Among humans, it is generally believed that males with bigger testicles have deeper voices (although there is not much scientific evidence backing this up). If you’re a howler monkey, however, it turns out the correlation between testicle size and louder/deeper voices does not hold true.

Howler Monkeys Have Loud Voice Or Big Balls, But Not Both: Study

New research has determined that howler money can have extra-loud voices or a lot of sperm—but not both as it apparently takes up too much energy. The findings of the new study are published in the September 29th edition of Cell Biology, and offer ground-breaking insights into evolutionary trade offs animals engage in to optimize species survival.

Details of the new study on howler monkeys

Howler monkeys are native to Central and South America, and are well known by locals for their ear-splitting roars. The roar of a male howler monkey is one of the loudest sounds produced by any animal in the world, and can be heard from more than thee miles away. Of note, primatologists have recorded roars lasting for more than 40 minutes, a very large investment of energy. It is theorized that the loud noise helps fend off competition from other males in the area.

Of interest, howler monkey larynxes (the noise-producing voice box of most mammals) vary a great deal among the 10 recognized species. The most common measure of larynx size is the size of the hyoid, a U-shaped bone supporting the tongue and larynx. The size of the hyoid in howler monkeys varies from only 8 cubic centimeters in some species to 110 cubic centimeters (14 times larger!) in others. Similarly, the size of howler monkey’s testes ranges from seven times between the species with the smallest balls, at around 3.5 cubic centimeters, and the species with the largest pair in the neighborhood of 23 cubic centimeters. Keep in mind that this broad variation is till seen even when differences in body size are compensated for.

Academic Jacob Dunn, an evolutionary biologist at the University of Cambridge (UK) and his team took a close look at the relationship between these two characteristics of male howler monkeys. The researchers used new and already published data on the hyoid and testes size of 144 male howler monkeys in nine of the 10 species. With the new data, the researchers used laser surface scanning to create 3D models of the hyoids. Finally, to prove earlier assumptions that hyoid size determined how deep and resonant the howls were, they studied the frequencies of recorded howler monkey calls.

As anticipated, there was a direct linear correlation between the size of a species’s hyoid and the “formant spacing” of its calls (the deepness and resonance of the calls), so it was confirmed that having a big hyoid generally means producing louder roars.

The data also showed there is a definite inverse linear relationship between the size of a monkey’s hyoid and the size of his balls. It turns out that howler monkey species with deeper calls had smaller balls.

Dunn and his fellow researchers also note that the two body measures are correlated to the number of males in a howler monkey’s social group, which typically varies from one to three animals. The data showed that social groups in which the males congregated into larger groups had smaller hyoids and larger testes, and species that lived in groups with just a single male generally had larger hyoids and smaller testes.

They suggest that in terms of an evolutionary strategy, howler monkey species living in small groups invest more energy in scaring off males from other groups by howling louder; if they are successful, then they can “get away” with smaller testes as they now have full access to the females around them