Quietest bulls walk away with more females than loudest rivals

December 17th, 2008 - 3:26 pm ICT by IANS  

Washington, Dec 17 (IANS) The quietest bull walks away with the most females and sires the most offsprings while studs bellowing the loudest are left with almost next to nothing, according to a new study. “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 geography student at the University of California, Davis, (UC-D) and co-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 vocalisations correlate with reproductive success.

Most studies of vocalised sexual signals among animals have focussed 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 vocalisations performed by bulls during the rut. They are most commonly used when one male challenges another, typically when the two are within 15 to 30 metres of each other.

Yet sometimes a bellow will attract she-bulls from further away, and this may be one reason that a herd’s dominant bulls keep their voices down, Wyman speculated, said a UC-D release.

“It could be that bulls provide information about their high quality through other signals - for example, the frequency or the duration of their bellows. So they don’t have to be louder, they just have to be heard,” she said. “If you bellow too loudly, it could bring in too many other bison to check you out.”

The bigger question raised by the study, Wyman said, is why lower-quality males don’t turn down the volume of their bellows to emulate their more successful rivals.

“That’s a lot harder to explain,” she said. “It could be that if you use a quieter volume, other bulls have to approach even closer to check you out, and any time you bring someone that close, there’s a higher risk of attack. And that’s the type of cost that these low-ranking bulls may not be able to bear.”

To learn how bison communicate with one another, Wyman and Michael S. Mooring of Point Loma Nazarene University, and a number of student interns spent two summers monitoring 325 wild bison in Fort Niobrara National Wildlife Refuge in the Sandhills region of north-central Nebraska. Their study was published in the November issue of Animal Behaviour.

Observing the herd for 14 hours each day during the two-month rut of July and August, the team was able to record each copulation and to detail the tangled web of connections between males and females as bulls lost and gained cows during their intense competitions.

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