Pigs, Peccaries and Hippos Status Survey and Action Plan (1993)
The Pygmy Hippopotamus
S. Keith Eltringham
Status and Action Plan Summary
H. l. liberiensis is status category 4 (vulnerable) and H. l. heslopi is 'Indeterminate' but probably critically endangered or extinct.
The pygmy hippopotamus is mainly confined to Liberia, where it is widely distributed although it nowhere occurs in large populations. A distinct subspecies has been described from Nigeria and may still survive there. Another isolated population has been described from Guinea Bissau but it was almost certainly based on a misidentification. The continued existence of the Nigerian population should be investigated and its current status established if it is found to be still present. The numbers of pygmy hippopotamus are unknown but are thought to total a few thousand at the most. It is a secretive, nocturnal animal that occurs in thick forest, usually alone or in pairs. It is hunted for meat throughout its range except in Guinea, where it is said not to be threatened. This may be because it is rarely encountered for it would be surprising if it were ignored by hunters unless it were associated with a taboo. It is not clear how far hunting for meat, or for trophies, is a threat to survival but given the low densities and scattered distribution of the species, any killing is likely to be detrimental to its survival. The main threat, however, is perceived to be loss of habitat through forest clearance and it is recommended that areas of relatively secure forests containing viable populations of the pygmy hippopotamus should be identified and given protected status. Attempts should also be made to devise methods for assessing population sizes. The species breeds well in captivity and the captive population of about 350 animals in 131 collections appears to be self-sustaining.
The pygmy hippopotamus shows considerable differences from its larger relative, the common hippo, in many ways other than in size. It is indeed considerably smaller, weighing between 180 and 270 kg. Unlike the common hippopotamus, it occurs alone or in small groups and it is confined to forested regions close to rivers. It is also much less aquatic with little webbing between the digits, allowing the feet to spread laterally to a greater extent. This is presumably a terrestrial adaptation. It is more pig-like in shape with proportionately longer limbs and neck and a proportionately smaller head. In its general ecology and social behaviour it shows a closer resemblance to tapirs than to the common hippopotamus (Robinson, 1970). It is nevertheless recognizably a hippopotamus with such aquatic adaptations as strong muscular valves to the ears and nostrils. Its aquatic nature is evident from its skin physiology, which is as complex as that of the common hippopotamus, and the animal suffers damage to the skin if exposed to sunlight. The ranges of the two species of hippopotamus do not overlap.
Two subspecies are recognized. Most members of the species belong to the nominate race H. l. liberiensis, which occurs in Liberia, but the small population in Nigeria has been given subspecific status. Four skulls acquired by Heslop (1945) were examined by Corbet (1969), who placed them into a new subspecies Choeropsis liberiensis heslopi. With the change in the name of the genus (see Grubb, Chapter 3.1), this subspecies should now be referred to as Hexaprotodon liberiensis heslopi (Corbet, 1969).
An attempt was made in 1989/90 to obtain up-to-date information on the status of the species through the African Suiform Questionnaire Survey and related correspondence. Replies were received from eight persons, whose comments have been incorporated into the following account. Mentions in the text of names not followed by a bracketed date refer to these correspondents. A bibliography of the species has been prepared by Robinson (1981).
Former and Present Distribution
The past distribution of the pygmy hippopotamus was not very different from what it is today, but it is likely that the populations have become fragmented, for the animal has disappeared from many of its former sites. The distribution of the species is centered on Liberia, which includes the bulk of the population (Anstey, 1991). The species also occurs in the three countries (Guinea, Ivory Coast and Sierra Leone) that are contiguous with Liberia, mainly close to the shared borders. A second, isolated population may occur in Nigeria some 1,800 km to the east on the other side of the Dahomey Gap. Such a discontinuous distribution is very rare amongst forest vertebrates in west Africa and Robinson (1970) considers that there is insufficient evidence to confirm that the species ever existed in Nigeria despite the account by Heslop (1945), who shot one near Omoku in the vicinity of the Niger Delta. Others have been equally skeptical but Ritchie (1930) gave measurements of two skulls that were obtained in 1928 from the Niger Delta so there seems little doubt that the species did once occur in the country. Another isolated population has been reported from the Corubal River in Guinea-Bissau by Cristino (1958), who claimed to have shot a specimen but it is more probable that the animal was a young common hippopotamus. The present distribution of the species in Liberia is shown in Fig. 6.
Link to Fig. 6: Approximate present known and reported distribution of the pygmy hippopotamus, Hexaprotodon liberiensis.
Habitat and Ecology
The pygmy hippopotamus is rarely seen because of its secretive, nocturnal habits and consequently not much is known of its ecology. The most detailed field study is that of Robinson (1970) and a general account of its biology is given by Lang (1975). The pygmy hippopotamus is much less gregarious than the larger species, being usually found in ones or twos. As it is largely nocturnal it tends to spend the day hidden in swamps, wallows or rivers and sometimes in hollows under the banks of streams, which it is said to enlarge. It favors heavily forested regions but it is dependent on water and usually remains close to streams. It also used to frequent forests fringing the rivers that extend into Transitional Woodland and the southern Guinea savanna. Within the forest it follows well-defined trails or tunnel-like paths through swamp vegetation, which it marks by spreading its dung by vigorously wagging its tail while defecating, like its larger relative.
The species is exclusively vegetarian, feeding on leaves and roots of forest plants as well as on fallen fruit. The stomach has four chambers (Langer, 1988). The first three are covered with tough keratinised epithelium, only the last containing glandular epithelial tissue. There is evidence that microbial breakdown of plant material takes place in the first three stomach chambers, no caecum being present in this species.
From studies of captive animals (Lang, 1975; Tobler, 1991), the estrous cycle has been shown to average 35.5 days with estrus itself being 24-48 hours long. The average gestation length is 188 days after which a singleton young is born weighing about 5.7 kg. Twins are born very rarely, the incidence being approximately one in every 200 births. The young are born on land, and there is no evidence from captive births that a nest constructed. A survey of over 800 births indicates that these occur throughout the year (Tobler, 1991).
The pygmy hippopotamus is identified as "vulnerable" in the IUCN list of threatened animals and the present survey suggests that this category is justified. Although its numbers have probably declined recently, there is, as yet, insufficient evidence to recommend that its status should be upgraded to "endangered".
Numbers are very difficult to estimate at all accurately but the species is present only in small numbers in each locality, although it is widespread throughout its range. In 1983 A. J. Peal considered that the population in Liberia was stable and was of the order of several thousands. He now considers that numbers are probably decreasing. Most correspondents did not attempt an estimate in the other countries but G. Teleki reports that his 1979/80 survey indicated a total population of 80 _ 10 for the whole of Sierra Leone, which effectively means Gola North and Gola East, near the Liberian border, and the Loma Mountains in the north-west. More recently, White (1986) reported the definite presence of the species on Tiwai Island, near Gola East, in the south-west of the country. Further sightings were made in 1986 including one of a female with a sucking calf, proving that the population was breeding. White estimated that the number using the island in 1984 was at least three adults and one sub-adult but that the total could be as high as ten. F. O. Amubode assumes that the pygmy hippopotamus still exists in Nigeria but provides no evidence that it does. Even fifty years ago, Heslop (1945) put numbers at no more than 30, split into several isolated groups, so it is likely that the subspecies has become extinct.
All correspondents in all countries reported that the species was present at low densities and was probably decreasing in numbers except in Guinea where the population was considered to be almost stable. All correspondents, again except in Guinea, also considered that the conservation status was giving cause for concern. The only optimistic feature is that there are many populations in Liberia, which although small, are widely dispersed over a large range. The species' biology precludes its occurrence in large numbers or at high densities and hence no population is too small to be worth conserving. Any long-term hope for the species relies on effective conservation within Liberia but the recent political disturbances in that country have probably made it difficult for any protective measures to be introduced.
Threats to Survival
The principal threat to the survival of the pygmy hippopotamus seems to be the destruction of forests in Liberia, particularly high forest. The civil war is unlikely to have had a direct effect, however, since the pygmy hippopotamus occurs in remote areas away from the main centers of fighting and is too secretive to be easily shot by the passing soldier. Human pressures are less in swamps and riverine areas. Deforestation also occurs elsewhere in its range but hunting for meat is a more immediate threat in the other countries where the species occurs. Hunting for meat was reported from Liberia as well. This was the only country where teeth were said to be the object of hunting. The animals in Sierra Leone may be persecuted by the local people because of the damage they cause in riverside gardens. A novel risk to survival, oil pollution, was suggested in Nigeria because the species occurs, or occurred, in estuaries close to the sea. The pygmy hippopotamus is not considered to be threatened in Guinea where there is no trade in meat or other products.
Conservation Measures Taken
The species is listed as vulnerable by IUCN (IUCN, 1990) and is included on Appendix II of CITES (The Convention on International Trade in Endangered Species of Wild Fauna and Flora). It is fully protected legally in all countries. It is protected in Liberia under the Wildlife and National Park Act of 1988 but enforcement of the regulations is loose except in Sapo National Park, where protection is good.
The latest edition of the International Studbook for the Pygmy Hippopotamus, which was published by Basel Zoological Gardens in 1991 (Tobler, 1991), lists a total of 820 animals. Of this number, 340 (131 males, 206 females and 3 of unknown sex) were alive on 31st December 1990 and were held in 131 collections throughout the world. The species breeds freely in captivity and most, if not all, of the specimens listed have been born in zoos to captive-bred parents. The world population of captive born animals has more than doubled since 1970.
Whilst the future of the species in captivity seems assured, the conditions under which it is kept need reconsideration given that most collections consist of a pair that are kept permanently together in a pool of water. Evidence from the wild suggests that pygmy hippopotamus come together infrequently and do not spend much time immersed in the water. The causes of death mentioned in the Studbook include many references to attacks by mates, maternal neglect and injuries inflicted by the mother. It is possible that many of these deaths are due to stress from the artificial conditions of captivity and greater attention to the way of life in the wild might help to reduce this mortality.
Conservation Measures Proposed:
An Action Plan
The future of the species in captivity seems more assured than in the wild. The biggest threat, although not immediate, is deforestation in Liberia. There is probably little that can be done at present to halt this universal trend, particularly given the anarchic situation in the country, but areas of forest least at risk and containing pygmy hippopotamus should be identified and conservation effort concentrated in such places. Sapo National Park is one such area. It is well protected and is known to contain the species for there have been two recent sightings in the park.
In view of the near-restriction of the pygmy hippopotamus to Liberia, it is in that country that any action plan should be concentrated but what is recommended for Liberia applies equally to other countries where the species occurs. There is little that can be done immediately and a conservation program cannot be effected until political stability returns to the country.
1. To ensure, as a first priority, that the species can continue to survive in the Liberian forests without further reduction or fragmentation of its range.
2. To establish more precisely the distribution and numbers of the species throughout its range but more particularly in Liberia, where the bulk of the population occurs.
3. To identify secure regions where conservation action can be concentrated.
4. To establish whether or not the isolated population reported from Nigeria still exists and if it does, to develop plans for its enhanced future protection.
(The alleged population in Guinea-Bissau is so improbable that the time and money that would be involved in an attempt to establish its existence are unlikely to be justified.)
1. Establish a reliable method for assessing the sizes of the various populations.
It is unlikely that such an elusive creature can be counted accurately and attention should be paid to developing indirect techniques that will provide an index of density, as has been done with forest elephants. These may include, for example, counts of dunging areas, trails or nest sites.
2. Identify and give special protection to areas containing adequate populations of the species and which appear free from the threat of deforestation.
This does not necessarily mean according them national park status, which might be difficult to achieve. In any case, even if new parks were created they might not be large enough to contain viable populations. As the only national park in Liberia, however, special attention should be given to Sapo National Park particularly as the species was recently recorded there.
3. Monitor the species in protected areas on a permanent basis using techniques developed for census purposes.
4. Identify potential threats to the species in each area and take steps to remove them.
Apart from the obvious problem of deforestation, attention should be paid to possible threats from meat hunting and the trophy trade. Education should play a prominent role in such projects in making local people aware of the rarity and uniqueness of the pygmy hippopotamus.
5. Mount expeditions to those regions of Nigeria where the species was last reported in order to look for evidence of its continued existence.
If it is shown to survive there, special efforts should be made to assist the development of management strategies for its enhanced future protection and to determine the taxonomic as well as the conservation status of this population.
6. Coordinate the international captive breeding effort to take advantage of recent computer programmes for analyzing stud book data and to ensure that maximum use is made of the genetic potential of the existing captive population.
7. Study the behaviour of the species under a variety of captive conditions in order to generate information of benefit to their enhanced future husbandry, with particular reference to the habits of the animals in the wild.
8. This project is suggested in the light of evidence that the species is not being maintained in captivity in the most appropriate conditions and social units.
I am grateful to William Oliver, Chairman of the IUCN/SSC Pigs and Peccaries Specialist Group, for his cooperation in the organization of this survey and to him and Alastair Macdonald for their critical reviews of earlier drafts. I am also greatly indebted to Paul Vercammen and Perter Cuypers for their assistance in preparing the range map, and to the following persons for providing information about the distribution and status of the pygmy hippopotamus: F. O. Amubode, W. F. H. Ansell, A. Blom, G. Davies, M. E. J. Gore, L. Macky, A. L. Peal and G. Teleki.
Anstey, S. 1991. Large mammal distribution in Liberia. (Unpubl.) rep. to W.W.F.- International, Gland: 81 pp.
Corbet, G. B. 1969. The taxonomic status of the pygmy hippopotamus, Choeropsis liberiensis, from the Niger Delta. J. Zool., 158: 387-394.
Cristino, J. J. de SA E. Melo. 1958. Statut des ongules en Guinee Portugaise. Mammalia, 22: 387-389.
Heslop, I. R. P. 1945. The pygmy hippopotamus in Nigeria Field, (Nigeria) 185: 629-630.
IUCN 1990. 1990 IUCN Red List of Threatened Animals. IUCN, Gland and Cambridge: pp. xviii-iv & 26.
Lang, E. M. 1975. Das Zwergflusspferd Choeropsis liberiensis. Zeiman Verlag, Wittenberg: 63 pp.
Langer, P. 1988. The Mammalian Herbivore Stomach. Gustav Fischer, Stuttgart: pp. 177-207.
Ritchie, J. 1930. Distribution of the pygmy hippopotamus. Nature, 126: 204-205.
Robinson, P. T. 1970. The status of the pigmy hippopotamus and other wildlife in West Africa. (Unpubl.) M. S. Thesis, University of Michigan: 80 pp.
Robinson, P. T. 1981. Bibliography for the pygmy hippopotamus Choeropsis liberiensis (Morton, 1844). (Unpubl.) rep. to IUCN/SSC Hippo Specialist Group: 8 pp.
Tobler, K. 1991. International Studbook for the Pygmy Hippopotamus Choeropsis liberiensis (Morten 1844). Zoologischer Garten, Basel.
White, L. 1986. Population survey of the pygmy hippopotamus (Choeropsis liberiensis) on Tiwai Island, Sierra Leone. (Unpubl.) rep., Sierra Leone Expedition 1986, University College London: 24 pp.