One of the first rules of the animal kingdom is simple: don’t get eaten. Unfortunately, for most animals, this is easier said than done. Save for a few lucky apex predators, almost every animal has to worry about predators. In order to counter this, evolution has given animals various anti-predation behaviors. These behaviors differ from species to species. Some animals use camouflage to avoid detection, while others will face predators head on in aggressive brawls. For animals that live in groups, the group’s survival is often as important as individual survival. Because of this, some animals, such as give certain calls to “sound the alarm.” “Alarm calls” have been studied extensively in various old world monkeys. An interesting thing about monkeys is that they are able to produce different alarm calls to differentiate between threats, such as predatory eagles or leopards.
Black-casqued hornbills are strange looking birds with double stacked beaks that have a habit of hanging out near Diana and Campbell’s monkeys. Importantly, they also share a predator: the crowned eagle. New research indicates that these hornbills are also able to understand the different monkey alarm calls; when they hear the monkeys yell about a crowned eagle, they respond in kind.
Researchers Hugo J. Rainey, Klaus Zuberbühler, and Peter J.B. Slater went to Côte d’Ivoirei to look into this interesting phenomenon. In a series of “playback experiments”, the three researchers located wild hornbill populations that tended to hang out near primate groups. They exposed each population of hornbills to one of six recorded sounds: 1) crowned eagle screeches, 2) leopard growls, 3) Diana monkey eagle alarm calls, 4) Diana monkey leopard alarm calls, 5) Campbell’s monkey eagle alarm calls, and 6) Campbell’s monkey leopard alarm calls. They then measured if the hornbills’ own calling behavior increased or decreased after exposure. They hypothesized two things: first, because hornbills face predation pressure from eagles but not leopards, calling behavior would increase in response to the eagle screeches but not the leopard growls; second, in a similar vein, the hornbills’ calling behavior would increase more in response to the monkey alarm calls associated with eagles than those associated with leopards.
After 592 trials, they found that their hypotheses were, in fact, correct. This table, taken from the original lab report, clearly displays that when it came to eagles, calling behavior tended to increase, whereas calling behavior would decrease when hornbills heard either leopard growls or leopard alarm calls.
While it is common for other mammals to pick up on the alarm calls of other mammals, it’s rare to see this interaction between birds and mammals. This research provides valuable insight into inter-species communication.
References:
Rainey, H. J., Zuberbühler, K., & Slater, P. B. (2004). The responses of black-casqued hornbills to predator vocalisations and primate alarm calls. Behaviour, 141(10), 1263-1277. doi:10.1163/1568539042729658
