The loudness of a howler monkey is relative to the size of its testicles. Researchers found that the smaller the testicles, the louder the monkey.
A howler monkey weighs little more than a pug but can roar as loud as a tiger. Researchers at Cambridge University have found a surprising trade-off across howler species: the louder a male’s roar, the smaller his reproductive organs. His testicles, to be specific.
The howler monkey’s roar originates in a U-shaped bone in the neck known as the hyoid bone. Humans have them too, but howlers’ hyoids are far bigger than those of any other primate. A larger hyoid means a deeper call.
A team of scientists from Utah, Cambridge and Vienna universities analyzed physical data from howler monkeys across different species and found an inverse relationship in size between the hyoid and the testes. The larger the hyoid, the smaller the howler family jewels, according to the findings published in the journal Current Biology.
The difference is the result of mating patterns in different howler species. The deep-voiced, small-balled ones tend to live in family groups with a single male and multiple females. A deep roar could be more attractive to females, or more threatening to competing males.
Those with smaller hyoids and bigger balls live in groups with multiple males and may need to produce more sperm to increase their chances of siring offspring.