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Although not strictly nocturnal, hippopotamuses typically forage for food at night, and spend the day digesting their food, sleeping and socialising (2) (8). Diet mainly comprises grass, although isolated incidence of scavenging on carcasses has been observed (4) (9). On land, hippos disperse individually to forage, with the exception of mothers and dependent offspring (2). Water habitat is partitioned into individual mating territories by mature bulls (2). Herds within these territories typically number 10 to 15 individuals, but vary from 2 to 50 and can number in the hundreds at highest density when standing water availability declines. The groups are primarily made up of females and their young and are headed by a dominant male, although some subordinate, non-breeding males may also be tolerated in the group (2) (7). Adult males vie for control of these herds with intense aggression, using their long canine teeth in threat displays and as weapons (8). Losing males are forced to retreat and live in bachelor herds or alone in marginal habitat (2) (8). Although capable of breeding year-round, seasonal birth peaks coincide with peak rainfall (8). After a gestation of around 240 days, the cow gives birth to a single calf, generally underwater (4) (8). Young remain with their mother for many years, with cows being observed with up to four successive offspring. However, small calves are also often left in 'creches', which are guarded by one to several cows while the mothers forage (2). Males reach sexual maturity at between seven and nine years of age, whilst females attain maturity at eight to ten years (9).

The hippopotamus is found in a number of protected areas (2) and breeds readily in captivity, making captive-breeding and reintroduction programmes a viable conservation measure in the future should numbers become critically low in the wild. However, habitat protection is currently a more pressing priority, including measures to prevent the drying-up of water courses and loss of grazing ground. An Action Plan has been drawn up for this species, which aims to ensure that viable populations survive in each of the states in which it is currently found. Sadly, many of the sub-populations in West Africa now contain fewer than 50 individuals each, well below the minimum considered viable in the long-term. A principle objective is therefore to increase these group sizes to 500, at which number they are considered reasonably free from the risk of extinction. However, with the current climate of hippopotamus-human conflict, which is surely set to grow as populations expand and proximity increases, conservation efforts must take into consideration the welfare of human populations and local economies. If conflict issues could be addressed and reconciled, and greater local support gained, this unusual amphibious mammal would have a much better chance of long-term survival in the wild (6).

Despite its massive bulk, this amphibious mammal moves underwater with grace, and trots on land with surprising speed (2) (4). Indeed, the name hippopotamus means 'river horse', pertaining to this species' semi-aquatic lifestyle (7). An extremely large animal with a round, barrel-shaped body, short legs and a large, broad head. The body is a greyish to muddy-brown colour on top and a pale pink colour underneath (3) (7) (8). The broad mouth can be opened extremely wide to expose large, curved canines, used in aggressive displays (3). The eyes, ears and nostrils protrude on the top of the head, allowing the animal to remain receptive to its surroundings and breathe while otherwise almost totally submerged underwater (3) (4). The hippopotamus' virtually hairless skin is moistened by a secreted pink, oily substance that protects the skin from sunburn and drying, and perhaps infection (2) (3).

Hippopotamuses require water deep enough to cover them, in rivers and lakes that are close to pasture for grazing (2) (8). Rapids are avoided, with gently-sloping, firm-bottomed beds preferred, where herds can rest half-submerged and calves can nurse without swimming. Submersion in water is necessary to prevent the thin, hairless skin from overheating and dehydration occurring (2).

Historically, hippopotamuses have been found throughout sub-Saharan Africa, but most populations have now disappeared or are greatly reduced in size. The largest current populations remain in the East African countries including Tanzania, Zambia and Mozambique (8) (9).

Classified as Vulnerable (VU) on the IUCN Red List (1), and listed on Appendix II of CITES (5). Five subspecies have been described, although the validity of these classifications has been questioned. These are H. a. amphibius, H. a. tschadensis, H. a. kiboko, H. a. constrictis and H. a. capensis (6).

The principle threats to the hippopotamus are loss of essential grazing lands to cultivation and encroaching human settlement and unregulated or illegal hunting (6) (9). Due to increased development and human population growth, these large animals have also run into frequent conflict with humans, and have been said to kill more people in Africa than any other animal, attacking when feeling threatened (3). The species is a notorious crop-raider and can cause extensive damage through grazing and trampling. In certain countries, farmers can file a complaint about hippopotamuses damaging their crops, after which officials can legally kill the offending animals. Unfortunately, farmers have sometimes been suspected of filing false claims of damage so as to not have to worry about this potential threat in the future (9). Unregulated or illegal hunting is also a significant threat, with this enormous animal being prized by hunters for its meat, skins and ivory (6) (8) (9). The ban on international trade in elephant ivory has led to the increased exploitation of the carvable canine teeth of hippopotamuses, which can measure upward of 60 centimetres in length and are not subject to the same import/export restrictions. An increase of 530 percent in the annual export of hippo teeth ensued within two years of the elephant ivory ban taking effect (9). Hippo populations have been decimated in former stronghold areas by unregulated hunting for bush-meat and for ivory, most notably in the Democratic Republic of Congo. Once home to more than 25,000 hippos, this population may have been hunted to less than 2000 over the past 15 years as a result of intense hunting pressure during more than eight years of civil unrest and fighting (9).