When an impala stops at a watering hole for a quick drink, it’s already on the lookout for predators. So when it hears the sound of a snarling lion, the reddish-brown African antelope will often bolt for safety. But an hour later, if it hears a cheetah instead, the herbivore will often just keep drinking water, unperturbed by the potential predator nearby.
Many South African ungulates, or hooved animals, have different fear-induced responses depending on what predator is nearby. That’s the key finding of a recent study in Behavioral Ecology in which researchers report that ungulates run away from lions most often, followed by African wild dogs, and then cheetahs. According to Liana Zanette, a coauthor of the study and wildlife ecologist at Canada’s Western University, this “hierarchy of fear” is important because fear affects every aspect of a prey animal’s behavior, and can have rippling effects throughout the ecosystem.
“You can see that if they have different fear responses, that affects their foraging behavior,” Zanette says. She adds that, “this can then affect the population numbers of the prey and have effects on their food further down the food chain.”
To test ungulates’ fear responses to different predators, the scientists first collected sound recordings of lions, cheetahs and African wild dogs, as well as bird calls to use as a non-scary control treatment. They used short-range sounds like snarls and growls rather than roars so they could simulate a predator being close by. Then they played these scary sounds to wild animals using a speaker connected to a camera trap. When the camera detected an animal moving nearby, it started recording a video and then triggered the speaker to play a predator sound.
The researchers set up the camera and speaker systems at 14 different sites and left them running day and night for several weeks in July 2017. To maximize their chances of finding wildlife, they installed the contraptions near watering holes. As Zanette explains, “The chances of catching anything on camera on a wildlife trail is pretty small, but almost everything needs to drink at some point or other.”
The scientists had three hypotheses about how the ungulates would respond to each predator. First, they could treat all predators as equally scary, in which case, they would run away just as often from all three of them. Second, they could show most fear of the predators that kill their species most often. And third, the prey might be most afraid of the predators with the highest likelihood of killing them if they do decide to attack, which would be the lions.
After collecting hundreds of videos of startled animals during their experiment, the researchers went through them and found the results aligned with the third hypothesis. “It was really a hierarchy of fear because animals feared lions most. So the king of beasts is definitely the king of beasts,” says Zanette, “and that was followed by wild dogs, that was followed by cheetahs, and then that was followed by a control to which they hardly ever ran.”
The animals caught most often on camera were impala, which clearly showed this hierarchy of fear. But Zanette explains that lions don’t actually go after impala very often. Instead, impala are more likely to be attacked and eaten by a wild dog or cheetah.“Even though impala don't end up in the belly of the lion, impala are still most afraid of lions, we think because the probability of being killed should a lion decide to catch you and eat you is really high,” Zanette says.
Warthogs provided an exception to this hierarchy of fear as they were equally likely to run away from all predators, a surprise finding as warthogs have a good chance of defending themselves against smaller predators like wild dogs and cheetahs.
The researchers note that the warthogs’ behavior illustrates the importance of directly testing each prey species’ fear behavior rather than assuming how they will react to predators.
Elizabeth le Roux, a large mammal ecologist at Aarhus University who was not involved in this study, says predators with different hunting behaviors might also trigger different types of responses in their prey. The best way to avoid getting eaten by a lion may be to run away, she says, whereas with a pack-hunting predator like an African wild dog, stopping and scanning the environment may be the smarter move.
Additionally, Kaitlyn Gaynor, a wildlife ecologist at the University of British Columbia who was not involved in this study, says that understanding this complexity in prey animals’ behavior may be critical for their conservation. She explains that humans have altered many ecosystems by removing some predators, adding others, and acting as novel predators ourselves. Those predators can have major impacts on the landscape. For example, a 2014 study showed that predation risk from leopards and wild dogs shapes impalas’ habitat preferences, which in turn alters the distribution of tree species in a Kenyan savanna.
So the loss or reintroduction of a predator can trigger cascading effects throughout the ecosystem, and Zanette says that killing alone doesn’t explain all those effects.“The reason predators can have such astronomical effects on prey populations and ecosystems is because they not only kill stuff, but they scare them as well,” says Zanette, “and that scaring has huge effects that we haven't really appreciated.”