Family
Hippopotamidae
Hippopotamuses |
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The Hippopotamidae is a small family containing two extant species, both
of which are restricted in their present distribution to the African continent.
Both the very large river hippopotamus (Hippopotamus amphibius,
weighing in at up to 4,500 kg) and the smaller pygmy hippopotamus
(Hexaprotodon liberiensis, weighing up to 270 kg) have heavy,
barrel-shaped bodies supported by short, stubby legs. Both species
are amphibious: most terrestrial activity (foraging) occurs at night, with
the days being spent submerged in water.
This family originated in Africa during the early Miocene, and rapidly expanded
into Europe and Asia by the late Miocene. Fossils of at least three
species of hippopotamus are known from Pleistocene deposits of Madagascar.
Of the two modern species, the pygmy hippo is the more conservative;
indeed, there is no fossil record to indicate its ancestry. The Hippopotamidae
are placed within the suborder Cetancodonta along with whales and dolphins
(Cetacea) as the result of molecular research. The early fossil record
of this family (and suborder) is scarce, and thus the common origins of these
two family groups remain obscure.
The Hippopotamidae share several features with the Cetacea, although whether
they are shared traits or the result of evolutionary convergence is uncertain.
Besides (or because of) their (semi)aquatic existence, hippos and cetaceans
both possess internal testes, virtually naked skin (the only hairs in hippos
are found on the face and tail), and give birth in the water, with the infant
being delivered rear-end first (the opposite of most mammals). Recent
observations of hippos underwater have revealed vocalizations which sound
similar to the echolocation clicks used by cetaceans.
Like many semi-aquatic animals, the head and skull of hippos are distinct
in form, with elevated orbits (eyes), ears, and nostrils; these features
protrude above the surface of the water while the rest of the head and body
remain beneath the surface. The nostrils and ears can be closed while
underwater. The dental formula is I 2-3/1-3, C 1/1, P 4/4, M 3/3 x
2 = 38-44. Both the incisors and canines are tusk-like and grow
continuously, being used for intraspecific competition, not food acquisition
(the lips have a tough edge which is used to rip off vegetation). The
stomach of hippos is multichambered but not ruminating. The limbs are
robust and have four well-developed digits, each capped with a hoof-like
nail. The last phalanx of each toe touches the ground, while the rest
of the foot is supported by a pad of connective tissue; the foot posture
is semi-digitigrade. Hippos have a very short gestation period for
their size (only 6-8 months) |
The Hippopotamus Family
Tree
Click on the species above to learn more,
or jump to the Hippopotamidae
Species List
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Literature Cited
Estes, R. D. 1991. The Behavior Guide to African Mammals: including
hoofed mammals, carnivores, primates. Los Angeles: University of California
Press.
Kingdon J. 1997. The Kingdon Field Guide to African Mammals. London
and New York: Academic Press, NaturalWorld.
Martin, R. E., R. H. Pine, and A. F. DeBlase. 2001. A Manual
of Mammalogy, Third Edition. Boston: McGraw-Hill Publishing.
Nowak, R. M. [Editor]. 1991. Walker's Mammals of the World. Fifth Edition.
Johns Hopkins University Press, Baltimore.
Vaughan, T. A., J. M. Ryan, and N. J. Czaplewski. 2000. Mammalogy. Fourth
Edition. Saunders College Publishing, Philadelphia.
Wilson, D. E., and D. M. Reeder [editors]. 2005. Mammal Species of
the World (3rd Edition). Johns Hopkins University Press, 2,142 pp.
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