U researchers have taken a special interest in howler monkeys because they are the easiest to observe, follow and collect DNA samples from.

Natalie Spendlove, a senior in anthropology and Leslie Knapp, professor and chair of anthropology, used howler monkeys to investigate DNA, social behaviors and reproductive habits. Their studies are based on rainforest observations and collecting feces samples.

“I thought I’d have some normal job where the constant fear of what’s lurking in the swamp is nonexistent,” Spendlove said. “But there I was — 21 and howling.”

Since her sophomore year, Spendlove has been involved in studying primates and evolutionary genetics under the guidance of Knapp. Spendlove’s experience also notes the importance of “howling.”

“When I started to look at the hyoid bones of different species of howlers,” Knapp said. “I was curious about how those bones varied and what that meant for howler monkey calls and the social and sexual relationships in their groups.”

Knapp observed that the hypoid bone — a bone that produces vocals — was bigger in males with smaller testes and vice versa. If a howler had a bigger hypoid, their howl was deeper and louder. To females looking to mate, this indicates that the males had larger reproductive organs and a bigger sperm count than they actually do.

“People like Charles Darwin have been wondering about the reason for the large hypoid bones and the loud howls that these monkeys make for hundreds of years,” Knapp said. “Now we better understand how large hypoids work and what advantages they provide to these monkeys.”

According to Spendlove’s experience in Costa Rica, howlers move slowly in comparison to other monkeys and are the safest choice for study. These primates are also slow breeders, which allows researchers to see how deforestation and hunting can affect reproduction levels and behaviors.

“When these monkeys are isolated, they have less potential mates and their genetic diversity drops way down,” Spendlove said. “They can start breeding with close relatives, and soon the population is left vulnerable to disease and disaster.”

These observations will hopefully allow humans to assist and help reduce the effects of deforestation. Knapp and Spendlove’s research show results of internal and external effects on howler monkey reproduction as time goes on. This may help in the study of other species.

“I hope that students will find this research interesting, and that it will stimulate them to ask questions and learn more,” Knapp said.

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