The impala inhabits savanna grasslands and woodlands in close proximity 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.
The common name "impala" (pronounced /im-ˈpa-lə, -ˈpä-/) comes from the Zulu name for the animal; the first recorded use of the name dates back to 1885. An alternative name for the impala is "rooibok". The scientific name of the impala is Aepyceros melampus; aepyceros comes from the Greek words αιπος aipos ("high") and κερος ceros ("horn"). The specific epithet melampus derives from the Greek words μελάς melas ("black") and πούς pous ("foot").
Taxonomy and evolution
The impala is the sole member of the genus Aepyceros and belongs to the family Bovidae. It was first described by German zoologist Martin Hinrich Carl Lichtenstein in 1812. In 1845, Swedish zoologist Carl Jakob Sundevall placed the springbok in Antidorcas, a genus of its own.
The impala's resemblance to the hartebeest led palaeontologist Elisabeth Vrba to consider the impala a sister taxon to the alcelaphines. A 1999 phylogenetic study by Alexandre Hassanin (of the National Centre for Scientific Research, Paris) and colleagues, based on mitochondrial and nuclear analyses, showed that the impala forms a clade with the suni(Neotragus moschatus). This clade is sister to another formed by the bay duiker (Cephalophus dorsalis) and the klipspringer(Oreotragus oreotragus). An rRNA and β-spectrin nuclear sequence analysis in 2003 also supported an association between Aepyceros and Neotragus. The following cladogram is based on the 1999 study:
Up to six subspecies have been described, although only two are generally recognised on the basis of mtDNA data. Though morphologically similar, the subspecies show a significant genetic distance between them, and no hybridsbetween them have been reported.
- A. m. melampus Lichtenstein, 1812: Known as the common impala. Occurs across eastern and southern Africa. The range extends from central Kenya to South Africa and westward into southeastern Angola.
- A. m. petersi Bocage, 1879: Known as the black-faced impala. Restricted to southwestern Africa, it occurs in northwestern Namibia and southwestern Angola.
According to Vrba, the impala evolved from an alcelaphine ancestor. She noted that while this ancestor has diverged at least 18 times into various morphologically different forms, the impala has continued in its basic form for at least five million years. Several fossil species have been discovered, including A. datoadeni from the Pliocene of Ethiopia. The oldest fossil discovered suggests its ancient ancestors were slightly smaller than the modern form, but otherwise similar in all aspects to the latter. This implies that the impala has efficiently adapted to its environment since prehistoric times. Its gregarious nature, variety in diet, positive population trend, defence against ticks and symbiotic relationship with the tick-feeding oxpeckers could have played a role in preventing major changes in morphology and behaviour.
The impala is a medium-sized, slender antelope similar to the kob or Grant's gazelle in build. The head-and-body length is around 130 cm (51 in). Males reach approximately 75–92 cm (30–36 in) at the shoulder, while females are 70–85 cm (28–33 in) tall. Males typically weigh 53–76 kg (117–168 lb) and females 40–53 kg (88–117 lb). Sexually dimorphic, females are hornless and notably smaller than males. Males grow 45–92 cm (18–36 in) long, slender, lyre-shaped horns. The horns, strongly ridged and divergent, circular in section and hollow at the base. Their arch-like structure allows interlocking of horns, which helps a male throw off his opponent during fights; horns also protect the skull from damage.
The glossy coat of the impala shows two-tone colouration – the reddish brown back and the tan flanks; these are in sharp contrast to the white underbelly. Facial features include white rings around the eyes and a light chin and snout. The ears, 1.7 centimetres (0.67 in) long, are tipped with black. Black streaks run from the buttocks to the upper hindlegs. The bushy white tail, 30 cm (12 in) long, features a solid black stripe along the midline. 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 glandscovered by the black tuft of hair on the hindlegs. Sebaceous glands are concentrated on the forehead and dispersed on the torso of dominant males; these glands males are most active during the mating season, while those of females are only partially developed and do not undergo seasonal changes. There are four nipples.
Of the subspecies, the black-faced impala is significantly larger and darker than the common impala; a recessive gene is responsible for the black colouration. Distinctive of the black-faced impala is the dark stripe on either side of the nose, that runs upward to the eyes and thins as it reaches the forehead. Other differences include the larger black tip on the ear, and a bushier and nearly 30% longer tail in the black-faced impala.
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 on the head and the neck and remove ectoparasites.
Ecology and behaviour
The impala is diurnal (active mainly during the day), though activity tends to cease during the hot midday hours; the night is spent in feeding and resting. Three distinct social groups can be observed – the territorial males, bachelor herds and female herds. The territorial males hold territories where they may form harems of females; territories are demarcated with urine and faeces and defended against juvenile or male intruders. Bachelor herds tend to be small, with less than 30 members. Individuals maintain distances of 2.5–3 m (8.2–9.8 ft) from one another; while young and old males may interact, middle-aged males generally avoid one another. The membership of female herds varies from 6 to 100; herds occupy 80–180 hectares (200–440 acres; 0.31–0.69 sq mi) large home ranges. The mother-calf bond is weak, and breaks soon after weaning; juveniles leave the herds of their mothers to join other herds. Female herds tend to be loose and have no distinct leadership. Allogrooming is an important means of social interaction in bachelor and female herds; in fact, the impala appears to be the only ungulate to display self-grooming as well as allogrooming. In allogrooming, females typically groom related individuals, while males associate with unrelated individuals. Each partner grooms the other six to twelve times.
Social behaviour is influenced by the climate and geography; as such, the impala are territorial at certain times of the year and gregarious at other times, and the length of these periods can vary broadly among populations. For instance, populations in southern Africa display territorial behaviour only during the rut, that lasts a few months, and are otherwise gregarious; whereas in eastern African populations, territoriality is relatively minimal despite a protracted mating season. Moreover, territorial males often tolerate bachelors, and may even alternate between bachelorhood and territoriality at different times of the year. 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.
The impala is an important prey for several carnivores, such as cheetahs, leopards and lions. The impala displays two characteristic leaps – it can jump up to 3 m (9.8 ft), over vegetation and even other impala, covering distances of up to 10 m (33 ft); the other type of leap involves a series of jumps in which the animal lands on its forelegs, moves its hindlegs mid-air in a kicking fashion, lands on all fours and then rebounds. It leaps in either manner in different directions, probably to confuse predators. At times, the impala may also conceal itself in vegetation to escape the eye of the predator. The most prominent vocalisation is the loud roar, delivered through one to three loud snorts with the mouth closed, followed by two to ten deep grunts with the mouth open, the chin and the tail raised; a typical roar can be heard up to a distance of 2 kilometres (1.2 mi). Scent gland secretions identify a territorial male. A notable feature of impala is their sedentary nature; adult and middle-aged males, in particular, can hold their territories for years.
Impala browse as well as graze; either may predominate, depending upon the availability of resources. The diet comprises monocots, dicots, forbs, fruits and Acacia pods whenever available. Impala prefer places close to water sources, and resort to succulent vegetation if water is scarce. An analysis showed that the diet of impala is composed of 45% monocots, 45% dicots and 10% fruits; the proportion of grasses in the diet increases significantly (to as high as 90%) after the first rains, but declines in the dry season. Browse predominates in the late wet and dry season, and diets are nutritionally poor in the mid-dry season, when impala feed mostly on woody dicots. Another study showed that the dicot proportion in the diet is much higher in bachelors and females than in territorial males.
Impala carefully feed on soft and nutritious grasses such as Digitaria macroblephora; tough, tall grasses, such as Heteropogon contortus and Themeda triandra, are typically avoided. Impala on the periphery of the herds are generally more vigilant against predators than those feeding in the centre; 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%).
Males are sexually mature by the time they are a year old, though successful mating generally occurs only after four years. Mature males start establishing territories and try to gain access to females. Females can conceive after they are a year and a half old; oestrous lasts for 24 to 48 hours, and occurs every 12–29 days in non-pregnant females. The annual, three weeks-long rut (breeding season) begins toward the end of the wet season, typically in May. Gonadal growth and hormone production in males begin a few months before the breeding season, resulting in greater aggressiveness and territoriality; the bulbourethral glandsare heavier and testosterone levels nearly double in territorial males in comparison to bachelors. Consequently, the neck of a territorial male tends to be thicker than that of a bachelor during the rut. Mating tends to take place between full moons, and might thus be influenced by the lunar cycle.
Rutting males fight over dominance, often giving out noisy roars and chasing one another; they walk stiffly and display their neck and horns. Males desist from feeding and allogrooming during the rut, probably to devote more time in garnering females in oestrus; the male checks the female's urine to ensure that she in oestrus. On coming across such female, the excited male begins the courtship by pursuing her, keeping a distance of 3–5 m (9.8–16.4 ft) from her. The male flicks his tongue and may nod vigorously; the female allows him to lick her vulva, and holds her tail to one side. The male tries mounting the female, holding his head high and clasping her sides 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.
Gestation lasts six to seven months; the mother may even delay giving birth for an additional month if conditions are harsh. Births generally occur in the midday; the female will isolate herself from the herd when labour pain begins. A single calf is born, and is immediately concealed in cover for the first few weeks of its birth. The fawn then joins a nursery group within his mother's herd. Calves are suckled for four to six months; young males are forced out of the group and subsequently join bachelor herds, while females may stay back.
Distribution and habitat
The impala inhabits woodlands due to its preference for shade; it can even occur on the interface (ecotone) between woodlands and savannahs; proximity to water sources is preferred. In southern Africa, populations tend to be associated with Colophospermum mopane and Acaciawoodlands. Habitat choices differ seasonally – Acacia senegal woodlands are preferred in the wet season, and A. drepanolobium savannahs in the dry season. Another factor that could influence habitat choice is vulnerability to predators; impala tend to keep away from areas with tall grasses as predators could be concealed there. 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 Baikiaeawoodland, that has now declined due to elephants, provided minimum browse to impala. The newly formed Capparis shrubland, on the other hand, could be a key browsing habitat. Impala are generally not associated with montane habitats; however, in KwaZulu-Natal, impala have been recorded at altitudes of up to 1,400 metres (4,600 ft) above the sea level.
The historical range of the impala, that spanned across southern and eastern Africa, has remained intact to a great extent, although the antelope has disappeared from a few places such as Burundi. The range extends from central and southern Kenya and northeastern Uganda in the east to northern KwaZulu-Natal in the south, and westward up to Namibia and southern Angola. The black-faced impala is confined to southwestern Angola and Kaokoland in northwestern Namibia; the status of this subspecies has not been monitored since the 2000s. The common impala has a wider distribution, and has been introduced in protected areas in Gabon and across southern Africa.
Threats and conservation
The International Union for Conservation of Nature and Natural Resources (IUCN) classifies the impala as Least Concern; the black-faced impala, however, is classified as Vulnerable because less than 1000 individuals are estimated in the wild. Though there are no major threats to the survival of the common impala, poaching and natural calamities have significantly contributed to the decline of the black-faced impala. As of 2008, the population of the common impala has been estimated at around two million. Studies opine that translocation of the black-faced impala can be highly beneficial in its conservation.
Around a quarter of the common impala populations occur in protected areas, such as the Okavango Delta (Botswana); Masai Mara and Kajiado (Kenya); Kruger National Park (South Africa); the Ruaha and Serengeti National Parks and Selous Game Reserve (Tanzania); Luangwa Valley (Zambia); Hwange, Sebungwe and Zambezi Valley(Zimbabwe). The rare black-faced impala has been introduced into private farms in Namibia and the Etosha National Park. Population densities vary largely from place to place; from less than one individual per square kilometre in Mkomazi National Park (Tanzania) to as high as 135 individuals per square kilometre near Lake Kariba (Zimbabwe).