Hippopotamus amphibius
TAXONOMY
Hippopotamus amphibius Linnaeus, 1758, Nile River, Egypt. Five subspecies have been described, though it is doubtful that they are valid. No external differences between them have been noted.
OTHER COMMON NAMES
English: River hippopotamus; French: Hippopotame; German: Grossflusspferd; Spanish: Hipopotamo.
PHYSICAL CHARACTERISTICS
The common hippo has a shoulder height of 54–60 in (137–152 cm), length up to 106 in (270 cm), and weighs up to 4,000 lb (1,800 kg). It has a rotund body with disproportionately short legs. The thick purple-brown skin appears hairless, with only a few bristles around the mouth and on the tail, but there is a covering of very fine hairs at low density over the whole of the body.
The head is large with a wide mouth that opens to a gape of nearly 180°. The number of teeth is slightly reduced in the common hippo to (I2/2, C1/1, P3/3, M3/3) × 2 = 36, although sometimes the fourth milk premolar is retained in the adult jaw.
DISTRIBUTION
The common hippopotamus occurs in some 35 sub-Saharan countries, but many populations are extremely small, especially in West Africa, where the hippos are fragmented into isolated groups each containing a few dozen at the most. Countries with substantial populations are situated in the eastern and southern parts of the continent of Africa and include Zambia, Democratic Republic of the Congo (Zaire), and Tanzania.
HABITAT
During the day, the hippo is found in shallow freshwater aquatic habitats, which may be rivers, lakes, wallows, or any wetland that keeps the skin moist. Wet mud is suitable, and hippos often occur in wallows that are drying out. At night, the hippo leaves the water to graze in grasslands that may be a mile (1.6 km) or more from its daytime retreat. It reaches the grazing areas by following well-worn trails.
BEHAVIOR
There have been few investigations into the behavior of the hippo as it is a difficult species to study and conclusions about its activities may not be of general applicability. The most detailed study, set in Uganda, reported that although the hippo is highly colonial in the water, it shows little in the way of a social life and is essentially solitary when grazing at night. The only social bond is that between a female and her dependent calves. Males are territorial in the water and maintain mating rights over the females within a defended length of shoreline. It is not a harem system, however, for the females do not necessarily return to the same territory every day. The territory may contain bachelor groups, which are tolerated by the territorial male provided they behave submissively on meeting. Fierce fighting may occur if a bachelor challenges the territory holder and the death of one of the combatants may follow. Hippos are not territorial on land. The spraying of bushes with feces occurs when the hippo vigorously wags its tail while defecating, thus spreading the droppings far and wide. The purpose is not clear but it may have a social function as a dominance display or possibly the deposits may serve to orientate the hippo during its nocturnal wanderings.
The bellows of hippos are well-known, though their role in communication is not clear. In addition to the aerial sounds, hippos vocalize under water, producing at least three types of call, including tonal whines, pulsed croaks, and clicks, all of which appear to be associated with communication. Some calls are amphibious and pass through the air and water simultaneously.
FEEDING ECOLOGY AND DIET
The common hippopotamus is predominantly a grazer. It plucks the grass with its lips, nipping it off close to the ground thereby creating short-grass patches known as hippo lawns. Relative to its weight, the hippo consumes less food than other herbivores, possibly reflecting its lower metabolic rate. The hippo is able to extend its grazing range in the wet season by making use of temporary wallows for daytime resting thus obviating the need to trek back to permanent water every day. However, it does not feed on aquatic vegetation to any extent. Carnivory, mainly through scavenging, has been reported several times, including a case of cannibalism.
REPRODUCTIVE BIOLOGY
Polygamous. The hippo mates in the water with the female remaining submerged other than when she breaks surface to
breathe. The gestation period of 240 days is short for such a large animal. The female withdraws from the herd before giving birth, which usually takes place in the water. The weight of the neonate is about 110 lb (50 kg). Normally, only one calf is produced at a time, though twins do occur at a low rate, which in Uganda was found to be 0.7%. The sex ratio at birth is 1:1.
Births occur throughout the year on the equator, but there are peaks associated with rainfall, with most births taking place at the beginning of the wet season. Away from the equator, where rainfall is more seasonal, there are some dry season months when there are no births at all.
The duration of lactation is unknown; it is probably of the order of one year. The calf remains with its mother after the second calf is born, but leaves before the onset of puberty, which in Uganda was estimated to be at about 7.5 years of age in the male and nine years in the female.
CONSERVATION STATUS
The common hippo is placed on Appendix II of CITES. It is not in immediate danger over much of its range in southern and east Africa, but the outlook in West Africa is bleak. The total number is low, and the remaining animals are scattered in small groups in which genetic problems resulting from inbreeding may be expected.
SIGNIFICANCE TO HUMANS
The hippo is a dangerous animal that is responsible for numerous human deaths each year, particularly of fishermen. It also raids crops, particularly rice. It is, in turn, hunted for meat and ivory; some of the human deaths occur during attempts to harpoon the animal. The hippo features in African folklore.
