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.
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.
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
Figure 3: Relationship between body weight and age
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.