During bison mating season, the quietest bulls score the most mates and sire the most offspring while studs with the loudest bellows see the least action, according to a surprising new study by researchers at University of California, Davis, and Point Loma Nazarene University in San Diego. The researchers also found that the volume of a bull's bellow was not related to its weight or age.
"We were expecting to find that the bigger, stronger guys the high-quality males would have the loudest bellows, because they can handle the costs of it," said Megan Wyman, a graduate student in geography at UC Davis and the lead author of the study. "But instead, we found the opposite. My collaborator in San Diego wanted me to call the paper 'Speak softly and carry a big stick.'"
The study is the first to examine how the amplitude, or loudness, of a mammal's vocalizations correlate with reproductive success. It was published in the November issue of the journal Animal Behaviour.
Most studies of vocalized sexual signals among animals have focused on the pitch characteristics, timing and duration of calls. Amplitude has received much less attention, Wyman said, largely because loudness is especially difficult to measure in the field. By the time a grunt or a roar reaches a sound-level meter, its amplitude may have been affected by the animal's distance from the meter, the direction the animal was facing when it called, wind conditions and a number of other factors.
Bison bellows are loud, low-frequency vocalizations performed by bulls during the rut. They are most commonly used when one male challenges another, typically when the two are within 45 to 90 feet of one another. Yet sometimes a bellow will attract bulls from further away, and this may be one reason that a herd's dominant bulls keep their voices down, Wyman speculates.
"It could be that bulls provide information about their high quality through other signals for
|Contact: Liese Greensfelder|
University of California - Davis