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Natural History, June, 2004 by Stephan Reebs

In the forests of West Africa, Diana monkeys make one kind of bark when they spot a crowned eagle, another when they see a leopard. Both animals prey on Dianas, and so the distinctive warnings provide crucial information to troopmates. Now Hugo J. Rainey and two other biologists at the University of Saint Andrews in Scotland show the Dianas' code can also be deciphered by eavesdropping birds.

Hornbills are large, canopy-dwelling birds that fear eagles but don't mind leopards--after all, the birds fly and the leopards don't. When the biologists played back the Dianas' two different alarm calls for hornbills to hear, the birds reacted only to "EAGLE NEARBY!"

It was already known that various species of monkeys can distinguish between different alarm calls of birds, but this study is the first to show that the tables can be turned. It is still unclear, though, how any of the eavesdroppers learns another species' vocabulary. ("Hornbills can distinguish between primate alarm calls," Proceedings of the Royal Society of London B 271:755-59, April 7, 2004)