|Male Impala with red-billed oxpeckers on it.|
The impala (Aepyceros melampus) is a medium-sized African antelope. It is the type species of the genus Aepyceros and belongs to the family Bovidae. It was first described by German zoologist Martin Hinrich Carl Lichtenstein in 1812. Two subspecies of the impala have been recognised: the common impala (A. m. melampus) and the black-faced (A. m. petersi). They are typically between 120–160 cm (47–63 in) long. Males stand up to approximately 75–92 cm (30–36 in) at the shoulder and weigh 53–76 kg (117–168 lb), while females are 70–85 cm (28–33 in) and 40–53 kg (88–117 lb). Both are characterised by a glossy, reddish brown coat. Only the males have the characteristic slender, lyre-shaped horns, which can grow to be 45–92 cm (18–36 in) long.
The impala inhabits savanna grasslands and woodlands close to water sources. It is a mixed forager, whose diet consists of grasses, forbs, monocots, dicots and foliage. It switches between grazing and browsing depending on the season and habitat. Water is an essential requirement. Impala are fast runners and are known for their leaping ability, reaching heights up to 3 m (9.8 ft). They communicate using a variety of unique visual and vocal cues. There are three distinct social groups during the wet season: the female herds, the bachelor herds and the territorial males. The mating season is the three-week-long period toward the end of the wet season in May. A single fawn is born after a gestational period of about six to seven months. The fawn remains with its mother for four to six months, after which it joins juvenile groups.
The impala is native to Angola, Botswana, Kenya, Malawi, Mozambique, Namibia, Rwanda, South Africa, Swaziland, Tanzania, Uganda, Zambia and Zimbabwe. Regionally extinct in Burundi, it has been introduced in two protected areas of Gabon. The black-faced impala is confined to Kaokoland (Namibia) and southwestern Angola. The common impala has been widely introduced in southern Africa. Though there are no major threats to the survival of the species as a whole, poaching and natural calamities have significantly contributed to the decline of the black-faced subspecies. While the common impala has been listed as of Least Concern by the International Union for Conservation of Nature (IUCN), the black-faced has been rated as Vulnerable.
Etymology and taxonomy
The scientific name of the impala is Aepyceros melampus. It is the type species of the genus Aepyceros and belongs to the family Bovidae. It was first described by German zoologist Martin Hinrich Carl Lichtenstein in 1812. The vernacular name "impala" comes from the Zulu language meaning "gazelle". The scientific name is derived from Greek words αιπος aipos ("high"), κερος ceros ("horn") and melas ("black"), pous ("foot").
Due to its close affinity to some species of gazelles, the kob and reedbucks, taxonomists had placed the impala in the same tribe as these alcelaphines. Its resemblance to the hartebeest led palaeontologist Elisabeth Vrba to consider the impala as a sister clade to the alcelaphines, and Aepyceros was included under subfamily Alcelaphinae in 1985. However, the impala was subsequently placed in its own tribe, Aepycerotini, in 1992, which has been elevated to subfamily status. An rRNA and β-spectrin nuclear sequence analysis supported an association between Aepyceros and Neotragus.
Up to six subspecies have been described, although only two are usually recognised, supported by mitochondrial DNA analysis. These are the common (A. m. melampus) and the black-faced (A. m. petersi) impala. While the former is abundant throughout southern and eastern Africa, the latter is restricted to southwestern Africa. Both subspecies show much genetic differentiation, and no hybrids are known. Several fossil species have been found, including A. datoadeni from the Pliocene of Ethiopia.
A study by Vrba revealed that while the common alcelaphine ancestor has diverged at least 18 times into various morphologically different forms of hartebeest and wildebeest, the impala has continued in its basic form for at least five million years. The oldest fossil discovered suggests its ancient ancestors were slightly smaller than the modern form, but otherwise similar in all respects to the latter. Even the two subspecies have only a few differences between them. This implies that the impala is very well adapted to its environment. Its gregarious nature, variety in diet, high population growth, defence against ticks and cooperation with oxpeckers who feed on ticks are some of the traits suggested as preventing major changes in morphology and behaviour.
It is a sexually dimorphic antelope; only males are horned, and they are noticeably larger than the females. The head-and-body length for the species as a whole is typically between 120–160 cm (47–63 in). Males reach approximately 75–92 cm (30–36 in) at the shoulder, while females reach 70–85 cm (28–33 in). Males typically weigh 53–76 kg (117–168 lb) and females 40–53 kg (88–117 lb). The tail is 30–45 cm (12–18 in) long. The impala is comparable in size to the kob.
The coat is a glossy reddish brown. The red hue fades away towards the animal's sides and the underside is white. Facial features include white rings around the eyes, as well as a light chin and muzzle. There are black stripes on the forehead, rump and tail. The ears are also tipped with black. The impala has a strong resemblance to the gerenuk in terms of colouration. However, the gerenuk has shorter horns and lacks the black thigh stripes of the impala. The impala has scent glands covered by the black tuft of hair on the back feet. Sebaceous glands are concentrated on the forehead and dispersed on the torso of dominant males. The forehead glands present in males are most active during the mating season, while those of females are only partially developed and do not undergo seasonal changes. The bulbourethral glands are heavier and testosterone levels are nearly double in territorial males in comparison to bachelors. There are four nipples.
Males grow 45–92 cm (18–36 in)-long slender, lyre-shaped horns. The horns have strong ridges and the tips are far apart. They are circular in section and hollow at the base. Their arch-like structure helps the animal to interlock the horns and throw off the opponent. These also protect the cranium from damage.
Like other small to medium-sized African antelopes, the impala has a special dental arrangement on the front lower jaw similar to the toothcomb seen in strepsirrhine primates, which is used during allogrooming to comb the fur and remove ectoparasites. In allogrooming, adult males and females groom one another orally on the head and neck. Each partner generally grooms the other six to twelve times. Allogrooming in females is generally between related individuals, and in males between unrelated individuals.
Of the subspecies, the black-faced is significantly larger as well as darker than the common impala. A recessive gene causes the black coloration in these animals. The black tip on the ear is much larger in the former, and the tail is bushier and almost 30% longer.
Ecology and behaviour
Impala are diurnal, most active shortly after dawn and before dusk. They spend the night feeding and resting. They use various kinds of unique visual, olfactory and auditory communication, most notably laying scent-trails and giving loud roars. The roaring process consists of one to three loud snorts with mouth closed, followed by two to ten deep grunts with an open mouth, lifted chin and upraised tail. The most characteristic movement of the impala is its unique leap. When alarmed, they run at very high speeds and jump to heights of 3 m (9.8 ft), over bushes and even other impala, covering distances of up to 10 m (33 ft). The impala has an average lifespan of about 15 years in the wild, and nearly 17 years in captivity.
Impala are important prey animals for several carnivores, including lions, leopards, cheetahs, Cape hunting dogs, spotted hyenas, crocodiles and pythons. An alert and wary animal, the impala turns motionless on sensing danger. It will scan the vicinity with its eyes to spot the predator, and rotate its ears to catch any tell-tale sounds. It stares at and moves its head to get a better view of any object it can not identify. The female who leads a file of impala on the way to drink often stops and surveys the surroundings for danger, while the rest stand relaxed. Unlike other antelopes, who run away in the open when disturbed, the impala tries to hide itself in dense vegetation in case of any alarm.
The social behaviour of the impala is influenced by the seasons. While their home ranges are heavily fortified in the wet season, they often overlap in the dry season. The southern impala are much likely to intermix in the dry season, while the eastern impala are territorial during this period. Three distinct social groups are formed in the wet season: the territorial males, bachelor herds and female herds. These groups continually break up into smaller herds and reunite. About a third of the adult males hold individual territories, which range from 0.2–0.9 km2 (2,200,000–9,700,000 sq ft) in size and may change according to the season. The males demarcate their territories with urine and faeces and defend them against any other male intruders. A study of impala in the Serengeti National Park showed that in 94% of the males, territoriality was observed only for a duration of less than four months. During the mating season males prefer small, easily defended territories, and will sometimes reclaim their old ones from previous mating seasons. These territorial males may or may not have breeding females in their territories. The male will try to control any female herds passing through his territory by herding them towards the centre, and will also chase away any bachelor males or juveniles who accompany them.
The bachelor herds comprise non-territorial adult as well as juvenile males, and can have about 30 members. Individuals maintain distances of 2.5–3 m (8.2–9.8 ft) from one another. Young and old males may interact, but middle-aged males usually avoid one another. The female herds consist of 15-100 individuals, comprising females and their young (including young males below four years). The females form clans, and inhabit home ranges 80–180 hectares (200–440 acres; 0.31–0.69 sq mi) in size. There is no distinct leader of the female herd, though animals aged five years or more may move independently. Membership in both bachelor and female herds is variable.
Unlike other antelopes, the impala is an adaptable forager and does not need to migrate long distances. They generally prefer to have water sources nearby, though they can survive on green succulent vegetation if water is scarce. Upper incisors and canines are absent in these ruminants, and the cheek teeth are folded and sharp. It eats both monocot and dicot plants, and also feeds on fruits and Acacia pods whenever available, providing an adequate and nutritious diet under most circumstances. It switches between grazing and browsing depending on the season and habitat. An analysis showed that the diet of impala consists of 45% monocots, 45% dicots and 10% fruits. The proportion of grasses in their diet increases significantly after the first rains (up to 90% of the diet), but declines in the dry season. In the late wet and dry season, forbs are the main item of the diet. Diets are nutritionally poor in the mid-dry season, when impala feed mostly on woody dicots. Another study revealed that the dicot proportion in the diet is much higher in bachelors and females than in territorial males.
Impalas most often feed in herds after dawn, before dusk and in the night. They stop feeding if it rains, and face away from the wind. The herd assembles nearer to one another, and begin chewing the cud. The animals who feed on the periphery of the herds are usually more vigilant against predation than those feeding in the centre of the herds. A foraging impala will try to defend the patch it is feeding on by lowering its head. A study revealed that time spent in foraging reaches a maximum of 75.5% in the late dry season, decreases through the rainy season, and is minimal in the early dry season (57.8%).
Though males are sexually mature by the time they are a year old, they actually mate only after four years. At this time they establish their own territories. Females can conceive after they are a year and a half old. The annual three-week-long breeding season of the impala, also called the rut, begins toward the end of the wet season in May. The males begin preparations for mating in March, including gonadal growth and hormone production, resulting in greater aggressiveness and territoriality. Males undergo several physical changes as well, such as darkening of the coat due to greasy secretions from the sebaceous glands, thickening of the neck and acquiring a musky odour. The rut is also influenced by the lunar cycle, with most mating taking place between full moons.
Rutting males fight over dominance, often giving out noisy roars and chasing one another. They walk stiffly and display their neck and horns. Territorial males usually reduce feeding and allogrooming during the mating season, probably in order to devote more time on the outlook for females in estrous. The male detects estrous in the female by testing her urine. The estrous cycle is 12 to 29 days and lasts for 24 to 48 hours. On coming across a breeding female, the excited male begins the courtship by chasing the female about. This gradually turns into a walk, with the female 3–5 m (9.8–16.4 ft) ahead of her mate. The male flicks his tongue and may nod vigorously. The female allows him to lick her vulva, and holds her tail to the side. The male tries mounting the female, holding his head high and clasping her with his forelegs. Mounting attempts may be repeated every few seconds to every minute or two. The male loses interest in the female after the first copulation, though she is still active and can mate with other males. He is also no longer interested in maintaining an individual territory, and may join the bachelor herds.
The gestational period is of six to seven months; however, the mother has the ability to delay giving birth for an additional month if conditions are harsh. Fawns are born singly. When giving birth (usually in the midday), the female will isolate herself from the herd, despite numerous attempts by the male to keep her in his territory. The female will keep the fawn hidden in an isolated spot for a few days, weeks, or sometimes more, before returning to the herd. There, the fawn will join a nursery group and will go to its mother only to nurse or when predators are near. Fawns are suckled for four to six months. Males which mature are forced out of the group and will join bachelor herds.
Distribution and habitat
The impala is native to Angola, Botswana, Kenya, Malawi, Mozambique, Namibia, Rwanda, South Africa, Swaziland, Tanzania, Uganda, Zambia, and Zimbabwe. The capital of Uganda, Kampala, derives its name from the large number of impala that once populated the area. Regionally extinct in Burundi, it has been introduced in Gabon. The current distribution of the impala is not much different from its historical range. The black-faced impala is confined to Kaokoland (Namibia) and southwestern Angola. The common impala has been widely introduced in southern Africa.
A typical ecotone species, the impala inhabits savanna grasslands and woodlands close to water sources. They show a preference for acacia savannas for their nutritious grasses and browse in the dry season. While they inhabit Acacia senegal woodlands in the wet season, they prefer A. drepanolobium savannas in the dry season. Impala in southern Africa have been found to be associated with Colophospermum mopane woodlands. High population densities are observed in places with short grasses. They may also choose habitats with shade, as they are not adapted to dry heat. Black-faced impala, who live in semi-arid environments, also select the interface between wooded savannas and open grassy wetlands. Impala are not known to be part of mountainous ecosystems.
A study found that the reduction of woodland cover and creation of shrublands by the African bush elephants has favoured impala population by increasing the availability of more dry season browse. Earlier, the Baikiaea woodland, that has now declined due to elephants, provided minimum browse to impala. The newly formed Capparis shrubland, on the other hand, can be a key browsing habitat.
Threats and conservation
Though there are no major threats to the survival of the impala, poaching and natural calamities have significantly contributed to the decline of the black-faced subspecies. The population of the common impala has been estimated to be around 2 million, while that of the black-faced impala is only 2,000-3,000. While the common impala has been listed as of Least Concern by the International Union for Conservation of Nature (IUCN), the black-faced has been rated as Vulnerable. Translocation of the black-faced impala can be highly beneficial in conserving the populations, especially when larger populations are introduced into a place at first.
The common impala is one of the most abundant antelopes in Africa, with about one-quarter of the population occurring in protected areas. The largest populations occur in the Masai Mara and Kajiado (Kenya); the Serengeti, Ruaha National Parks and Selous Game Reserve (Tanzania); Luangwa Valley (Zambia); Okavango Delta (Botswana); Hwange, Sebungwe and Zambezi Valley (Zimbabwe); and Kruger National Park (South Africa). The rare black-faced impala has been reintroduced from Kaokoland into private farms in Namibia and the Etosha National Park. Population densities vary largely from place to place; from less than 1/km² in Mkomazi National Park (Tanzania) to as high as 135/km² near Lake Kariba (Zimbabwe).
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