Biology is thin on the ground these past few weeks, so let’s look at a video of antelopes pronking. No, I didn’t say “bonking”! The behavior of this young Dama gazelle (Niger dama, a denizen of the Sahara and surrounding regions), is also called “stotting,” from the Scots word “stots,” apparently meaning to walk jauntily. When you see stotting in the video below, you’ll recognize it instantly if you have any familiarity with animal shows on television. The paper by Tim Caro at the bottom defines stotting as “vertical leaping with all four legs off the ground simultaneously, with the legs being held stiff and straight.” It also gives all the hypothesis for why stotting could have evolved (I list a few below).
Watch this three-week-old gazelle stot—and then rush about in normal running mode—at the Smithsonian (presumably the National Zoo in Washington, D.C. but perhaps at their reserve in Virginia):
Other quadrupeds do this, especially other species of gazelles. But what does it denote? We’re not sure. It seems maladaptive in an evolutionary sense, since it’s often done when fleeing from predators, and it’s not only inefficient, wasting energy when you could be running, but also makes you more visible. Yet, contra Larry Moran (who will probably weigh in here now!), I think the behavior is an adaptation. For one thing, many species show similar behavior under similar circumstances. But how could it benefit the genes that produce the behavior?
The Wikipedia article on stotting, and the two papers below, detail some of the theories and their tests. Here’s a short list:
- It allows an animal to jump out of high grass to look for predators
- The behavior startles the predator, giving the gazelle more time to escape
- It’s an alarm signal (like bird alarm calls), alerting herd members that a predator is nearby. This would probably evolve only if herd members were closely related, so the behavior could evolve via kin selection (assuming it’s individually maldaptive, which isn’t proven).
- It’s simply play behavior. But not only the young do it: adults pronk too when they’re chased by predators..
- It’s a way, in young gazelles, of letting the mother know the baby has been disturbed. This may be one function, but doesn’t explain stotting in adults.
- It confuses the predator. Presumably a herd of gazelle, all pronking, would puzzle a pursuing cheetah or wild dog, making it hard to pick out a given individual to chase. I don’t believe this for a second; predators aren’t that dumb, and in fact a predator would probably either learn to or evolve to concentrate on the stotting individuals because they might be easier to catch. (This “confusion” explanation was once used to explain zebra stripes: it might be hard to single out one zebra in a mass of fleeing stripey equids. But see my earlier post on another explanation for stripes.)
- It’s a way to attract mates, possibly by showing how fit you are. Sage grouse in the western U.S. form “leks” in which males group together and jump up and down for hours (making loud noises at the same time) while the females watch from nearby. Invariably it is the males who jump the longest that are chosen as mates. Females want a fit father for several reasons. This doesn’t wash for gazelles since both sexes do it, and not in a sexual context.
- This is a favored hypothesis: the “honest signal” theory. This posits that the behavior is saying to potential predators, “Don’t bother trying to catch me as I can bounce really high, so imagine how fast I could run if I wanted to!” In other words, the behavior deters the predator from attacking that individual.
- This is the hypothesis I find most credible: stotting warns the predator that it has been seen, thus discouraging it from pursuing the stotting animal. (Predators like to sneak up on a prey, getting as close as possible before they’re detected.) That is, stotting evolved via individual selection. Remember that predators often don’t go after a whole pack of quadrupeds at once, but single out certain individuals—often young or weak ones—to pursue.
Now all or some of these explanations might operate simultaneously, or they might all be wrong. The paper by FitGibbon and Fanshawe supports the last and the “confusion” hypotheses by showing that pursuing wild dogs were less likely to kill a gazelle when pursuing a pack in which more individuals stotted, and also that individuals who stotted for a longer proportion of the time during pursuit were less likely to be killed. But the data are scanty and the results barely significant in a statistical sense. (Of course it’s very hard to do this work in the field!) So, for right now, there’s no highly convincing explanation for stotting.
h/t: Matthew Cobb
Caro, T. M. (1986) The functions of stotting in Thomson’s gazelles: Some tests of the predictions. Animal Behaviour 34:663-684. (free download)
FitzGibbon, C. D., and Fanshawe, J. H., (1988), Stotting in Thomson’s gazelles: an honest signal of condition. Behavioral Ecology and Sociobiology, Volume 23, Number 2 / August, pages 69–74.