Pick from the Past
Why Do Tommies Stott?
THOMSONS GAZELLES, or Tommies as they are known colloquially in East Africa, are a familiar sight on the African plains. These small gazelles are largely taken for granted by visitors in search of bigger animals, such as elephants and rhinos, and of photogenic predators, such as cheetahs and wild dogs. Yet Tommies support many of the populations of East Africas notorious carnivores. Their behavior in the face of these predators has caused a great deal of discussion by big game hunters, naturalists, and scientists because Tommies have an impressive repertoire of antipredator behavior. The most notable of these is stotting, a conspicuous, stiff-legged, bounding gait that propels the gazelle almost straight up as much as two feet off the ground. This and similar acts also performed by other animals, such as springboks, wildebeests, and hartebeests, puzzle observers. One would presume that stotting would slow the Tommies down as they run away from would-be predators. Surely, therefore, to do so in the face of danger would be foolhardy. So mysterious is the function of this behavior that numerous theories have been put forward to explain it.
Beginning in 1980, I spent three and a half years in the Serengeti National Park in Tanzania studying cheetahs as part of a long-term project on the behavioral ecology of this species. I also kept records of Tommy behavior in the face of predators in an attempt to decipher the circumstances and consequences of stotting and thereby discover the reasons why Thomsons gazelles stott.
Slender and graceful, with dark brown backs, white bellies, and a black flank stripe, Tommies are representative of the genus Gazella. They are typical of relatively dry savanna habitats and abound on the East African plains. Males weigh about fifty-five pounds and have ringed, S-shaped horns more than a foot long. Females are smaller, under forty-five pounds, and most have shorter, straight, ear-length horns. They can give birth twice a year to a single dark brown infant weighing about four and a half pounds.
Serengeti gazelles are migratory. Almost a third of a million trek annually from the shortgrass plains in the southeast, where they spend the wet season between November and May, to the long grass plains and woodland areas of the northwest during the June to October dry season. Cheetahs and wild dogs follow this migration. I, too, followed in a Land-Rover, tracking individually known cheetahs and noting predator-prey interactions.
Gazelles have many ways of reacting to predators. They continually interrupt their feeding to look up and monitor the surrounding vegetation. If they spot a predator, especially their prime attacker, the cheetah, they turn toward it with neck erect and back stiff and stare at it continuously for as long as it remains in view. They may stamp a front foot or emit one or more loud snorts, which are probably alarm signals. If the predator is approaching slowly, the Tommy will often run for thirty to sixty feet, stotting a variable number of times. This initial flight usually describes a J-shaped path, ending with the gazelle turning to face the predator again. But a gazelle rarely stotts during its getaway if a cheetah is making a fast approach when first seen. Instead, the gazelle takes off at a full gallop.
Several East African naturalists had suggested that stotting imposes costs of time, energy, or survivorship on gazelles. I wanted to check on these hypotheses. Because I was unable to measure the energy cost in the field, I looked only at time and survivorship. Contrary to previous fieldworkers reports, I could find no time cost to stotting in situations in which it normally occurred. By calculating the gazelles rate of flight away from the cheetah and away from my Land-Rover, I found that during a slow flight gazelles did not cover fewer feet forward per second the more stotts they performed. This may be because stotting involves a slight forward motion. In addition, Tommies stott in the middle or at the end of a flight whether the flight is for as little as 30 feet or as much as 250 feet. Thus stotting did not seem to delay escape from potentially dangerous situations; either it does not slow down running or is a slow form of travel used when the gazelle is not running flat-out. But I found that stotting did slow down the Tommies flight when they were running at top speed.
As far as survivorship goes, I did not find that cheetahs were more successful in catching the gazelles that stotted the most. For one thing, gazelles never stotted until they were at a safe distance from a cheetah. Cheetahs hunt under concealment, slowly stalking their unwary prey and freezing each time the quarry looks up, in natures version of a childrens game. When less than about ninety feet from their intended victim, cheetahs make an open rush toward it that usually results in capture. If the cheetahs reveal themselves at greater distances, the hunt usually fails. On the average, the gazelles first stott occurred when a cheetah was as far as 200 feet away, a distance at which the cheetah will almost certainly be unsuccessful. So stotting appeared to inflict no survivorship cost.
I turned my attention next to the benefits that Tommies might signal to other gazelles, usually kin; and as a means of surveying the environment but with no signaling function at all. One of the several signaling hypotheses is that since stotting occurs at safe distances from cheetahs, it informs the predator that it has been seen. The detection hypothesis, as this is called, is predicated on the cheetahs reliance on a concealed approach and predicts that they will abandon hunts once they know they have been seen. This is exactly what I found. For example, a cheetah might approach a single male Tommy in the open, stalking the gazelle every time it lowered its head to feed. If it spotted the cheetah, the gazelle often ran about fifty to sixty feet, stotting a few times. The cheetah would then lie down or continue to walk openly, rather than stealthily, toward the prey. That meant the cheetah had lost the advantage of surprise and the hunt had ended. And when only one gazelle in a group stotted and several stared, the cheetah also seemed to get the message that it had been seen by the whole group. Other reasons also influencing whether a cheetah will give up the hunt depend on the predators level of hunger, whether it has cubs of its own to feed, and the possibility of making a capture somewhere else.
Another version of the signaling hypothesis is that gazelles stott to inform the predator that they are fit enough to outrun it in a long chase. In this case, many gazelles would be expected to stott simultaneously, each trying to outdo the others in displaying its good condition. Since only a single gazelle in a group usually stotts in response to a cheetah, this hypothesis is unlikely to apply here.
The Tommies response to another predator, the wild dog, however, differs from that to the cheetah. Unlike cheetahs, wild dogs hunt their prey without concealment, counting on outrunning the prey in a long chase. In this instance, many gazelles will stott at the same time, and as Clare Fitzgibbon and John Fanshawe, scientists at Cambridge University in England, have found, stotting in response to wild dogs does signal the gazelles ability to outrun the predator in a long and demanding race.
All gazelles are so-called hider species. Mothers leave their newborns hidden in the grass for the first two weeks of life, returning periodically to nurse and lick them. These neonates also stott and some people have suggested that their stotting is meant to signal to their mothers that they are moving from one location to another.
Another suggestion is that they require their mothers aid to fend off a predator. But young Tommies do not always recognize cheetahs as predators. I have seen neonates attempt to play with young cheetahs or even try to suckle from them. I also found that concealed neonates that I disturbed with my Land-Rover stotted much more often than unconcealed neonates that could be seen by their mothers. I also noted that the farther concealed youngsters were from their mothers, the more they stotted. Both findings give some support to the idea that neonate stotting provides the mothers with information of their offsprings whereabouts.
I found that mothers themselves stott at a high rate in front of predators when their neonates are in danger. Positioning themselves between their fleeing neonates and the pursuing cheetahs, the mothers zigzag and stott during the ensuing chase. On one occasion, when a Thomsons gazelle with a fawn saw an adolescent cheetah running at them from about 300 feet away, she jumped very high straight up, almost three times as high as a normal stott in response to a cheetah, then stotted eight times and stopped. Again, she jumped high, stotted, and then ran zigzag between the cheetah and her offspring But after a chase of only some 130 feet, the young gazelle was caught.
Separating instances in which a young gazelle was caught from those in which it escaped, I found that the stotting rates of mothers were significantly higher when the neonates escaped. That convinced me that the mothers somehow affected the outcome of the hunt by their stotting. Hunts were unsuccessful in most cases not because mothers incited the cheetah to chase them instead of their offspring but because the fawn outran the cheetah or suddenly disappeared from view. A mothers stotting seemed to distract the predators attention just long enough to enable the neonate to hide in thick vegetation during the chase, making it almost impossible for the cheetah to locate it.
Some fieldworkers believe that stotting involves no signaling at all. They hypothesize that the practice permits gazelles to get a better view of their surroundings or that it is a form of play. But gazelles dont stott more frequently in high vegetation that may block their view nor do they play in dangerous situations. Even the idea that gazelles stott to warn other gazelles in a group or to warn close kin, such as neonates, of danger from a predator does not hold much water. Since the only permanent gazelle grouping is a mother and her infant, and they may stay together for several months, one might expect female gazelles to stott more often than males, which are unlikely to be nearby their relatives. Yet adult females did not stott more often than adult males, nor did their stotting increase when they were in larger groups that might include relatives.
Other types of antipredator behavior thought to function as alarm signals-foot stamping, snorting, and perhaps even flank flashing, in which the gazelles black lateral stripe is rapidly twitched before the animal runs offoften occur, but rarely in conjunction with stotting. This implies that stotting is not really part of the alarm repertoire. These findings indicate that stotting probably does not serve to warn others of danger, as was once supposed.
I conclude, therefore, that stotting may serve several different functions during a gazelles lifetime, depending on the individuals age, what predator it is facing, and whether it has a baby.
Up to now scientists and the public alike have concentrated on what predation means for the predators, but we should consider the effects of predation on the behavior of the hunted as well.
Tim Caro is a professor in the Center for Population Biology; and the Wildlife, Fish, and Conservation Biology departments at the UC Davis.
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