The next time you hear a howler monkey making a loud, deep-throated call in the jungle, you might be right to wonder, "Is he compensating for something?"
A study in the journal Current Biology found that male howler monkeys with the most powerful voices – a trait that is useful for attracting females – also had the smallest testicles.
For scientist Leslie Knapp, who describes the work as her "calls and balls paper", the findings offer an intriguing look at an evolutionary tit for tat.
"The idea has been around since Charles Darwin, but this is the first time that anyone actually has demonstrated a trade-off between vocal characteristics before mating and sperm competition after mating," said Knapp, professor and chair of anthropology at the University of Utah.
"Our study shows that Darwin was probably right when he suggested that the roars of howler monkeys are important for reproduction."
Males with the deepest voices tended to live with a harem of a few females, and no other males.
Those with larger testicles and less powerful voices competed for females with other males. The females in these groups mated with multiple partners.
Under such competitive conditions, the ability to create more sperm, via bigger testes, could be a boon toward fertilising an egg, researchers say.
Knapp believes the trade-off came about through "different solutions to the same problem", she said.
The study, co-authored by Jacob Dunn, an evolutionary biologist at the University of Cambridge, was based on a bone in the voice box known as the hyoid, which when bigger helps monkeys make a louder, deeper call.
Researchers used 3D laser scans to calculate the volumes of 255 howler monkey hyoid bones, gathered from museums in the United States and Europe.
They collected data on testicle sizes from published literature on 66 howler monkeys.
Researchers also used calipers to measure the testicles of 21 more monkeys at zoos in Brazil and Germany.
"It is not possible to produce a large hyoid and large testes. This probably arose because individuals within one species produced more offspring if they had large hyoids. And in another species they were more successful if they had large testes," said Knapp.
Whatever the reason for differences, both female monkeys and female humans seem to prefer a deep voice when choosing a mate.
"A large hyoid might make a male more attractive to females or make other males think he is large and scary, which may be the best way for him to keep his harem," said Knapp.