Monkeys With The Loudest Voices ‘Have The Smallest Testicles’

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.

Howler monkeys with the loudest roars are all mouth and no pants, scientists have learned.

Males use their roars to attract females and scare males. To determine how roars relate to other reproductive traits, researchers examined the hyoid bone, which expands the vocal tract and amplifies calls. The hyoid bone, bone in the primates’ throat, works as a resonator.

Species living in a “harem” structure with one male mating with several females had larger hyoids and smaller testes. This indicates that exclusive access to females means the male can “afford” a bigger voice.

“We have strong evidence that howler monkey species that invest in larger vocal organs produce less sperm”, said lead author Jacob Dunn of the University of Cambridge. The bone creates a sort of resonance chamber that makes howler monkey hoots and hollers really ring out. It is the first time where a relation has been observed between sperm production, fertility and vocalising. They measured testes size across a range of howler monkey species and collected the data to back their findings. Their goal: to discern what features might correlate with the sound of a howler monkey’s love call. The research has been published in the journal Current Biology. “Is there evidence that males with the very largest hyoids are the ones having the most kids?” But a recent study found that males with the deepest and loudest calls typically had the smallest testes and thus were less reproductively blessed, so to speak.

This trend suggests an evolutionary trade-off between the two: monkeys with limited resources might have to choose whether to invest their energy in roaring or making sperm. Dunn continues relating this as another example of sexual selection as described by Charles Darwin almost 150 years ago. Darwin suggested that the cries of the males could be used to attract females who would determine their mates based on both the resonance as well as the depth of their calls. While all the species howl to interact with each other, a few howl louder and more deeply than others. Male Howler monkeys literally just scream until a DTF female shows up. Larger hyloids produce sounds of lower frequencies, giving the impression of a larger body behind the sound. To see if this hypothesis is correct, the researchers studied information on hyoid volume in nine of 10 species, groups of social circles in eight species and volumes of testes in five species. They also analyzed the size of the hyoid, a curvy bone in the neck that supports the tongue to power howling, in 250 males. When it comes to sex and reproduction, evolution says you can’t have it all. Alternatively, such a roar may be so effective at deterring rival males that there’s no need for capacious, hard-working testes to generate large amounts of quality sperm. Male howler monkeys are famed for their deep, powerful roars, which are among the loudest noises made by terrestrial animals, and may help them compete with other males.