Howler monkeys are loud. (If you doubt me, just click on the video and listen.) They can bring on the noise because they have a built-in amplifier: a huge cup-shaped throat bone that resonates to their song. But a study up in today's Current Biology suggests that the males with the biggest voices also have the smallest testes.
When an international team of researchers led by University of Utah anthropologist Leslie Knapp compared the average size of the bony resonator, called the hyoid, to the average size of the testes in males from nine of the twelve species of howler monkey, they found that species with larger resonators had smaller testes than species with small hyoids.
That's interesting because the size of the males' testes is also correlated to how many males live in a howler monkey troop. In species where several males hang out with females, males typically have large testes; in species with just one adult male per troop, testes tend to be small. Knapp says they think sex is affecting both traits.
When female howler monkeys are fertile, they're also promiscuous, mating with many males both inside and outside the troop. In species where females and males mate freely, males that can put lots of reproductive tickets into a female's fertilization lottery have a better chance of becoming fathers. That favours larger testes that can pump out loads of sperm.
But in some species of howler, Knapp explained, one male keeps other males away, leaving him with a harem of females. In those species, males have evolved smaller testes: their sperm don't have to beat out the emissions from other males.
The open question is why species of howler monkey that keep harems also make deeper calls (and have the large hyoid bones to help them do it). Knapp and her team are eager to test how that trait affects a male's reproductive success. She explains, "We want to know if males in a species that have the largest hyoids (and the deepest calls) also have the most mating and offspring compared to other males in the social group." [Kowalewski et al. 2014 [Strier 2015 [Dunn et al. 2015]] Video credit: La Senda Verde Animal Refuge, Bolivia.