In the forests of West Africa, bands of handsome primates called Diana monkeys roam the tree branches. Each group has just one male and several females with their babies. The tradeoff for his apparently cushy living situation is that the male has to chase off predators. His female companions use specific calls to tell him what kinds of threats are nearby. And he responds to whatever they tell him—even if it goes against his own judgment. Diana monkeys (Cercopithecus diana) of both sexes use all-purpose alert calls to tell each other about dangers, explains Claudia Stephan, a biologist at the University of Neuchâtel in Switzerland. They might call out because another group of monkeys is approaching, or a tree is falling. The monkeys also have calls that are specific to certain predators. One call means "Look out, leopard below!" Another means "Heads up, eagle above!" When one of these cries goes up, it's time for action. "Usually the male directly approaches the predator," Stephan says. First he scans above or below, depending on the alarm, to find the threat. Then he charges toward the animal while calling out. Ideally, he can harass or chase the predator away. But it doesn't always work. "This type of anti-predator behavior is extremely dangerous," Stephan and her coauthor Klaus Zuberbühler write in Current Biology. Unsuccessful monkeys sometimes turn up as a heap of remains beneath an eagle's nest. That may be why the males approach predators "only after considerable delays," the authors add. Male Diana monkeys make alarm calls, but they typically don't approach a predator until the females in their group take up the cry. To find out more about who's calling the shots, the researchers tracked down 36 monkey groups in the forests of the Ivory Coast. They played audio recordings of either leopard growls or eagle shrieks, then watched to see what the monkeys did. Even though all the monkeys in a group could hear the predator, females were consistently the first to raise the alarm. Sometimes it took nearly a minute for the male to make his own "Leopard!" or "Eagle!" call. But once the male started calling, female calls tapered off. Had they been waiting for the male to confirm he'd gotten the message? The researchers did a second experiment. This time, after playing the predator recording, they played alarm calls they'd previously recorded from that group of monkeys. They tested the male or females from each group independently, playing recordings from the opposite sex. Sometimes the alarm calls they played matched the predator in the recording; at other times, they didn't match (for example, a male monkey might hear an eagle's shriek followed by females calling "Leopard! Leopard!"). Again, the researchers watched to see what the monkeys did in response. Females always made an alarm call that matched the predator they'd heard. But males matched their calls to the females' alarms—regardless of what animal they'd just heard. If a male heard females calling "Eagle!" he called "Eagle!" too, even when the researchers had played a leopard's growl. "I am positive that the male is capable of correctly identifying a predator," Stephan says. "But, surprisingly, he decides against better knowledge if females call differently." Why go along with an alarm he knows is wrong? Stephan thinks the answer is likely a combination of factors. But the most important one is probably a male's need to impress the females. "Reassuring his position as the group's only reproducing male seems to depend highly on the acceptance by the females," she says. If the male shows that he'll provide whatever defensive services the females ask for—dashing up to the canopy to scare off an eagle, or down to the ground to fight a leopard—then he can stay in the group. As long as he doesn't become a pile of bones under a nest, anyway.

Image: by Hamish Irvine (via Flickr)

Stephan, C., & Zuberbühler, K. (2016). Persistent Females and Compliant Males Coordinate Alarm Calling in Diana Monkeys Current Biology, 26 (21), 2907-2912 DOI: 10.1016/j.cub.2016.08.033