Howler monkeys that roar the loudest have the lowest sperm count and smaller testicles.
New research shows that the males making the most noise are trying to make up for smaller testicles and lower sperm count, at least in howler monkeys, according to an article on themarketbusiness.com.
The report on the study published in Current Biology, says researchers found that there was a connection between the size of a species’ hyoid and the “formant spacing” of its mating calls. The hyoid is a u-shaped bone that supports the tongue and larynx. Formant spacing is a measure of the deepness and resonance of the callings made by the species. So, having a large hyoid generally means the louder the call.
Ironically, in the howler monkeys research, there is also an inverse liner correlation between the size of the hyoid and the testes, with the smallest testicles being able to produce the deepest calls.
Apparently in the howler monkey universe, you can use the body’s energy to produce a lot of noise, or a lot of sperm, but not both.
Howler monkeys, native to Central and South America, produce among the loudest roars of any animals and can be heard for miles. They have been observed making the loud roars for as much as 40 minutes, possibly to keep other males away from potential mates.
A team of researchers led by Jacob Dunn, an evolutionary biologist at the University of Cambridge in the United Kingdom, found that the measurements of the hyoid and testes also related to the size of the monkey’s social group. The group with smaller testicles had more males in the group than the ones with the larger testes.
The team believes that the males in the groups with smaller testes spend more time and energy fending off other male suitors from other groups, while those males in larger groups invest their energy in producing more sperm to increase their chances of successfully mating.
Dunn says, “You can’t invest in everything at once. Perhaps surprisingly, given sperm’s reputation as an abundant resource, there is strong evidence that sperm production is actually quite costly.” He adds that howling for as much as 40 minutes is a very strenuous activity as well.
Dawn Kitchen, a physical anthropologist at Ohio State University, Columbus, who was not involved with the work said that instead of using their roars to compete within their own social group, small group males expend their energy to discourage males from outside the group, while the males in larger groups invest their energy in sperm production to mate with local females.