Thomson's gazelle

Taxonomy and etymology

The scientific name of Thomson's gazelle is Eudorcas thomsonii. It is a member of the genus Eudorcas and is classified under the family Bovidae. Thomson's gazelle was first described by British zoologist Albert Günther in 1884. The relationships between Thomson's gazelle and the congeneric Mongalla gazelle (E. albonotata) remain disputed; while some authors such as Alan W. Gentry of the (Natural History Museum, London) consider the Mongalla gazelle to be a subspecies of Thomson's gazelle, others (such as Colin Groves) consider the Mongalla gazelle to be a full-fledged species. Zoologist Jonathan Kingdon treated Heuglin's gazelle, sometimes considered a species of Eudorcas (E. tilonura) or a subspecies of the red-fronted gazelle (E. r. tilonura), as a subspecies of Thomson's gazelle. Thomson's gazelle is named after the Scottish explorer Joseph Thomson; the first recorded use of the name dates to 1897. Another common name for the gazelle is "tommy".

Antilope, Eudorcas, Gazella and Nanger form a clade within their tribe Antilopini. A 1999 phylogenetic analysis showed that Antilope is the closest sister taxon to Gazella, although the earliest phylogeny, proposed in 1976, placed Antilope as sister to Nanger. In a more recent revision of the phylogeny of Antilopini on the basis of nuclear and mitochondrial data in 2013, Eva Verena Bärmann (of the University of Cambridge) and colleagues constructed a cladogram that clearly depicted the close relationship between Nanger and Eudorcas. Antilope and Gazella were found to have a similar relationship.

Two subspecies are identified:

Description

Thomson's gazelle is a relatively small gazelle; it stands 55–82 centimetres (22–32 in) at the shoulder. Males weigh 20–35 kilograms (44–77 lb), while the slightly lighter females weigh 15–25 kilograms (33–55 lb). The head-and-body length is typically between 80 and 120 centimetres (31 and 47 in). Kingdon noted that physical measurements show a decrease from the northern to the southern parts of the gazelle's range. Facial characteristics of the gazelle include white rings around the eyes, black stripes running from a corner of the eye to the nose, rufous stripes running from the horns to the nose, a dark patch on the nose and a light forehead.

The coat is sandy brown to rufous; a distinctive black band runs across the flanks, from the upper foreleg to just above the upper hindleg. A buff band can be seen just above the black stripe. Short black streaks mark the white rump. The black tail measures 15–27 centimetres (5.9–10.6 in). Males have well-developed preorbital glands near the eyes, which are used for scent-marking territories. Both sexes possess horns, that curve slightly backward with the tips facing forward. The horns, highly ringed, measure 25–43 centimetres (9.8–16.9 in) on males and 7–15 centimetres (2.8–5.9 in) on females. However, females have highly fragile horns; some are even hornless. Grant's gazelle is very similar to Thomson's gazelle, but can be told apart by its larger size and a large white patch on the rump.

The two subspecies differ markedly in their appearance. The eastern Thomson's gazelle is the larger of the two, with fainter facial markings. The Serengeti Thomson's gazelle, on the other hand, has a whiter face with more conspicuous markings. The horns of females are shorter than those of males to a greater degree in the eastern Thomson's gazelle; moreover, the horns are more divergent in the eastern Thomson gazelle.

Ecology

The Thomson's gazelle lives in Africa's savannas and grassland habitats, particularly the Serengeti region of Kenya and Tanzania. It has narrow habitat preferences, preferring short grassland with dry, sturdy foundation. It does, however, migrate into tall grassland and dense woodland. Gazelles are mixed feeders. In the wet seasons, they eat mainly fresh grasses, but during the dry seasons, they eat more browse, particularly foliage from bushes, forbs, and clovers.

Thomson's gazelles are dependent on short grass. Their numbers are highly concentrated at the beginning of the rains since the grass grows quickly. They follow the larger herbivores, such as plains zebras and blue wildebeests as they mow down the tall grasses. Then, the gazelles spread out more. In the wild, Thomson's gazelles can live 10–15 years. Their major predators are Tanzanian cheetahs, which are able to attain higher speeds, but gazelles can outlast them in long chases and are able to make turns more speedily. This small antelope-gazelle can run extremely fast, from 80 km/h (50 mph), to 96 km/h (60 mph) and zigzag, a peculiarity which often saves it from predators. Sometimes they are also chased by leopards, lions and hyenas, but the gazelles are faster and more agile; these predators attack especially the young or infirm individuals. They can also be devoured by crocodiles and pythons, and their fawns are sometimes the prey of eagles, jackals, and baboons. A noticeable behaviour of Thomson's gazelles is their bounding leap, known as stotting or pronking, used to startle predators and display strength.

Social behavior

During the wet season, a time when grass is abundant, adult male gazelles will graze extensively. They spread out more and establish breeding territories. Younger males usually spend their time in bachelor groups, and are prevented from entering the territories. Females form migratory groups that enter the males' territories, mostly the ones with the highest-quality resources. As the female groups pass though and forage, the territorial males may try to herd them, and are usually successful in preventing single females from leaving, but not whole groups. Subadult males usually establish dominance through actual combat, while adults are more likely to do rituals. If a bachelor male should be passing through a territorial male's region, the male will chase the offender out of his territory.

When patrolling his territory, a male may use his horns to gore the grass, soil, or a bush. Males will also mark grass stems with their preorbital glands, which emit a dark secretion. Territories of different males may share a boundary. When territorial males meet at the border of their territories, they engage in mock fights in which they rush towards each other as if they are about to clash, but without touching. After this, they graze in a frontal position, then in parallel and them in reverse, and move away from each other while constantly grazing. These rituals have no victor, but merely maintain the boundaries of the territories. Territorial males usually will not enter another male's territory. If a male is chasing an escaping female, he will stop the chase if she runs into another territory, but the neighboring male will continue the chase.

Reproduction and parental care

A male gazelle will follow a female and sniff her urine to find out if she is in estrus, a process known as the Flehmen response. If so, he will continue to court and mount her. Females will leave the herd to give birth to single fawns after a five- to six-month gestation period. They give birth twice yearly with 1–2 fawns. When birthing, a female gazelle crouches as the newborn fawn drops to the ground, tearing the umbilical cord. The mother then licks the fawn clean of amniotic fluid and tissues. In addition, licking possibly also serves to stimulate the fawn’s blood circulation, or to "label" it so its mother can recognize it by scent.

In the first six hours of the fawn'’s life, it moves and rests with its mother, but eventually spends more time away from its mother or hides in the grass. The mother stays in the vicinity of the fawn and returns to nurse it daily. Mother and fawn may spend an hour together before the fawn goes and lies back down to wait for the next nursing. Mother gazelles may associate with other gazelle mothers, but the fawns do not gather into "kindergartens". Mothers will defend their young against jackals and baboons, but not against larger predators. Sometimes, a female can fend off a male baboon by headbutting him with her horns to defend her fawn.

As the fawn approaches two months of age, it spends more time with its mother and less time hiding. Eventually, it stops hiding. Around this time, the fawn starts eating solid food, but continues to nurse from its mother. The pair will also join a herd. Young female gazelles may associate with their mothers as yearlings. Young males may also follow their mothers, but as they reach adolescence, they are noticed by territorial males, so cannot follow their mothers into territories. The mother may follow and stay with him, but eventually stops following him when he is driven away; the male will then join a bachelor group.

Status

The population estimate is around 550,000. The population had declined 60% from 1978 to 2005. Threats to Thomson’s gazelles are tourist impacts, habitat modification, fire management, and road development. Surveys have reported steep declines (60-70%) over periods of about 20 years dating from the late 1970s in several places, including the main strongholds for the species: Serengeti, Masai Mara, and Ngorongoro.