Pigs, Peccaries and Hippos Status Survey and Action Plan (1993)
The Afro-tropical Hippopotamuses - Hippopotamidae
Hippopotamus and Hexaprotodon
3.1 Taxonomy and Description
The hippos, Family Hippopotamidae, are separated from all other suiformes in the Superfamily Anthracotheroidea. All Holocene or Recent hippos belong to two genera, Hexaprotodon (= Choeropsis) and Hippopotamus. Only one species of each of these are usually recognized, but three more species became extinct within the Holocene on Madagascar (Stuenes, 1989; Faure & Guerin, 1990). The two living and three recently extinct forms may be described as follows:
The Pygmy Hippopotamus
The West African pygmy hippo has usually been referred to the genus Choeropsis but Coryndon (1977) has shown that it is essentially one of the hexaprotodonts - more generalized hippos, previously thought to be completely extinct. In spite of differences between this animal and the hexaprotodonts in the number of teeth, Coryndon believed they should no longer be placed in separate genera.
Although it is much smaller than the common hippo, the pygmy species has relatively longer limbs. It also has a proportionally smaller, narrower head with the orbits not raised above the skull roof. There are 38 teeth, as against 42-44 in the larger species, owing to differences in the numbers of incisors. Dorst & Dandelot (1970) give the weight as 270 kg and the shoulder height as 80cm.
There are two subspecies. The nominate race, H. l. liberiensis occurs in Guinea, Sierra Leone, Liberia and Ivory Coast. In the Ivory Coast it has been recorded as far east as between the Sasandra and Bandama Rivers (Dekeyser, 1954) but recently Bosman & Hall-Martin (1989) reported it from the Azagny National Park in the southeast corner of the country. Whether it naturally occurs there or has been introduced is not clear. The second subspecies, H. l. heslopi, is known only from the Niger Delta east to the vicinity of the Cross River in Nigeria (Corbet, 1969). It differs from the nominate subspecies in skull proportions. It may be extinct, but Oates (in litt.) reports that residents in the Niger Delta still know of the species, and possibly it still survives.
The Common Hippopotamus (Hippopotamus amphibius)
The characteristic features of this species are well known: its great bulk and short limbs; its huge head with nostrils, eyes and ears placed on top; and its broad snout and enormous gape. Weight is 1,100-2,600 kg, shoulder height 140-160 cm (Dorst & Dandelot, 1970).
No serious attempt has been made in recent years to assess geographic variation in the species. The supposed subspecies have been listed below (from Lydekker, 1915) but unless more specimens are examined and from a wider area, it is impossible to say whether diagnostic skull characters are any more than peculiarities of particular specimens.
1. H. a. amphibius: skull with moderate preorbital constriction, convex upper surface, long mandibular symphysis and relatively large cheek teeth. Said to occur in Egypt (where it is now extinct) south to Sudan, northern Zaire and Ethiopia, and west to Gambia; also Tanzania and Mozambique. Populations in the Nile Delta and on the lower Nile were geographically isolated from other hippos (Kock, 1970) and may have represented distinguishable subspecies. If this is the case, the Delta population should bear the subspecific name amphibius and the naming of other populations will have to be re-appraised.
2. H. a. tschadensis: similar to nominate race but with orbits more prominent. Distinguished from H. a. capensis by much shorter and wider facial region and more forward direction of orbits. Range: Chad and Nigeria. Synonymised with H. a. amphibius by Haltenorth (1963).
3. H. a. kiboko: skull with very broad nasals, relatively small rostral constriction, and great elevation of orbits and occipital crest above deeply hollowed interorbital region. Orbits more nearly circular than in H. a. capensis and more prominent than in H. a. constrictus - which also differs by greater rostral constriction and shorter mandibular symphysis (see below). Also said to differ in color and hairiness of ears and tail! Range: Kenya and Somalia.
4. H. a. constrictus: skull lighter than in typical race with preorbital constriction deeper, upper surface more flattened, muzzle less expanded, mandibular symphysis shorter and cheek teeth smaller. Range: Angola, southern Zaire and Namibia. Synonymised with H. a. capensis by Ellerman et al. (1953).
5. H. a. capensis (syn. australis): skull still more flattened than in H. a. tschadensis, so that width of orbit is greater than height. Range: Zambia south to South Africa.
Extinct Malagasy Hippopotamuses
Stuenes (1989) showed that two kinds of dwarf hippopotamuses (Hippopotamus lemerlei and H. madagascariensis) survived late into the Quaternary on Madagascar, and Faure & Guerin (1990) have described another and larger species, H. laloumena. H. lemerlei occurred on the island at least until 980 + 200 yrs. B. P., but neither the H. madagascariensis or H. laloumena material has been dated, though both are believed to be Holocene in age. The latter species is known only from a lower jaw and limb bones obtained near Mananjary on the east coast. It was largest of the three species but was not as big as H. amphibius, which it otherwise appears to have resembled closely. H. lemerlei was somewhat larger than H. madagascariensis and differed from it in that the skulls of adults were markedly (presumably sexually) dimorphic in size. These two smaller hippos also appear to have differed in their ecology. Like H. laloumena, H. lemerlei (Plate ///) resembled a small H. amphibius and probably had similar habits. Its remains have been found mostly in the coastal lowlands. H. madagascariensis was a more terrestrial species, subfossils of which have been located in the central highlands. Recently, Harris (1991) has reassessed the systematics of the two smaller hippos and concluded that the more terrestrial species, madagascariensis, should be placed in the genus Hexaprotodon. Inferences to be drawn are firstly that each of the surviving species of hippos had close relatives on Madagascar, and secondly that Madagascar had been colonized three times by hippopotamuses! Early human colonization of Madagascar, which occurred c. 1,500 yrs BP, is implicated in their extinction, though climatic change may also have been a factor.
Bosman, P. and Hall-Martin, A. 1989. Elephants of Africa. New Holland, London.
Corbet, G. B. 1969. The taxonomic status of the pygmy hippopotamus Choeropsis liberiensis, from the Niger Delta. J. Zool. 158: 387-394.
Coryndon, S. C. 1977. The taxonomy and nomenclature of the Hippopotamidae (Mammalia, Artiodactyla) and a description of two new fossil species. Proc. K. Nederlandse Akad. Wetensch. Ser. B. 80: 61-88.
Dekeyser, P. L. 1954. L'hippopotame nain. Notes afr. 63: 91-92.
Dorst, J. and Dandelot, P. 1970. A Field Guide to the Larger Mammals of Africa. Collins, London.
Ellerman, J. R., Morrison-Scott, T. C. S. and Hayman, R. W. 1953. Southern African Mammals 1758 to 1951: A Reclassification. British Museum, London.
Faure, M. and Guerin, C. 1990. Hippopotamus laloumena nov. sp., la troisi�me �spece d'hippopotame holocene de Madagascar. Comptes Rendus de l'Academie des Sciences, Serie 11, 310: 1299-1305.
Haltenorth, T. 1963. Die Klassifikation der S�ugetiere Artiodactyla 1(18). Handb. Zool. 8(32): 1-167.
Harris, J. M. 1991. Family Hippopotamidae. Pp. 31-85, in Koobi Fora Research Project. Vol. 3. The Fossil Ungulates: Geology, Fossil Artiodactyls and Palaeoenvironments. Clarendon Press, Oxford: xvi + 384 pp.
Kock, D. 1970. Die Verbreitungsgeschichte des Flusspferdes, Hippopotamus amphibius Linne, 1758, im unteren Nilgebiet. Saugetierk. Mitt. 18: 12-25.
Lydekker, R. 1915. Catalogue of the Ungulate Mammals in the British Museum (Natural History). Vol. 5. British Museum, London.
Stuenes, S. 1989. Taxonomy, habits and relationships of the sub-fossil Madagascan hippopotamuses Hippopotamus lemerlei and H. madagascariensis. J. Vert. Paleontol. 9: 241-268.