Monkeys With Smaller Testicles Scream Louder to Compensate

Howler monkey in Santa Maria de Jetiba, Brazil. Photo by Paulo Chaves. It’s a long-held belief that loudmouths overcompensate for something, but in the case of howler monkeys, science has confirmed it’s a biological fact. A landmark study by Dr. Jacob Dunn at Cambridge University, along with 2026 follow-up research , has established that monkeys who scream the loudest effectively “pay” for that volume with significantly smaller testes and lower sperm counts. Howler monkeys are one of the few nest-building monkeys, and one of the loudest creatures in the animal kingdom. They generally live in groups of 6-12 individuals, with only a few males and many more females, in a harem style of arrangement. Biological anthropologist Jacob Dunn and his team from Cambridge University wanted to see how the size of the monkey testicles plays into social arrangements, and how they correlate with their screams. Specifically, the team calculated how loud monkeys scream depending on the size of their testicles. “The results of our acoustic analyses show that howler monkeys produce roars at a similar frequency as tigers, which is far lower than we would have predicted from their body size, yet exactly what would be predicted from measuring their giant vocal folds’ which are three times bigger than in a human.” Dunn’s team analyzed the biological “budget” of these primates and found a direct negative correlation: males with large hyoid bones (the hollow throat bone that acts as an echo chamber) produce deeper, louder roars but have smaller testes. Males with smaller hyoids have quieter calls but significantly larger testes. This trade-off dictates their social lives. Males with “loud mouths and small testes” are typically the ones who live in harems (one or few males, many females). They use their terrifying roar to keep rival males away, meaning they don’t need to compete via sperm count. Conversely, males in large, mixed groups rely on “sperm competition” mate with the same females as other males so they invest their energy in testicular size rather than vocal volume. Howling Like a Tiger Credit: Pexels. Why is the roar so expensive to produce? It turns out the monkeys are engaging in “acoustic deception.” A recent study revealed that the enlarged hyoid bone lowers the sound’s frequency, allowing an 8kg (17 lb) monkey to sound like a tiger. This phenomenon, known as “honest exaggeration,” tricks rivals into thinking the caller is physically massive, preventing dangerous physical fights. They do this through a unique adaptation of a vocal membrane . This is a tiny ribbon of tissue above the vocal cords; we humans have lost this membrane. This membrane creates chaotic, non-linear sounds (similar to a death metal scream or an erratic yodel), which are biologically designed to be impossible to ignore. “It may be that investment in developing a large vocal organ and roaring is so costly that there is simply not enough energy left to invest in testes,” Dunn explains . “Alternatively, using a large vocal organ for roaring may be so...

Preview: ~500 words