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.
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 relativesa 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 experimentas opposed to fleeingbacks 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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