When danger approaches, many creatures seem to follow the ancient proverb that “the enemy of my enemy is my friend.” Researchers have recently been finding subtle ways that animals communicate with other species in this kind of cooperative defense pact.
For example, a recent study in Nature Ecology & Evolution documented more than 20 bird species on four continents that emit virtually identical “whining” calls when they spot brood parasites such as cuckoos. That call is essentially “the word for ‘cuckoo,’” says study co-lead author James Kennerley, an ornithologist at Cornell University. “And it’s recruiting individuals to come together [against] this common enemy.”
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Brood parasites lay eggs in other birds’ nests, manipulating the host parents into raising their chicks for them. At a field site in Australia, Kennerley has witnessed individuals from a dozen or more species mob a cuckoo in response to a chorus of whining calls. These mobs can be so ferocious that Kennerley and his colleagues need to cage the taxidermy cuckoo used in their experiments to protect it. Otherwise the attacking birds would have “just completely shredded it to pieces,” Kennerley says.
Many birds also share a common vocabulary for predators. Research by wildlife ecologist Erick Greene, an emeritus professor 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 raptors to hear well, so the predators remain oblivious as info about their arrival shoots through the forest. If the raptor perches, songbirds switch to “mobbing” calls, a distinct vocalization that, as Greene puts it, “draws in the troops [to] drive that raptor out of Dodge.”
Monkeys, lemurs and chipmunks also recognize other species’ alarm calls. And in coral reefs, unrelated fish seem to swap visual and chemical cues as protection against dangers such as hungry barracudas. But cooperative defense is not the only reason for cross-species communication. Among other things, it may help birds migrate and enhance food intake among mixed-species monkey troops and dolphin pods. A recent study in the Proceedings of the National Academy of Sciences USA found that when seabirds with good vision, such as black-browed albatrosses, forage with seabirds with strong senses of smell, such as white-chinned petrels, they both have far greater success at catching krill. Unlike with the seet and whining calls, however, it’s unclear whether they’re deliberately signaling to one another or “just randomly following other birds,” says study lead author Jesse Granger, an organismal biophysicist at Duke University.
But clearly, “very complex multispecies communication networks are pervasive,” Greene says. “It really behooves [animals] to pay attention to one another,” he adds. “It can save their lives.”