How Animals Form Unlikely Alliances to Keep Predators Away

When danger approaches, many creatures seem to follow the ancient proverb that “the enemy of my enemy is my friend.” Researchers have recently discovered subtle ways that animals can communicate with other species in this type of cooperative defense pact.

For example, a recent study in Ecology and evolution of nature have documented more than 20 species of birds on four continents that emit virtually identical “whining” calls when they spot brood parasites such as cuckoos. This call is essentially “the word for ‘cuckoo,'” says study co-lead author James Kennerley, an ornithologist at Cornell University. “And it’s about recruiting individuals to come together [against] this common enemy.

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Brood parasites lay eggs in the nests of other birds, manipulating the host parents into raising their chicks for them. At a field site in Australia, Kennerley saw individuals of a dozen or more species swarming a cuckoo in response to a chorus of whining calls. These crowds can be so fierce that Kennerley and his colleagues have to cage the taxidermy cuckoo used in their experiments to protect it. Otherwise, the attacking birds would have “just completely shredded it,” says Kennerley.

Many birds also share a common vocabulary for referring to predators. Research by wildlife ecologist Erick Greene, professor emeritus at the University of Montana, and others shows that various songbirds — and even red squirrels — produce recognizable “seet” calls to warn of a raptor in flight. The calls are too high-pitched for the raptors to hear well, so the predators remain oblivious as news of their arrival spreads through the forest. If the raptor perches, the songbirds utter “mobbing” calls, a distinct vocalization that, as Greene puts it, “attracts the troops.” [to] chase that dodge raptor away.

Monkeys, lemurs and chipmunks also recognize the alarm calls of other species. And on coral reefs, unrelated fish appear to exchange visual and chemical signals to protect themselves against dangers such as hungry barracudas. But cooperative defense is not the only reason for communication between species. Among other things, it could help birds migrate and improve the food consumption of troops of mixed-species monkeys and pods of dolphins. A recent study in the Proceedings of the National Academy of Sciences USA found that when seabirds with good vision, such as black-browed albatross, feed on seabirds with a keen sense of smell, such as white-chinned petrels, they are both much more successful in capturing krill. However, unlike screeching and whining, it’s not clear whether they deliberately signal to each other or whether they “just randomly follow other birds,” says the study’s lead author, Jesse Granger, an organismal biophysicist at Duke University.

But it’s clear that “very complex multi-species communication networks are ubiquitous,” says Greene. “It is really up to [animals] “Look out for each other,” he adds. “It can save their lives.”

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