Howler Monkeys

Howler Monekys are found in tropical Central and South America. Their name comes from the loud cries they use to send messages to other monkeys that this territory is already occupied. Howlers are omnivores that have an average live span of 15 to 20 years, growing to be about 22-36 inches and weighing anywhere from 5 to 22 pounds.

Deep calls or Small Balls?

Studies have shown that the smaller a Howlers testes are, the larger their hyoid bone is. A monkey’s hyoid bone is the structure allows them to create that deep, booming call they’re known for. According to a new study published just a few days ago in the journal of Current Biology, “the relative sizes of the hyoid bones and testes appear to be related to how the animal lives and reproduces.” For example, the scientists found that male Howler monkeys that live in groups multiple other males tend to inherit larger testes. Unlike the single male Howlers that live amongst a group of females who invest in booming, strong calls.

Charles Darwin, even suggested that the male Howlers used their cries  to attract females. Concluding that the females would choose their mates based on the depth of these calls.  This theory was studied years later by an anthropologist named Leslie Knapp and her colleagues at the University of Utah where they discovered that male howlers hyoid bones are about five times larger than those found in the females. Which implies that Darwin’s proposition that Howlers calls play a reproductive role was accurate.

Discovery

Knapp and her fellow researchers also measured 10 different species of monkeys hyoid bones across Mexico and Central America. Knapp told Live Science that “the hyoids and testes varied widely across species: The species with the largest hyoids had bones 10 times larger than those with the smallest. And the largest testes were about seven times larger than the smallest testes.” But, specifically among Howlers, the researchers said that these monkeys were either found to have large testes and smaller hyoids or smaller testes and larger testes which was linked to social structure. For example, it was discovered that male monkeys that were mating with multiple females were found to have larger hyoids and smaller testes. Meanwhile, the monkeys living in groups with numerous males and females all mating together were found to have larger testes and smaller hyoids.

This could be explained by Darwin’s theory that Howlers use their call to attract more mates or scare off other males from taking their females. “Resulting in a lower sperm load because they compete for females before the actual mating act” said Knapp.

On the other hand, for the male Howlers living in groups, they have a much greater volume of sperm because their goal is to impregnate a female monkey that they’ve mated with multiple times. It’s probably impossible for howler monkeys to evolve both a large hyoid and large testes, Knapp said.

Does this also apply to humans, after all we are long distant cousins? According to researchers from University College London men with deeper voices are considered more attractive by women than men with higher-pitched voices. Their study consisted of  32 participants, and concluded that  “high-pitched female voices females were found to be attractive because they indicated the speaker had a small body and deep male voices were judged as more attractive because they conveyed that the speaker had a large frame.”

So what do you think? Is this just another similarity between humans and monkeys we haven’t discovered yet?

Work Cited

http://animals.nationalgeographic.com/animals/mammals/howler-monkey/

http://www.livescience.com/52560-monkeys-with-deeper-calls-smaller-testes.html

http://www.smithsonianmag.com/science-nature/why-women-like-deep-voices-and-men-prefer-high-ones-41492244/