Howler Monkeys With Loudest, Deepest Growls Have the Smallest Testes, New

Researchers from Cambridge University found that the trade-off also corresponded to how the different species of howlers lived.

Another important finding, Dunn and his colleagues report, was how these two body measures are related to the number of males in a howler monkey’s immediate social group, which ranges from one to three animals depending on the species. They’re communicating about where they are and what’s going on.

You can see how much effort it takes. If this data for a few reason calls to mind the image of a grown male human driving a flashy auto blaring a club-sized sound system to compensate for the tiny, tiny gonads you are certain he has-well, that’s on you. The focus of the research was on a bone in the shape of a cup, the hyoid that is located above the larynx and creates a resonating chamber for amplification of vocalizations. Its size is thought to dictate if a primate’s call has a low or high pitch. The hyoid bone acts as an amplifier of the otherwise less impressive howls. This suggests that the effect of the howl is to make the males seem larger than they really are. It appears that males living in Central and South America need deep voices to attract partners and scare potential rivals. They found that larger testicles interfere with immune functions. Our new research aimed to try and describe this variation quantitatively, as well as understand both the evolutionary pressures that led to the observed variation, and the acoustic consequences of having large versus small hyoids.

A new study reveals how male howler monkeys who possess the loudest, booming voices, apparently reveal an evolutionary strategy suggesting that these loud monkeys have smaller testicles and produce lesser sperm.

Researchers used 3D laser scans to calculate the volumes of 255 howler monkey hyoid bones, gathered from museums in the United States and Europe. “A large hyoid might make a male more attractive to females or make other males think he is large and scary, which may be the best way for him to keep his harem”. Per the study, most mammals that are deep-voiced have larger bodies and hyoid bones.

It’s actually kind of similar to the way male humans call out to women on the street, in that the louder ones are often the more “insecure” individuals. Those they studied also produced less sperm. But this also comes with an interesting catch – according to the researchers, the depth of a male’s mating call tends to be inversely proportional to the size of their testicles.

“This is probably because the females in these species will mate with several males, and so the males have evolved to produce more sperm to increase their chances of fertilizing an egg”, Dunn said. More sperm means that they can spread their seed further throughout their community, elevating the odds that their genes will be passed onto to the next generation. Species with larger hyoid bones have smaller testes and vice versa. At the same time, his balls shrank, since there is less competition.

For those who don’t know: the howler monkeys are primarily found in the region between southern Mexico and Argentina.