Thomson's Gazelle

The Thomson's gazelle is a small antelope of the East African plains. Its name honors Joseph Thomson, a Scottish explorer who crossed Maasai country in 1883 and 1884, the same journey that gave his name to Thomson's Falls in Kenya, and the species was scientifically described in 1884. Safari guides almost always call it the "tommy", and in Swahili it is swala tomi. For many years it was placed in the genus Gazella, which is why older books write Gazella thomsoni, but today it belongs to Eudorcas, a small group of gazelles that includes the red fronted gazelle of West Africa. The Mongalla gazelle of South Sudan was once counted as part of the same species and is now usually treated as a separate one, so the true Thomson's gazelle lives only in Kenya and Tanzania. On safari it is often confused with the larger Grant's gazelle, whose white rump patch climbs above the tail, while the tommy has a bolder flank stripe and a black tail that flicks almost all the time.

Thomson's gazelles live only in Kenya and Tanzania. Their home is open grassland with short grass, where they can see a predator coming from far away, and they avoid tall grass, thick bush and forest. The heart of their range is the Serengeti and Maasai Mara ecosystem, which holds the largest population, followed by the Ngorongoro Crater and the plains around it. In Kenya they are also found in Amboseli, Nairobi National Park, Laikipia and other grasslands of the Rift Valley. Tommies do not travel as far as wildebeest, but they do move with the seasons. When the rains arrive, they gather on the short grass plains of the southern Serengeti, where fresh grass grows quickly after each storm. As the dry season sets in, they spread out to places with taller grass and woodland edges, often trailing behind wildebeest and zebras, which trample and crop the tall grass and leave the short, green shoots that tommies like best.

The Thomson's gazelle is one of the smaller gazelles of Africa. Adults stand about 55 to 82 cm at the shoulder, and the head and body measure roughly 80 to 120 cm, with a short tail of 15 to 27 cm. Males weigh about 20 to 35 kg, while females are lighter, at around 15 to 25 kg. Its coat is a warm sandy brown on the back, with a white belly and a wide black band along each flank separating the two colors. The face has a reddish stripe down the middle, white rings around the eyes and a dark line from each eye toward the nose. Males have long, strongly ridged horns that grow 25 to 43 cm long. They rise almost straight up and curve slightly back, with the tips bending forward, and the ridges help rivals lock horns when they push against each other. Some females have short, thin horns of about 7 to 15 cm, often broken or bent, and many have none at all. Newborn fawns weigh only 2 to 3 kg and have a grayer coat with weaker markings.

Thomson's gazelles are grazers, and grass makes up most of what they eat. They clearly prefer short, young, green grass over tall and dry stems, because the fresh shoots have more protein and are easier to digest. Their small mouths let them pick out the best leaves and shoots one by one. This is why they follow the rains across the plains and often feed where wildebeest and zebras have just been, since those larger animals cut and trample the tall grass and expose the short growth underneath. When the dry season arrives and green grass becomes hard to find, tommies eat more leaves from low shrubs, herbs, clover and seeds, which keeps them going until the next rains. They drink when water is nearby, but they also get a lot of moisture from fresh plants and morning dew. Because they need the same short grass as wildebeest and cattle, competition for food is one of the reasons their numbers have dropped.

Thomson's gazelles are active during the day and live in loose herds of a handful to several hundred animals, which change all the time as animals join and leave. Females and their young form one type of group, young males without territories form bachelor herds, and adult males try to hold a territory of their own. A territorial male marks his patch with piles of dung and with a dark, sticky secretion from glands in front of his eyes, which he rubs on grass stems. He chases away rivals and tries to keep passing females inside his ground, and fights are usually short pushing matches with locked horns. Females can give birth twice a year, which is unusual for antelopes, after a pregnancy of about five and a half months. The single fawn spends its first two to three weeks hidden in the grass while the mother grazes nearby and returns to nurse it. Mothers will chase off jackals, but about half of all fawns die in their first year, most of them taken by predators.

Stotting, also called pronking, is one of the most famous behaviors of the Thomson's gazelle. When a predator shows up, the gazelle bounces high into the air with all four legs held stiff and straight, landing and taking off again as if it were on springs, often with its white rump flashing. At first sight it looks like a waste of energy at the worst possible moment. The best known explanation comes from a study by Clare FitzGibbon and John Fanshawe, published in 1988 and based on African wild dogs hunting tommies. The dogs were more likely to chase gazelles that stotted less, and gazelles that stotted at higher rates were more likely to get away. Stotting is an honest signal: a gazelle that can afford to bounce like that is telling the hunter it is healthy and fast. Tommies stot far more in front of hunters that run their prey down, like wild dogs, than in front of stalkers such as cheetahs. It may also warn the herd, and young gazelles sometimes stot in play.

Thomson's gazelles are among the fastest animals on the African plains. Most researchers put their top speed at around 80 km/h. Just as important is how they run: when a predator gets close, they zigzag and make sudden sharp turns, and they can keep a fast pace for longer than a cheetah. The cheetah is faster over a short sprint, but it tires after a few hundred meters, so if it does not catch a tommy quickly, the chase is usually over. Even so, Thomson's gazelles are the cheetah's main prey on the Serengeti plains, where studies have found that they make up well over half of what cheetahs eat. Lions and leopards hunt them by ambush, spotted hyenas and African wild dogs run them down in open country, and black backed jackals and servals are a serious danger to fawns hidden in the grass. With so many hunters around, tommies depend on sharp eyes and ears, open ground and the safety of the herd, where many animals watch for danger at once.

Thomson's gazelles are still common, but there are far fewer than there used to be. Published counts vary a lot depending on the year and the method used, and totals that include young animals are higher than counts of adults alone. Most tommies live in the Serengeti and Maasai Mara ecosystem, which holds by far the largest population, and the rest are spread across other grasslands of Kenya and northern Tanzania. Between the 1970s and the early 2000s, numbers fell by as much as 70 percent in some key areas, including the Serengeti, the Maasai Mara and the Ngorongoro Crater. The good news is that the decline has slowed: one review found a drop of only about 8 percent between 2002 and 2014. Part of the fall in the Serengeti is linked to the recovery of wildebeest, which grew into huge herds and now compete with gazelles for the same short grass. Outside parks, the loss of open plains to farms, fences and settlements, along with more livestock, has had a bigger effect.

The IUCN lists the Thomson's gazelle as Least Concern, an improvement on the Near Threatened status it held in 2008, because it is still common and large populations live inside protected areas. That does not mean all is well, though. The species lives in only two countries, so what happens in the Serengeti and Maasai Mara decides much of its future. The biggest threat is the spread of farmland, fences and settlements into the open plains, which shrinks the space the gazelles need to move with the rains. Growing herds of cattle, sheep and goats compete with them for short grass, especially on the edges of parks. Poaching for meat takes a toll outside reserves, and more frequent droughts can reduce the fresh grass they depend on. Inside parks such as the Serengeti, Ngorongoro, the Maasai Mara and Amboseli, numbers are much more stable, which shows how important these protected areas are, and keeping wildlife corridors open between them is key in the long term.