Impala (Aepyceros melampus)

Taxonomy

Aepyceros is a genus of African bovid currently represented by a single species, Aepyceros melampus, commonly known as the impala. The Aepyceros lineage was suggested to originate with the species Aepyceros premelampus from the Late Miocene (6), however this was recently reclassified to three separate genera: Afrotragus and Turkanatragus and Aepyceros (9). It is somewhat awkward that the type specimen for Aepyceros premelampus was reassigned to Afrotragus  premelampus, as it leaves the other Miocene Aepyceros remains in taxonomic limbo without an official species designation (9). The Miocene Aepyceros was succeeded in the Pliocene by several distinct species (6, 7, 8), most prominent of which was A. shungurae which eventually transitioned into the modern species in the Early Pleistocene (6). Additionally, two other Pleistocene species have been recognised by Faith et al 2014, these are A. helmoedi of the early Pleistocene (Brink et al 2012) and an unnamed Late Pleistocene species (Aepyceros sp. Nov.) which may constitute a direct descendant of A. melampus. The genus is morphologically conservative, with the Miocene specimens closely resembling modern impalas (6).

The impala is subdivided into two subspecies, the black-faced impala (A. m. petersi) and the common impala (A. m. melampus) (24). Several additional subspecies were recognised in the past, but these are not corroborated by genetic evidence (13).

The impala has quite a unique placement amongst the bovids. Despite superficially resembling other antelopes of similar size, the impala is the sole member of the subfamily Aepycerotinae (23). The closest relatives of the Impala are the dward antelopes of the genus Nesotragus (23), together they form an outgroup to all other non-bovine bovids (23).

Table 1. Species of Aepyceros recognised by Faith et al 2014, revised with newer information from Geraards 2019 and the temporal ranges of each taxa.

Distribution and Age

The species Aepyceros melampus dates back to the onset of the Pleistocene some two million years ago in Eastern Africa (6). Impalas are distributed throughout most of Eastern Africa, from Kenya down to Northern South Africa, and range westwards into the Namibian panhandle, these are all assigned to the common impala, which is very abundant in these areas and considered Least Concern on the IUCN redlist (24). The common impala populations can be broken down along genetic lines into a southern and eastern population, which are quite distinct but do not constitute different subspecies as they are morphologically near identical. Additionally, there are a few disconnected populations in South Western Africa (24) which are attributed to the black-faced impala (13) a much rarer subspecies classed as vulnerable with a population of only some 3000-4000 adult individuals (24). Overall, the Impala is a very abundant megafaunal species in Africa with about 2 million estimated individuals and is therefore listed as Least Concern on the IUCN redlist.

Variously during the Late Pleistocene and much of the Holocene the range was greater. Particularly of note, are the remains of imapala fossils from Central Ethiopia during the Pleistocene (25), far northwards of the current range, and further specimens have been found in the mid-Holocene of Southern Ethiopia (17). Additionally, fossil, and historical evidence suggests a broader range in many other parts of their range (24, 25). No fossil evidence the South-West African subspecies has been reported, leaving its former range to be somewhat of a mystery, but undoubtedly the fragmented range of the taxon were once united.

Interestingly, modern day East African impalas appear to have been descended from a Southern population, this region acted as a refugia for the species during times of climatic shifts (13, 14, 15). Perhaps cycles of range expansion and retraction took place throughout the Pleistocene. In any case, impalas appear to have been continuously present from at least 57,000 years ago in the region (6).

Morphology and Ecology

Impalas are medium sized antelopes, with males generally weighing 53-76kg and females 40-53kg with shoulder heights of 75-92cm and 70-85cm respectively (20). Males alone possess horns, which are S-shaped and typically reach 45-92cm in length (20).

Carbon isotope data (both dental and fecal) suggests impalas are mixed-feeders and quite generalistic, being equally capable of grazing and browsing (3, 4). The propotion of either vegetation type consumed varies heavily between populations and it seems that their diet is selective, reflecting nutrient richness rather than local abundance, particularly with respect to protein content (3, 4). For this reason, diet may also vary between seasons (4). Due to their high consumption of protein, the species also produces high amounts of nitrogenous waste, requiring them to drink large quantities of water (20). Curiously, despite their mixed feeding habits Impalas exhibit dental morphology very similar to that of grazing bovids, they are highly hypsodont with a long row of premolars (4).

Impalas live in herds comprised of a single dominant male and a multitude females and juveniles; these groups typically comprise about 16 to 35 individuals, but large groups may exceed 60 (19). Young males which lack a harem will form smaller bachelor groups which tend to be made up of only a handful of animals (19). The formation of large groups is probably an adaptation of vigilance, impalas spend copious amounts of time feeding, especially when conditions are unfavourable (4) and so to reduce the time spent scanning for predators they form herds (19). Males of breeding herds appear to exempt from this time reduction, because they are alone in scanning for rival males (19).

Impala habitat types vary somewhat, but they are described as an ecotone species, which mean they prefer the transition zones between habitats (20) but seem comfortable both in woodland and grassland habitats. Their ecological flexibility does not extend to montane areas though nor too densely vegetated areas (20), they are also dependent on a constant watersource precluding them from overly arid areas (20).

Across their range Impalas overlap with a large abundance of other megafaunal herbivorous, these include an abundance of other antelope species, rhinceroses (Diceros bicornis & Ceratotherium simum), the plains zebra (Equus quagga), cape buffalo (Syncercus caffer), giraffe (Giraffa camelopardalis), African warthog (Phacochoerus africanus) and the African elephant (Loxodonta africana). Landscape transformation by the African elephant in particular appears to benefit the impala by enabling a greater abundance of shrubs during dry seasons due to the clearing of dense vegetation (18). Two key species, which have formed a co-operative behaviour with the impala are the vervet monkey (Cercopithecus pygerythrus) and yellow babbon (Papio cynocephalus), both of which have superior eyesight to the impala and thus assist in detecting threats. The impalas compliment this with better hearing and smell, lessening the threat of undetected predators for the monkey as well (20). Impalas are a key prey item of Leopards (Panthera pardus)(21), African wild dog (16), Cheetah (Acionyx jubatus)(5) and to a lesser extent lion (Panthera leo) in the areas where they co-occur (10). Other predators such as the spotted hyaena (Crocuta crocuta)(29) and even the black-backed jackal (11) will also hunt them on occassion.

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Citations

1.      Averbeck, C., Apio, A., Plath, M., Wronski, T.. (2009). Hunting differentially affects mixed-sex and bachelor herds in a gregarious ungulate, the impala (Aepyceros melampus: Bovidae). Afican Journal of Ecology 48, 255-264.

2.      Brink, J. S., Herries, A. I. R., Moggi.Cecchi, J., Gowlett, J. A. J., Bousman, C. B., Hancox, J. P., Grun, R., Eisenmann, V., Adams, J. W., Rossouw, L.. (2012). First hominine remains from a 1.0 million year old bone bed at Cornelia-Uitzoek, Free State Province, South Africa. Journal of Human Evolution 63, 527-535.

3.      Codron, D., Codron, J., Lee-Thorp, J. A., Sponheimer, M., Ruiter, D., Brink, J. S.. (2006). Dietary variation in impala Aepyceros melampus recorded by carbon isotope composition of feces. Acta Zoologica Sinica 52(6), 1015-1025.

4.      Copeland, S. R., Sponheimer, M., Spinage, C. A., Lee-Thorp, J. A., Codron, D., Reed, K. E.. (2009). Stable Isotope evidence for impala (Aepyceros melampus diets at Akagera National Park, Rwanda. African Jounral of Ecology, 74, 490-501.

5.      Craig, C. A., Brassine, E. I., Parker, D. M.. (2017). A record of cheetah (Acionyx jubatus) diet in Northern Tuli Game Reserve, Botswana. African Journal of Ecology 55, 697-700.

6.      Faith, J. T., Tryon, C. A., Peppe, D. J., Beverly, E. J., Blegen, N.. (2014). Biogeographic and Evolutionary Implications of an Extinct Late Pleistocene Impala from the Lake Victoria Basin, Kenya. Journal of Mammal Evolution 21, 213-222.

7.      Geraads, D., Melillo, S., Haile-Selassie, Y.. (2009). Middle Pliocene Bovidae from Hominid-bearing sites in the Woranso-Mille Area, Afar region, Ethiopia. Palaeontologia Africana 44, 59-70.

8.      Geraards, D., Bobe, R., Reed, K.. (2012). Pliocene Bovidae (Mammalia) from the Hadar Formation of Hadar and Ledi-Geraru, Lower Awash, Ethiopia. Journal of Vertebrate Paleontology, 32(1), 180-197.

9.      Geraards, D.. (2019) A reassessment of the Bovidae (Mammalia) from the Nawata Formation of Lothagam, Kenya, and the late Miocene diversification of the family in Africa. Journal of Systematic Palaeontology 17(2), 169-182.

10.  Hayward, M. W., Kerley, G. I. H.. (2005). Prey preferences of the lion (Panthera leo). Journal of Zoology 267, 309-322.

11.  Kamler, J. F., Foght, J. L., Collins, K.. (2009). Single black-backed jackal (Canis mesomelas) kills adult impala (Aepyceros melampus). African Journal of Ecology 48, 847-848.

12.  Klein, R. G.. (1984). Later Stone Age Faunal Samples from Heuningneskrans Shelter (Transvaal) and Leopard’s Hill Cave (Zambia). South African Archaeological Bulletin 39(140), 109-116.

13.  Lorenzen, E. D., Arctander, P., Siegismund, H. R.. (2006). Regional Genetic Structuring and Evolutionary History of the Impala Aepyceros melampus. Journal of Heredity 97(2), 119-132.

14.  Lorenzen, E. D., Masembe, C., Arctander, P., Siegismund, H. R.. (2010). A long-standing Pleistocene refugium in Southern Africa and a mosaic of refugia in East Africa: insights from mtDNA and the common eland antelope. Journal of Biogeography 37, 571-581.

15.  Lorenzen, E. D., Heller, R., Siegismund, H. R.. (2012). Comparative phylogeography of African Savannah ungulates. Molecular Ecology 21, 3656-3670.

16.  Pole, A., Gordon, I. J., Gorman, M. L., MacAskill, M.. (2004). Prey Selection by African Wild Dogs (Lycaon pictus) in Southern Zimbabwe. Journal of Zoology 262, 207-215.

17.  Rowan, J., Faith, J. T., Gebru, Y., Fleagle, J. G.. (2015). Taxonomy and paleoecology of fossil Bovidae (Mammalia, Artiodactyla) from the Kibish Formation, Southern Ethiopia: Implications for dietary change, biogeography, and the structure of the living bovid faunas of East Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 420, 210-222.

18.  Rutina, L. P., Moe, S. R., Swenson, J. E.. (2005). Elephant Loxodonta africana driven woodland conversion to shrubland improves dry-season browse availability for impalas (Aepyceros melampus). Wildlife Biology 11(3), 207-213.

19.  Shorrocks, B., Cokayne, A.. (2005). Vigilance and group size in impala (Aepyceros melampus, Lichtenstein): A study in Nairobi National Park, Kenya. African Journal of Ecology 43, 91-96.

20.  Spies, K. S.. (2015). Ecology of Impala (Aepyceros melampus) and waterbuck (Kobus ellipsiprymnus) in Majete Wildlife Reserve, Malawi. Masters Thesis. Stellenbosch University, Stellenbosch.

21.  Stein, A. B., Bourquin, S. L., McNutt, J. W.. (2015). Avoiding intraguild competition: Leopard feeding ecology and prey caching in Northern Botswana. African Journal of Wildlife Research 45, 1-11.

22.  Wronski, T.. (2002). Feeding ecology and foraging behaviour of impala Aepyceros melampus in Lake Mburo National Park, Uganda. African Journal of Ecology 40, 205-211.

23.  Yang, C., Xiang, C., Qi, W., Xia, S., Tu, F., Zhang, X., Moermond, T., Yue, B.. (2013). Phylogenetic analysis and improved resolution of the family Bovidae based on complete mitochondrial genomes. Biochemical Systematics and Ecology 48, 136-143.

24.  IUCN SSC Antelope Specialist Group. 2016. Aepyceros melampus. The IUCN Red List of Threatened Species 2016:

25.  Palaeobiology Database. (2021). Aepyceros melampus. https://paleobiodb.org

26.  Plug, I., Badenhorst, S.. (2001). The distribution of Macromammals in Southern Africa Over the Past 30,000 Years as reflected in animal remains from archaeological sites. Transvaal Museum Monograph 12, 161-170.

27.  Bushoff, A., Landman, M., Kerley, G.. (2015). Filling the gaps on the maps: historical distribution patterns of some larger mammals in part of Southern Africa. Transactions of the Royal Society of South Africa.

28.  Berger, L. R., Brink, J.. (2007). An Atlas of southern African mammalian Fossil Bearing Sites – Late Miocene to Late Pleistocene. Chapter 6: The Sites Dolomitic and Other Cave Deposits South Africa. Johannesburg: University of the Witwatersrand.

29.  Cooper, S. M., Holekamp, K. E., Smale, L.. (1999). A seasonal feast: Long-term analysis of feeding behaviour in the spotted hyaena (Crocuta crocuta). African Journal of Ecology 37, 149-160)