A young cheetah learns to hunt. (Photo: Volt Collection/shutterstock.com)
For many years, humans thought simplistically about how animals escape their predators. Because, if another animal wants to eat you, the best course of action seemed simple enough.
You RUN. As fast as you can. In the opposite direction.
But that’s not actually what happens. Most prey, when they detect a predator, do not try to escape immediately. “This is a decision, like many other decisions,” says Dan Blumstein, a UCLA professor of ecology and evolutionary biology, and co-editor of the new book Escaping From Predators.
Say, for instance, that you’re a low-status bird. No one likes you, and you’re not always going to have great access to food. But, at this moment, you’re feasting on a rich source of nutrition. A predator approaches. Maybe the other birds take off. But not you. “Lower status birds will stick around longer because that’s their chance to eat,” says Blumstein.
It’s only in the past decade or so, though, that biologists have really begun to understand the factors that contribute to animals’ escape plans. When Ronald Ydenberg and Lawrence Dill published the first paper on the economics of flight in 1986, “few thought of fleeing as a ‘decision’ or saw that there were costs, most notably lost opportunity,” they write in the foreword to the new book. And even after that first paper, few scientists looked closely at how animal fled their would-be killers.
But in the past ten years, the study of escape behavior has been a booming field of research, and scientists are beginning to better understand how animals decide to run, walk, jump, dive, sprint or saunter for their lives.
“This is one of those hidden success stories of taking a very economic approach,” says Blumstein. It’s clear now that the decision to flee can be influenced by rank, hunger level, sexual drive, season, location, or prior experience with predators. And they can begin before animals are even born.
Karen Warkentin first started studying the eggs of red-eyed tree frogs back in the early 1990s, when she was just beginning her doctoral work. She was in Costa Rica and looking for, in her words, “a frog that did something cool that Canadian frogs don’t do.” (She grew up in Canada.)
At the particular pond where she was working, red-eyed tree frogs were abundant, but so were snakes. About the half the eggs that the frogs laid—little translucent globes with wide-eyed tadpoles peering out—were being eaten by snakes. Given how many were dying, it seemed to Warkentin like these soon-to-hatch tadpoles should have a strategy for running away, once they were under attack.
To test her hypothesis, she gathered up eggs, put them in a cage with a snake and let the snake into the eggs’ side of the cage. When the snake attacked the egg clutch, the uneaten tadpoles started escaping—instead of waiting to spontaneously hatch, they sensed the danger and—plop—fled from their eggs and into the water below.
This isn’t just a random reaction. In the years that followed, Warkentin, now an associate professor at Boston University, and her collaborators were able to show that escape hatching is a specific response to being attacked. Sometimes, the eggs wouldn’t have hatched for days otherwise.
There are trade-offs to escape hatching, though. There may be a risk, for instance, of trying to hatch too early and failing, leaving the poor tadpole in danger, now in a damaged egg. It’s like being “trapped in a deflated water balloon,” says Warkentin. “You better be really sure you’re about to die if you’re going to risk that.”
Of course, once the tadpoles escape the snake, they’re not home free. Waiting for them in the water are aquatic predators, happy to suck up vulnerable tadpoles.
Animals face the possibility of predation throughout their life, but they do have one advantage. “The prey’s life is at stake. The predator misses a meal,” Blumstein says. This is called the life/dinner principle, and the stakes are much higher for the animals on the “life” side of that equation.
Even once an animal decides to flee, though, it won’t necessarily devote all its energy to the escape. It might be running for its life, but that doesn’t mean sprinting as fast as possible to the nearest refuge.
“The escape response was thought of as being an all or none response, either on or off,” says Paolo Domenici, a researcher at the Italian National Research Council. “But there’s more and more evidence that it’s not really so black and white. If there is a response, there can be different levels. It can be a half hearted response or a full response.”
If your predator is far enough away, for instance, you can run or swim a little slower—save energy and still get away from the danger. If the refuge is near by, you can walk towards it. Or maybe you’re just tuckered out from some other activity, and you can’t muster the will to speed away as fast as might be wise.
When animals resolve to escape, they don’t always turn away from their predators, either. “You’d think it’d all be away,” says Domenici. “But that’s not the case. Not all animals will. The proportion varies from 50 to 90 percent.”
Scientists aren’t sure why, exactly, but it may have something to do with the element of surprise. Predictability is dangerous in its own way. One snake will signal an attack from one direction, and its prey, a fish, will turn away from that wave of pressure. But the sneaky snake has actually positioned its head behind the fish—in the direction its prey will most likely escape.
“The fish, by escaping from the pressure wave, will go straight into the predator’s mouth,” Domenici says.
Some animals do always run away: schools of fish, in particular, tend to all turn one way—away from the danger—probably in order to stay together. Not all animals flee in a straight line from their predator, either. If the predator is faster than the prey, there’s some advantage to running at an angle from the predator, as sharp as 90 degrees. A cockroach depends on the element of surprise: it might turn 90, 120, 150, or 180 degrees away from its potential predator.
And some animals flee at an angle where they can keep an eye on their pursuer: “If I’m chasing you and you go straight away from me, you have a hard time checking out what I’m doing,” says Domenici. “You can’t turn your head while you’re running, but if you run 120 degrees away from me—at a bit of an angle—with the corner of your eye, you can be checking out what i’m doing, if I’m throwing rocks or something.”
Biologists are still studying the mechanism and genetics of this. Different populations of the same species, for instance, have different tendencies to escape. “You can walk right up to an urban squirrel,” Blumstein points out. Scientists are still trying to understand if different populations have actually evolved to be less flight-prone, or if they’re simply sorting themselves out based on their preferences. Individual animals, too, can be more skittish.
Blumstein runs a long-term study of marmots, and his wife, a collaborator, once noted that across 10 different experiments, one animal ran away immediately from any stimulus. “We couldn’t include it in anything,” he says. “That was a nervous nelly.”
Once the decision is made to flee, though, there’s a whole world of ways to actually run away. In Escaping from Predators, the author of one chapter note that mammals “show tremendous variation in how they flee from predators: rapid running, jumping, dropping from trees to the ground, fleeing into a burrow or other covers, climbing trees moving into water, and even flying away. In the Reptiles chapter, we learn that lizards often prefer refuges, “most commonly trees, logs, crevices in rocks, and animal burroww.”
To get there, they might jump, glide, swim or dive. When they run up trees, they often choose “the side of the tree opposite that of the approaching predator, where their movements are invisible.” In an earlier book on antipredator defenses, the scientist Tim Caro describes animals “zigzagging, looping, wild bouncing, and sudden twisting”—everything “from ponderous moment to extremely fast dashes.”
Every animal has its own particular strategy. Blumstein has also studied hermit crabs, and, he says,” I feel a little sorry for the work I’ve done with these guys.” In the wild, he says, they’ll slowly make their way away from the water, moving slowly up steep mountain slopes in the Virgin Islands. “They’ll be pretty far away from the water,” says Blumstein. “But as soon as they detect you, they pull their legs in, and they start rolling down,” losing all the ground they’ve covered in an instant.
On the upside, at least they have a chance of getting away.