The hippopotamus (Hippopotamus amphibius Linnaeus 1758) evolved with the other live-bearing mammals (Theria) from the Cynodonts (mammal-like reptiles) of the Triassic (225-195 million years ago). First to diverge from the stem were egg-laying mammals (Monotremes) during the Jurassic age. In the early Cretaceous era (about 100 million years ago), the Theria diverged into 3 major groups, one of which was the Marsupials. The other two became the placental mammals (Eutheria).

The southern landmass Gondwana became isolated from the northern continent at this time and the Atlantogenata (the Afro-Arabian group including the Afrotheria and Xenarthrans) evolved separately from the northern group of the Laurasiatheria, the Glires and the Euarchonta. These last two groups diverged from the Laurasiatheria in the mid- Cretaceous period. Further divergences took place amongst the Laurasiatheria before the end of the Cretaceous era, the first being the separation of pangolins and carnivores followed by the Perissodactyla (odd-toed ungulates) and bats. The progenitors of the modern Artiodactyla (even-toed ungulates) separated from the Cetacea (whales and dolphins) before the start of the Paleocene.

Within the Artiodactyla order, the suborders Tylopoda (camels and llamas) and Ruminantia (ruminants) separated from the suborder Suina during the mid-Eocene (34-55 million years ago), followed by the Suidae (pigs) and Tayassuidae (peccaries). The earliest recognisable hippo forebears date back to the Miocene (Smithers 1983) and it may have been that Hippopotamus and Hexaprotodon (the Pygmy Hippo - a separate genus) diverged at that time. No subspecies of Hippopotamus are currently recognised although Lydekker (1915) listed five and Ansell (1972) listed four.

Physical description

The common hippopotamus has a barrel-shaped body, smooth hairless skin and short stout legs. It is amongst the largest mammals with males achieving body weights greater than 2,500kg (Parker 2005). The tail is abbreviated and flattened with a sparse fringe of bristles at the tip.

The head is broad and massive with the eyes, ears and nostrils (which can be closed) on top of the skull - an adaptation to spending most of the time semi-submerged in water. The skin has a unique structure which causes a high rate of water loss when exposed to the air - a further reason for remaining in water during the day. The colour of the body skin is greyishblack with a pink tinge; the skin around the eyes and ears is pinkish-yellow and the gape of the mouth is flesh-coloured.

The alimentary canal of the hippo is able to break down the tough cellulose which makes up a large part of its diet. The stomach consists of 4 chambers which function like those of ruminants with micro-organisms fermenting and producing enzymes which break down cellulose (Arman & Field 1973). Hippos do not 'chew the cud' and are known as 'pseudoruminants'. An 'average' hippo requires about 150kg of food (28 kg dry plant matter) daily (Pienaar et al 1966) which, although it may seem high, is less than 1.5% of their body weight and about half of that consumed by animals such as a white rhino.

The dentition of hippo is fully described by Laws (1968). The canines and incisors are enormously enlarged with the former being used exclusively for fighting and the latter primarily for digging. The lower canines are long and kept very sharp by continuous vertical wear against the short upper canines. There is considerable sexual dimorphism in the canine and incisor growth and the large protuberances on the front of the upper jaws of male hippo can be used to determine the sex of adult hippo when in water (Parker pers.comm.). Unlike the elephant, where a progression of six molars erupt from the posterior of the jaw and move along the mandible during the animal's lifetime, hippo molars are not replaced and only the premolars, canines and incisors go through an early deciduous stage. Laws (ibid) developed age criteria for hippo from the dentition (Analysis 1).

Using the data of Pienaar (et al 1966) of body lengths and total weight and the data of Laws (1968) of age and body length, Martin suggests a relationship between body weight and age of male and female hippo (Analysis 2) (Figure 3), which can be used to predict potential meat production from a hippo population.