![]() ![]() Monkeys with larger hyoids produce louder, deeper calls, suggesting a larger body size, which indicates an ability to get food and resources.īut the hyoid is an energetically expensive organ to make, so males that grow large hyoids may have to compensate by having smaller structures elsewhere. All howler monkeys howl to communicate with each other, but some species are able to howl more loudly and more deeply. Male howler monkeys are unusual among primates in that they have an enlarged hyoid bone in their voice box that acts like an echo chamber to amplify sounds. "This was a very serious business because the monkeys were passive - but not that passive," Knapp says. To confirm the scans were accurate, the team made CT and MRI images of two adult male howlers.ĭata on testicle sizes for 66 howler monkeys were collected from published literature, but Knapp says researchers visited zoos in Brazil and Germany, using calipers to measure testes of 21 more monkeys while the animals were sedated for health exams. ![]() Researchers used 3-D laser scans to calculate the volumes of 255 cup-shaped howler monkey hyoid bones from museums in the United States and Europe. ![]() "Our study shows that Darwin was probably right when he suggested that the roars of howler monkeys are important for reproduction." "The idea has been around since Charles Darwin, but this is the first time that anyone actually has demonstrated a trade-off between vocal characteristics before mating and sperm competition after mating," she says. And in another species they were more successful if they had large testes." This probably arose because individuals within one species produced more offspring if they had large hyoids. "It is not possible to produce a large hyoid and large testes. "They are different solutions to the same problem," says Leslie Knapp, professor and chair of anthropology at the University of Utah and one of the study's two senior authors. In the cover story of Current Biology's new issue, the research team provides the first evidence of a tradeoff between two traits evolved by different howler monkey species so that males could mate with females and pass their genes to offspring: large gonads to make more sperm, or a large vocal-tract hyoid bone to produce a deep call that scares competitors and attracts females - similar to how women like deep male voices. Now, a new study finds that male howler monkeys with deeper calls have smaller testicles - and vice versa, according to researchers from universities of Utah, Cambridge and Vienna and other institutions. 22, 2015 - Across the animal kingdom, males hoot and holler to attract females and ward off competing suitors. “Whereas with animals, it’s always difficult to assess that.Image: A chorus of howler monkeys of the species Alouatta caraya. “With humans, we can ask, ‘Are you having fun?’,” she says. Just as kittens and puppies pounce and wrestle to build hunting skills, the mental and physical exercise of play may be especially beneficial during bumblebees’ youth, says Galpayage.īecause ball-rolling appears spontaneous and voluntary and doesn’t have immediate or long-term benefits for the insects, they could be doing it to have a good time, says Galpayage. Younger bees rolled more balls than older bees, and males rolled balls for longer than females of the same age. Not all bees had the same knack for playfulness. “When you look at the videos of the bees on the balls, if you saw that in a dog, or a monkey, or even a bird, we’d have no problem calling it play,” says Gordon Burghardt at the University of Tennessee, Knoxville, who was not involved in the work. Each bee rolled balls between one and 117 times throughout the study despite having no obvious incentive to do so. Over 18 3-hour sessions, the bees opted to enter the zone with free-moving balls 50 per cent more often than the zone with stationary balls.
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