While perhaps not as widely distributed as Grant's, Thomson's are still the most common gazelles in East Africa. Though their numbers have diminished in some areas, in others they have persisted on ranches and farmlands long after other species have disappeared.

Physical Characteristics
The graceful "tommie" is noticeably smaller than the Grant's gazelle, which it resembles in shape and color. It is also distinguished from a Grant's by the dark side stripe that runs from the shoulder to the flank and the white patch on the rump. The tommy is a dark fawn or cinnamon color on the topside and white on the underside. The black tail seems to be constantly in motion.

The males are larger than the females and have strongly ridged, almost parallel horns that curve backwards, with the tips curving forward. Female tommies have short, smooth, pencil-slim horns, or none at all. The face is accented by a black stripe running down from the eye, a dark marking on the nose and a light patch on the forehead.

Habitat
Although more reliant on water than Grant's gazelle, the tommy has adapted to the open plains and grasslands of southern Kenya and northern Tanzania.

Behavior
During migration, as tommies spread out over the plains in the wake of zebra and wildebeest herds, the strongest males set up territories. They use an exaggerated display posture when urinating or defecating on dung piles. To mark the boundaries the males deposit a small amount of secretion from their scent glands (located beneath the eyes) onto a blade of grass, leaving these markers daily about every 20 feet. As the herd migrates, new territories are established.

The females along with their immature offspring form groups of five to 50 that wander through male territories. The groups change members and numbers from hour to hour, so no obvious patterns of hierarchy or leadership emerge. Nonterritorial males gather together in small groups along the outskirts of the larger herd, generally avoiding other male territories unless one of the group attempts to take one over.

In the early morning and again in the evening, the herd, which may have spread out during the day, comes together. This is playtime for the younger gazelles, when they engage in stotting and pronking (bouncing along on stiff legs) and sprint around the perimeter of the herd.

The relatively silent tommies rely on visual awareness of one another to stay in contact. Their distinctive coloring may help-they can contract the skin so the black side stripe becomes more obvious. They also stamp their front feet to signal when they are disturbed.

Diet
Tommies congregate not only with Grant's gazelle but with larger ungulates such as wildebeest and zebra-and even cattle, which trample and graze on tall grass, making it easier for the tommy to feed on short grass. Although grasses make up about 90 percent of the tommy's diet in dry season, it also eats seeds and browses on shrubs. When the tiny new green shoots of grass begin to grow in areas that have been burned, tommies often gather in large numbers to feed.

Caring for the Young
Tommies breed twice a year. Although births occur throughout the year, they peak right after rainy seasons. After giving birth the mother hides the newborn in the grass, returning several times a day to nurse it. With their tawny coloring and ability to remain motionless for long periods, the young are surprisingly invisible when hidden in open country.

Nevertheless, predation on the young is heavy, and many predators feed on nothing else during the calving peaks.

Predators
Cheetahs, lions, leopards, hunting dogs and hyenas prey on young and adults alike, with adult tommy males three times more susceptible than females. The young are also taken by serval cats, jackals, baboons, eagles and pythons.

Did you know?

• The Thomson gazelle is exceptionally alert to sounds and movements, and its fine senses of hearing, sight and smell balance its vulnerability on the open plains.
• Males vigorously defend their territories. If challenged, the defending male and his rival clash horns, with the winner claiming the territory.