If you’re an impala or zebra on the South African savannah, there is no shortage of predators to worry about. You’d make a tempting meal for leopards, cheetahs, African wild dogs, and lions, especially if you are less bulky than the animals around you. How do you stay one step ahead of all those hungry mouths?
To avoid being caught by surprise, you'd best keep your ears open. That means listening for any cries of warning from your fellows—and eavesdropping on the other species around you. It turns out that impalas, wildebeests, and zebras can recognize each other's calls. What's more, they don't pay the same amount of attention to each alarm cry, scientists reported February 19 in the journal Animal Behaviour. When the researchers broadcast zebra calls, all three species went on the alert. But when zebras heard impala or wildebeest calls, they were less perturbed. This could be because impalas are smaller and targeted by more predators, making their calls less concerning for the other, larger, herbivores.
“They know what [the calls] mean, that there’s a predator around,” says coauthor Meredith Palmer, an ecologist at the University of Minnesota’s St. Paul campus. “But they also know to temper the degree to which they respond to these calls based on how much of a threat this call is actually signaling to them.”
There are plenty of animals that listen in on other species' chatter, including birds, primates, and rodents. Scrubwrens and fairy wrens flee when they hear each other's alarm calls, while nuthatches eavesdrop on chickadees, and vervet monkeys heed superb starling warnings. Eavesdropping lets an animal stay in the know without much effort. "You're not out there gathering that information firsthand, you're just intercepting it from your environment," Palmer says.
It’s common for wildebeests, zebras, and impalas to group together while grazing, and all three have distinctive alarm calls. Startled impalas let out a series of short barks, while wildebeests make sneeze-like snorts and zebras bray, whinny, and snort. Since they spend so much time together, Palmer suspected these herbivores might recognize their neighbor’s warning noises.
She and her colleague, Abby Gross, also of the University of Minnesota, gathered recordings of each of these cries with a little help from a life-sized photograph of a stalking lion mounted on plywood. They wheeled the ersatz lion through the bush in hopes of surprising zebra, wildebeest, and impala, then filmed their reactions. Later, the researchers played the alarm calls at grazing herds of each animal. They then watched to see how long the animals spent scanning their surroundings for predators, as well as how swiftly they began to sound the alarm or flee.
For every species, zebra calls provoked strong reactions. Generally, though, zebras ignored impala or wildebeest calls or only stayed vigilant for a brief time after hearing them. This is likely because zebras, which weigh in at more than 400 pounds, don't have to worry about as many threats as other animals. Impalas, on the other hand, max out at around 170 pounds. While hefty herbivores like zebras and wildebeests are mostly hunted by lions, impalas are often on the menu for smaller carnivores such as cheetahs as well.
That means an impala distress cry is less likely to signal imminent danger to a zebra. Impalas themselves were alarmed by every animal’s distress call. They also were more likely to flee if they heard zebra or wildebeest calls than those of their own kind. This may be because impalas are skittish and prone to sounding false alarms.
“If you’re an impala and you know that other impala are probably responding to a predator but there’s also a 25 percent chance that they are alarm calling at some waving grass, maybe you would give more weight to an alarm call from something like a zebra which perhaps is a little more discriminatory,” Palmer says.
Strangely, wildebeests fled more quickly after hearing impala calls than wildebeest ones. This might be because it’s actually easier for wildebeest to leave an area and return once they realize there is no threat than trying to focus on eating and looking around for a stalking lion at the same time.
In future, Palmer plans to investigate how zebras, impalas, and wildebeests learn to link other animals’ distress calls with danger.
