ANN ARBOR-Humans who cheat on a loved one aren't the only ones to change their behavior to avoid discovery.

New research documents for the first time how wild gelada monkeys tactically conceal their infidelity from dominant leader males.

Researchers from the University of Michigan, University of the Free State-Qwaqwa in South Africa and University of Pennsylvania observed a herd of geladas in the Simien Mountains National Park of Ethiopia. For three years, they noted the details of the sexual behavior—and misbehavior—of more than 200 wild geladas. Their findings are published in the current issue of Nature Communications.

While females typically only mate with the dominant leader male of their unit or harem, the opportunity for infidelity always exists. Subordinate follower males hang out with some of the units, while young bachelor males are also hovering around the edges of every gelada herd.

However, monkeys that would try to commit adultery while deceiving the leader male have to overcome an obstacle presented by their open grassland habitat—there is simply no place to hide, says Aliza le Roux, the study's lead author and researcher at the University of the Free State-Qwaqwa.

Researchers suspected that cheating geladas might still try to conceal their indiscretions. Leader males and females typically give loud calls while mating—a vocal duet that can be heard from more than 30 meters away. Therefore, the simplest way for cheaters to avoid discovery would be to stay quiet and keep an eye on the whereabouts of the leader male, researchers said.

After carefully studying more than 1,000 sexual relationships among geladas, the research team found that cheating individuals do, indeed, make sexual noises less frequently. Most cheating between the female and subordinate happened while the leader male was at a safe distance away. This combination of behavior suggests that the couple tactically alter their behavior to avoid detection.

Further, the study showed that cheating has consequences: the leader male violently chased the follower male and female apart. This aggression was not direct competition, as the leader male seldom mated with the female after the attack. The researchers, therefore, surmise that gelada leader males punish cheaters whenever they can.

"By studying the existence of both deception and punishment in the wild, especially among primates, it allows scientists to better understand cooperative and competitive strategies," said Thore Bergman, U-M assistant professor in the departments of Psychology and Ecology and Evolutionary Biology.

The study's other authors were Noah Snyder-Mackler of the University of Pennsylvania and Eila Roberts and Jacinta Beehner of U-M.

To view the slideshow of Gelda Monkeys, please visit the U-M News Service Website.