analyzed those sound readings, she found that the bulls with the lowest number of matings and offspring bellowed at least 50 percent louder than their rivals who were more successful at mating and producing offspring.

"We were expecting to find that the bigger, stronger guys—the higher-quality maltes—would have the loudest bellows, because they can handle the costs of it, but instead we found the opposite. It could be that bulls provide information about their high quality through other signals—for example, the frequency or duration of their bellows. So they don't have to be louder, they just have to be heard." ~ Megan Wyman, UC Davis.

Find out more: Quiet Bison Sire More Calves Than Louder Rivals (Eurekalert)

Photo (top) © Paul Haverkamp / UC Davis Photography / UC Regents. UC Davis geography graduate student Megan Wyman measures the amplitude of a bison's bellow in Nebraska's Fort Niobrara National Wildlife Refuge. Photo (bottom) © Megan Wyman / UC Davis Photography / UC Regents. A bull bison (right) bellows as he guards a female from other males.