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Diversity of Living Hippopotamuses

The family Hippopotamidae (hippopotamuses or hippos) is one of a number of families in the mammal order Artiodactyla (even-toed ungulates). Molecular phylogenetic studies have suggested that the closest living relatives of the hippos are the whales. These studies and other evidence (some going back to the 1880s) indicating a close relationship between artiodactyls and whales have led to the naming of the group Cetartiodactyla, a clade composed of the Cetacea (whales) + Artiodactyla.

There are just two extant hippo species, the Common Hippopotamus (Hippopotamus amphibius) and the Pygmy Hippopotamus (Choeropsis liberiensis). Common Hippos have short limbs and massive barrel-shaped bodies. Both sexes have extremely large jaws and can open their mouths to 180 degrees. The meaning of "hippopotamus" is " river horse", and both species require moist condition, but Pygmy Hippos spend much less time in the water than do Common Hippos and are more adapted to terrestrial life, with proportionately longer limbs and a smaller head. Pygmy Hippos have a single pair of lower incisors whereas Common Hippos have two or three pairs. Hippos are mostly hairless, with sparse hair on the snout and tail. Hippo skin contains large amounts of subcutaneous ("beneath the skin") fat and dense sheets of collagen; subdermal glands secrete "sweat". In Common Hippos, analysis of this "sweat" has indicated that it serves as both an antibiotic and a sunscreen (hippo skin dries out quickly and cracks when exposed to the sun). Both hippo species have strong muscular valves in the ears and nostrils that close when the animal is submerged. The nostrils, eyes, and ears are all at the top of the head, facilitating longer periods of submergence. Upon emerging from water, Common Hippos oftem produce a loud, forcible expulsion of air.  

Hippo Ecology and Behavior

Common and Pygmy Hippos have clearly different habitat preferences. Common Hippos are seen widely across sub-Saharan Africa, typically wallowing in large social groups in wetlands, rivers, and lakes. Herd size ranges from around 15 to 300. A typical herd consists of a dominant bull with a harem of cows and their young. "Bachelor herds" of adult males can often be found near mixed-sex herds. Often associated with hippos are birds such as Cattle Egrets (Bubulcus ibis), Sacred Ibises (Threskiornis aethiopicus), and Red-billed Oxpeckers (Buphagus erythrorhynchus). Pygmy Hippos are rarely seen as they are found, alone or in pairs (often a female and her young), in heavily forested lowlands that are remnants of once far more extensive forest complexes. Common Hippos typically spend most daylight hours submerged, emerging at night to feed on short green grass, as well as aquatic vegetation and farm crops. They may increase water flow in some wetlands by lumbering through loose sediment, preventing the establishment of vegetation that would otherwise reduce water flow.  Pygmy Hippos also spend the day near or in water sources such as swamps or streams, using game trails to move through the forest. Pygmy Hippos may use cavities from riverbank erosion as hiding places. In size, shape, behavior, and habitat, Pygmy Hippos bear a striking resemblance to tapirs, although they are not closely related and these similarities are the result of evolutionary convergence.

Common Hippos can reach 4500 kg, with adult males larger than females. Males also have much larger canines and incisors (these teeth are not used by either sex for feeding, but rather only for display, defense, and attack). When walking on land, Common Hippos help support their massive bodies by keeping three legs on the ground at all times. Underwater, hippos do not swim, but rather move along the substrate in a slow-motion gallup. Because their heads account for such a large proprtion of their weight (60% for Common Hippo), hippos use their forelegs more than their hindlegs to propel themselves over the bottom. As in other semi-aquatic mammals, hippo limb bones are denser than those of terrestrial mammals, helping to compensate for the buoyancy effect of water.

Common Hippos have an elaborate foregut fermentation system. They are thought to consume 20-45 kg of grass during a typical nocturnal foraging bout of five to eight hours. Pygmy Hippos are believed to be largely browsers, feeding on a wide variety of vegetation. Hippos use their muscular lips, rather than their teeth, to tear vegetation.

Common Hippo females become sexually mature between around seven and nine years old and males mature between around nine and eleven years. Females typically bear a single young after a gestation period of eight months, with a lactation period of eight (up to 18) months; young stay with their mothers for several years. Based on data from captive animals, Pygmy Hippo gestation is around six months and calves weigh about 5 kg at birth. Pygmy Hippos are believed to become sexually mature at three to five years (athough it may be that, as is the case for Common Hippos, captive animals reproduce at a younger age than wild ones). There is some evidence for infanticide in hippos, possibly by males assuming the role of dominant male in a new territory, as is seen in lions.

Conservation Status of Hippos

In some areas, Common Hippos are hunted for meat and for "ivory" from the large canine teeth (this ivory harvest has cost the lives of many thousands of hippos). Water diversion for agriculture and large-scale development in and around wetland areas have caused significant habitat loss. Pygmy Hippos have also suffered from both habitat destruction and hunting, athough habitat loss is clearly the major threat to this species. Common Hippos are found in at least 29 countries in sub-Saharan Africa, with the largest populations persisting in East Africa. Population declines are documented for at least half the countries in which they currently occur (most dramatically in DR Congo, thought at one time to have harbored the largest populations). Forests in the Pygmy Hippo's range have been steadily logged, farmed, and settled over the past century and, especially, from the late 20th century onward and this habitat loss has, in turn, increased access for poachers. Wide-ranging armed conflicts across Africa during the past half-century have devastated not only human populations, but many other species as well, including Pygmy Hippos. The range of the Pygmy Hippo is much reduced from historical times. It is now known from a fragmented range that includes parts of Guinea, Sierra Leone, Liberia, and Ivory Coast. The IUCN Red List lists Common Hippo as Vulnerable and Pygmy Hippo as Endangered.

(Lewison 2011 and references therein)

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