The Comparative Behavior of Grant's and Thomson's Gazelles

Abstract

The comparative behavior of two associated gazelles, Gazella granti and G. thomsonii was observed in Ngorongoro Crater, Tanzania during a 2½-year antelope behavior study. Though superficially alike, the two species actually differ so much in details that morphological characters alone are probably sufficient to prevent interspecific confusion. Ecologically, the gazelles have quite different requirements: thomsonii, a grazer, prefers short-grass steppe and requires water; granti is mostly a browser and water independent. They share the same six predators in Ngorongoro, of which two jackals are considered the most important, purely on the basis of fawn predation. The same elaborate system for concealing newborn fawns is found in both gazelles and mothers sometimes cooperate in pairs to drive away pair-hunting jackals. Adult thomsonii are preferred prey of East African wild dogs, and territorial male gazelles may be most vulnerable to their hunting methods. Behavioral interaction between associated ungulates is discussed and found to extend no further than mutual response to warning signals, probably even in the gazelles. The gazelle warning signals are practically identical, featuring snorting, stamping, twitching of the side stripe, and most important, stiff-legged bounding (stotting). Gazelles and many other gregarious, territorial antelopes have the same basic social system. Differences between the gazelles may be largely based on their habitat preferences; for example, granti males defend much larger territories than do thomsonii males. Grant's nursery herds are correspondingly widely spaced and also smaller, on the average. The two seem to differ most in territorial behavior, including fighting styles. Male thomsonii are more vigorously territorial, and besides scent-marking by urination-defecation like granti, mark extensively by means of a secretion from preorbital glands, which are less developed in granti. Agonistic encounters between granti males are usually settled by means of a neck intimidation display, whereas conflicts between thomsonii males routinely end in fighting. Neck development is very important in the granti fighting style, where males lock horns and push, attempting to twist one another out of position—this is the basis for the intimidation display. In thomsonii natural selection has operated on horn configuration and fighting style to produce a relatively safe type of parry-thrust combat, thus obviating the need for a display substitute. Compared to territorial behavior, epigamic behavior in the gazelles appears remarkably similar. However, subtle-looking differences in emphasis, sequences, and combinations of the basic displays turn out to be possibly as divergent as the epigamic displays that serve to sexually isolate some other genera. But in the absence of any demonstrated tendency for these gazelles to interbreed, morphological differences alone may be considered adequate species-specific isolating mechanisms. In this respect, olfactory differences may well be more important than visual characters: thomsonii have and granti lack strong-smelling inguinal glands.