Inside the roaring sex lives of howler monkeys

Researcher dissects howler monkey roars

DEEP ROARS, SMALL SPERM STORES In nine species of howler monkey, researchers found an inverse relationship between testes size and size of the hyoid bone, part of the vocal tract.

By Helen Thompson

November 30, 2015 at 5:12 pm

Just after dawn, barbershop quartets of male howler monkeys echo over the canopy of Mexico’s forests. Jake Dunn remembers them well from his early fieldwork in Veracruz. “Most people who don’t know what they’re listening to assume it’s a jaguar,” says Dunn, a primatologist at the University of Cambridge.

The calls serve as a warning to male competitors and an alluring pickup line for females. While studying primates in Mexico, Dunn heard drastic differences between resident howler monkeys. He and his colleagues decided to pin down the origin and evolution of this well-known variation among species.

After reading a 1949 paper that classified howlers based on a vocal tract bone called the hyoid, Dunn paired up with Lauren Halenar of the American Museum of Natural History in New York City, who was studying the hyoid’s role in howler biology.

**NOW IN 3-D** Scans of hyoid bones from different howler monkey species provided scientists with digital models from which to compute the actual size of the bones, which correspond to the frequency of calls. Jacob C. Dunn/University of Cambridge

Scouring collections at museums and zoos in the United States and Europe, the team used laser scanners to create 3-D models of hyoids from nine howler species. The work required a lot of digging through cupboards for skeletons. “Some of these specimens are hundreds of years old,” says Dunn, who recalls imagining “the early naturalists hunting these animals and bringing back the collections.”

Real pay dirt came from the National Museums of Scotland, which had preserved the remains of two howlers that had died of natural causes in zoos. CT and MRI scans of the two specimens provided a rare peek at the howler vocal system’s layout.

Dunn thinks the container-like hyoid functions as a resonating chamber for calls. “It’s a bit like when you blow over the top of a bottle,” he says. Based on acoustic measurements, the team found species with larger hyoids roar at lower frequencies. Their findings appear in the Nov. 2 Current Biology.

Howler species with bigger hyoids have smaller testes and live in harem-style social groups (SN Online: 10/22/15). In this less-crowded playing field, reproductive success depends heavily on luring females rather than producing plenty of sperm to compete with sperm from other males living in the group. Harem species invest energy in vocals over testicles. Bigger hyoid bones give males deeper calls, fooling females, rivals and unsuspecting humans into thinking they are more fearsome, virile animals.