African Birds Understand Monkey Communication, Study Says

John Pickrell
for National Geographic News
March 18, 2004
Monkeys and birds in West Africa may be inadvertently pitching together to protect themselves. A new study says the hornbill can understand different diana monkey alarm calls and respond accordingly. Also, monkeys have been found to use birdcalls as early-warning systems.

Various yelps, hoots, screams, squawks, and other vocalizations are used by many species of primates and birds to herald the approach of predators to relatives and other members of the same species.

In addition, researchers know that some monkeys, lemurs, and other mammals also eavesdrop on neighboring species. Now a new study reveals that cross-species eavesdropping may not just be for mammals. The brightly decorated yellow-casqued hornbills (Ceratogymna elata) are able to tap into monkeys' early-warning systems, it says.

According to behavioral biologists behind the findings—all from St. Andrews University in Scotland—this is the first study to show that birds are able to make sense of mammalian alarm calls.

"We were surprised to find that hornbills not only distinguish between the threatening noises of leopards and eagles, but also between monkey alarm calls given to these predators," said co-author and monkey-communication expert Klaus Zuberbühler. "If potentially valuable, lifesaving information is available, it makes sense to listen out for it," he said.

Incredible Din

Zuberbühler has been studying the behavior of the endangered diana monkey (Cercopithecus diana) in Côte d'Ivoire's Tai National Park for the past 13 years. Zuberbühler began to notice that when prerecorded eagle and monkey vocalizations were played during tests of primate behavior, nearby hornbills would also start to call.

Hornbill alarm calls can carry for up to 2 kilometers (1.2 miles). A hollow casque-like, or helmet-like, structure on the bird's bill is thought to amplify the sound.

"In the beginning we found the hornbills pretty annoying," Zuberbühler said.

The birds' cacophony often obscured the monkeys' roaring alarm calls, which Zuberbühler was attempting to record. Though loud, the monkeys' roars rarely carry farther than half a kilometer (a third of a mile).

But the biologists soon realized that a link existed between the two species' behaviors. This observation led to work on the hornbills themselves by graduate student Hugo Rainey. The teams findings are reported in the April issue of the science journal Proceedings: Biological Sciences.

Flocks of hornbills are often found feeding alongside troops of diana monkeys, sometimes even in the same trees, and the species are both at risk from attack by crowned eagles (Stephanoaetus coronatus). To study the association in more detail, Rainey spent 18 months recording hornbill behavior in Côte d'Ivoire (Ivory Coast), beginning in 2001.

Rainey found that when he played either prerecorded eagle shrieks or a monkey's eagle alarm calls, hornbills would react defensively, producing their own noisy alarm squawks and approaching the loudspeaker. In contrast, when Rainey played recordings of leopard growls or the subtly different calls diana monkeys produce in response to that predator, the hornbills rarely reacted.

Hornbills are not commonly a leopard prey item. Therefore, the researchers argue that the birds have learned to distinguish between the two types of monkey warning.

Previous work has shown that diana monkeys themselves respond to the alarm calls of chimpanzees, other monkeys, and guinea fowl, Zuberbühler said.

Attracting Attention

Scientists are still not agreed as to the benefits of all these alarm calls. "You might think that acting conspicuously in front of a predator is not the wisest thing to do," Zuberbühler said.

Traditionally alarm calls have been thought of as warnings to genetic relatives—a kind of last-ditch attempt to pass at least some of your family's genes on by increasing the chances of survival of your closest relatives.

However, alarm calls may have evolved for another reason too. Many predators rely on the element of surprise to snatch a young monkey or other tasty morsel. Signaling to an eagle that it has been spotted, and robbing it of the element of surprise, is often enough to send the predator packing, Zuberbühler said.

Rather than warning relatives of impending doom, hornbills might be calling to predators to make it clear that the game is up. The fact that groups of birds were found to approach the loudspeaker during Rainey's experiment—as opposed to fleeing—backs up that idea, Zuberbühler added.

No previous studies have shown that a bird is able to distinguish between the different calls of a single mammal, commented Marc Hauser. Hauser is a director of Harvard University's Mind, Brain, and Behavior Initiative in Cambridge, Massachusetts.

Hauser said the hornbill findings back up his own work that shows that another African jungle bird, the great blue turaco (Corythaeola cristata), can distinguish among the calls of different species, if not among different calls of the same species. The turaco was found to differentiate among the calls of predators, such as chimps and eagles, and those of competitors, such as fellow fruit-eaters like hornbills and monkeys.

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