The
pygmy hippopotamus (
Choeropsis liberiensis or
Hexaprotodon liberiensis) is a small
hippopotamid which is native to the forests and swamps of
West Africa, primarily in
Liberia, with small populations in
Sierra Leone,
Guinea, and
Ivory Coast.
The pygmy hippo is reclusive and
nocturnal. It is one of only two extant species in the
family Hippopotamidae, the other being its much larger relative, the common
hippopotamus (
Hippopotamus amphibius) or Nile hippopotamus. The pygmy hippopotamus displays many terrestrial
adaptations,
but like the hippo, it is semiaquatic and relies on water to keep its
skin moist and its body temperature cool. Behaviors such as mating and
giving birth may occur in water or on land. The pygmy hippo is
herbivorous, feeding on ferns,
broad-leaved plants,
grasses, and
fruits it finds in the forests.
A rare
nocturnal
forest creature, the pygmy hippopotamus is a difficult animal to study
in the wild. Pygmy hippos were unknown outside West Africa until the
19th century. Introduced to zoos in the early 20th century, they breed
well in captivity and the vast majority of research is derived from zoo
specimens.
[3] The survival of the species in captivity is more assured than in the wild; the
World Conservation Union estimates that fewer than 3,000 pygmy hippos remain in the wild.
[1]
Pygmy hippos are primarily threatened by loss of habitat, as forests
are logged and converted to farm land, and are also vulnerable to
poaching, hunting for
bushmeat, natural predators, and war. Pygmy hippos are among the species
illegally hunted for food in Liberia.
[4]
Taxonomy and origins
Nomenclature of the pygmy hippopotamus reflects that of the
hippopotamus. The plural form is pygmy hippopotami (
hippopotamuses is also accepted as a
plural form by the
OED, or
pygmy hippos for short). A male pygmy hippopotamus is known as a
bull, a female as a
cow, and a baby as a
calf. A group of hippopotami is known as a
herd or a
bloat.
[5]
The pygmy hippopotamus is a member of the family
Hippopotamidae where it is classified as a member of either the
genus Choeropsis ("resembling a
hog") or, the genus
Hexaprotodon ("six front teeth"). Members of Hippopotamidae are sometimes known as
hippopotamids. Sometimes the sub-family
Hippopotaminae is used. Further, some taxonomists group hippopotami and
anthracotheres in the superfamily
Anthracotheroidea or
Hippopotamoidea.
A sister species of the pygmy hippopotamus may have been the little-studied
Malagasy pygmy hippopotamus (
Hexaprotodon madagascariensis or
Hippopotamus madagascariensis), one of three recently extinct
species from Madagascar.
C. madagascariensis was the same size as
C. liberiensis
and shared its terrestrial behavior, inhabiting the forested highlands
of Madagascar, rather than open rivers. It is believed to have gone
extinct within the last 500 years.
[6][7][8]
The
taxonomy of the genus of the pygmy hippopotamus has changed as understanding of the animal has developed.
[1][9][10][11] Samuel G. Morton initially classified the animal as
Hippopotamus minor, but later determined it was distinct enough to warrant its own genus, and labeled it
Choeropsis. In 1977,
Shirley C. Coryndon proposed that the pygmy hippopotamus was closely related to
Hexaprotodon, a genus that consisted of prehistoric hippos mostly native to Asia.
[12]
This assertion was widely accepted,
[1][9][10][11] until Boisserie asserted in 2005 that the pygmy hippopotamus was not a member of
Hexaprotodon,
after a thorough examination of the phylogeny of Hippopotamidae. He
suggested instead that the pygmy hippopotamus was a distinct genus, and
returned the animal to
Choeropsis.
[9][13][14] All agree that the modern pygmy hippopotamus, be it
H. liberiensis or
C. liberiensis, is the only extant member of its genus.
[9][12]
Nigerian subspecies
A
distinct subspecies of pygmy hippopotamus existed in Nigeria until at
least the 20th century, though the validity of this has been questioned.
[1] The existence of the subspecies, makes
Choeropsis liberiensis liberiensis (or
Hexaprotodon liberiensis liberiensis under the old classification) the full
trinomial nomenclature
for the Liberian pygmy hippopotamus. The Nigerian pygmy hippopotamus
was never studied in the wild and never captured. All research and all
zoo specimens are the Liberian subspecies. The Nigerian subspecies is
classified as
C. liberiensis heslopi.
[10]
The Nigerian pygmy hippopotamus ranged in the
Niger River Delta, especially near
Port Harcourt,
but no reliable reports exist after the collection of the museum
specimens secured by I. R. P. Heslop, a British colonial officer, in the
early 1940s. It is probably extinct.
[1] The subspecies is separated by over 1,800 km (1,100 mi) and the
Dahomey Gap,
a region of savanna that divides the forest regions of West Africa. The
subspecies is named after I. R. P. Heslop, who claimed in 1945 to have
shot a pygmy hippo in the Niger Delta region and collected several
skulls. He estimated that perhaps no more than 30 pygmy hippos remained
in the region.
[15]
Heslop reportedly sent four pygmy hippopotamus skulls he collected to
the British Museum of Natural History in London. These specimens were
not subjected to taxonomic evaluation, however, until 1969 when G. B.
Corbet classified the skulls as belonging to a separate subspecies based
on consistent variations in the proportions of the skulls.
[16] The Nigerian pygmy hippos were seen or shot in
Rivers State,
Imo State and
Bayelsa State,
Nigeria. While some local populations are aware that the species once existed, its history in the region is poorly documented.
[10]
Evolution
The evolution of the pygmy hippopotamus is most often studied in the
context of its larger cousin. Both species were long believed to be most
closely related to the family
Suidae (
pigs and hogs) or Tayassuidae (
peccaries), but research within the last 10 years has determined that pygmy hippos and hippos are most closely related to
cetaceans (
whales and
dolphins). Hippos and whales shared a common semi-aquatic ancestor that branched off from other artiodactyls around
60 mya.
[17][18]
This hypothesized ancestor likely split into two branches about six million years later.
[19] One branch would
evolve into cetaceans, the other branch became the
anthracotheres, a large family of four-legged beasts, whose earliest member, from the Late
Eocene, would have resembled narrow hippopotami with comparatively small and thin heads.
[13]
Hippopotamids are deeply nested within the family
Anthracotheriidae. The oldest known hippopotamid is the genus
Kenyapotamus, which lived in Africa from
16 to 8 mya.
Kenyapotamus is known only through fragmentary fossils, but was similar in size to
C. liberiensis.
[14]
The Hippopotamidae are believed to have evolved in Africa, and while at
one point the species spread across Asia and Europe, no hippopotami
have ever been discovered in the Americas. Starting
7.5 to 1.8 mya the
Archaeopotamus, likely ancestors to the genus
Hippopotamus and
Hexaprotodon, lived in Africa and the Middle East.
[9]
While the fossil record of hippos is still poorly understood, the lineages of the two modern genera,
Hippopotamus and
Choeropsis, may have diverged as far back as
8 mya. The ancestral form of the pygmy hippopotamus may be the genus
Saotherium.
Saotherium and
Choeropsis are significantly more
basal than
Hippopotamus and
Hexaprotodon, and thus more closely resemble the ancestral species of hippos.
[9][14]
Extinct pygmy and dwarf hippos
Several species of small hippopotamids have also become extinct in the Mediterranean in the late
Pleistocene or early
Holocene. Though these species are sometimes known as "pygmy hippopotami" they are not believed to be closely related to
C. liberiensis. These include the
Cretan dwarf hippopotamus (
Hippopotamus creutzburgi), the Sicilian hippopotamus (
Hippopotamus pentlandi), the Maltese hippopotamus (
Hippopotamus melitensis) and the
Cyprus dwarf hippopotamus (
Hippopotamus minor).
[20]
These species, though comparable in size to the pygmy hippopotamus,
are considered dwarf hippopotamuses, rather than pygmies. They are
likely descended from a full-sized species of
European hippopotamus, and reached their small size through the evolutionary process of
insular dwarfism which is common on islands; the ancestors of pygmy hippopotami were also small and thus there was never a dwarfing process.
[20] There were also several species of pygmy hippo on the island of Madagascar (see
Malagasy hippopotamus).
Description
Pygmy hippos share the same general form as a hippopotamus. They have
a graviportal skeleton, with four stubby legs and four toes on each
foot, supporting a portly frame. The pygmy hippo, however, is only half
as tall as the hippopotamus and weighs less than 1/4 as much as its
larger cousin. Adult pygmy hippos stand about 75–100 cm (2.46–3.28 ft)
high at the shoulder, are 150–175 cm (4.92–5.74 ft) in length and weigh
180–275 kg (397–606 lb).
[21] Their lifespan in captivity ranges from 30 to 55 years, though it is unlikely that they live this long in the wild.
[10][22]
The skin is greenish-black or brown, shading to a creamy gray on the
lower body. Their skin is very similar to the common hippo's, with a
thin
epidermis over a
dermis that is several centimeters thick. Pygmy hippos have the same unusual
secretion
as common hippos, that gives a pinkish tinge to their bodies, and is
sometimes described as "blood sweat" though the secretion is neither
sweat nor
blood. This substance,
hipposudoric acid, is believed to have antiseptic and
sunscreening properties. The skin of hippos dries out quickly and cracks, which is why both species spend so much time in water.
[10]
The skeleton of
C. liberiensis is more
gracile
than that of the common hippopotamus, meaning their bones are
proportionally thinner. The common hippo's spine is parallel with the
ground; the pygmy hippo's back slopes forward, a likely
adaptation
to pass more easily through dense forest vegetation. Proportionally,
the pygmy hippo's legs and neck are longer and its head smaller.
[22]
The
orbits and
nostrils
of a pygmy hippo are much less pronounced, an adaptation from spending
less time in deep water (where pronounced orbits and nostrils help the
common hippo breathe and see). The feet of pygmy hippos are narrower,
but the toes are more spread out and have less webbing, to assist in
walking on the forest floor.
[22]
Despite adaptations to a more terrestrial life than the common hippopotamus, pygmy hippos are still more aquatic than all other
even-toed ungulates.
The ears and nostrils of pygmy hippos have strong muscular valves to
aid submerging underwater, and the skin physiology is dependent on the
availability of water.
[10][11]
Behavior
The behavior of the pygmy hippo differs from the common hippo in many ways. Much of its behavior is more similar to that of a
tapir, though this is an effect of
convergent evolution.
[11]
While the common hippopotamus is gregarious, pygmy hippos live either
alone or in small groups, typically a mated pair or a mother and calf.
Pygmy hippos tend to ignore each other rather than fight when they meet.
Field studies have estimated that male pygmy hippos range over 1.85 km
2 (460 acres), while the range of a female is 0.4 to 0.6 km
2 (99–148 acres).
[10]
Pygmy hippos spend most of the day hidden in rivers. They will rest
in the same spot for several days in a row, before moving to a new spot.
At least some pygmy hippos make use of dens or
burrows
that form in river banks. It is unknown if the pygmy hippos help create
these dens, or how common it is to use them. Though a pygmy hippo has
never been observed burrowing, other artiodactyls, such as
warthogs, are burrowers.
[10]
Diet
Like the
common hippopotamus, the pygmy hippo emerges from the water at dusk to
feed. It relies on game trails to travel through dense forest
vegetation. It marks trails by vigorously waving its tail while
defecating to further spread its feces. The pygmy hippo spends about six
hours a day foraging for food.
[10]
Pygmy hippos are
herbivorous.
They do not eat aquatic vegetation to a significant extent and rarely
eat grass because it is uncommon in the thick forests they inhabit. The
bulk of a pygmy hippo's diet consists of
ferns,
broad-leaved plants and
fruits
that have fallen to the forest floor. The wide variety of plants pygmy
hippos have been observed eating suggests that they will eat any plants
available. This diet is of higher quality than that of the common
hippopotamus.
[10]
Reproduction
A study of breeding behavior in the wild has never been conducted;
the artificial conditions of captivity may cause the observed behavior
of pygmy hippos in zoos to differ from natural conditions. Sexual
maturity for the pygmy hippopotamus occurs between three and five years
of age.
[11] The youngest reported age for giving birth is a pygmy hippo in the
Zoo Basel,
Switzerland which bore a calf at three years and three months.
[10] The
oestrus cycle of a female pygmy hippo lasts an average of 35.5 days, with the oestrus itself lasting between 24–48 hours.
[1][23]
Pygmy hippos consort for mating, but the duration of the relationship
is unknown. In zoos they breed as monogamous pairs. Copulation can take
place on land or in the water, and a pair will mate one to four times
during an oestrus period. In captivity, pygmy hippos have been conceived
and born in all months of the year.
[11] The gestation period ranges from 190–210 days, and usually a single young is born, though twins are known to occur.
[10]
The common hippopotamus gives birth and mates only in the water, but
pygmy hippos mate and give birth on both land and water. Young pygmy
hippos can swim almost immediately. At birth, pygmy hippos weigh
4.5–6.2 kg (9.9–13.7 lb) with males weighing about 0.25 kg (0.55 lb)
more than females. Pygmy hippos are fully
weaned
between six and eight months of age; before weaning they do not
accompany their mother when she leaves the water to forage, but instead
hide in the water by themselves. The mother returns to the hiding spot
about three times a day and calls out for the calf to suckle. Suckling
occurs with the mother lying on her side.
[10]
Conservation
Pair at the Mount Kenya Wildlife Conservancy
The greatest threat to the remaining pygmy hippopotamus population in
the wild is loss of habitat. The forests in which pygmy hippos live
have been subject to logging, settling and conversion to agriculture,
with little efforts made to make logging sustainable. As forests shrink,
the populations become more fragmented, leading to less genetic
diversity in the potential mating pool.
[1]
Pygmy hippos are among the species
illegally hunted for food in Liberia.
[4]
Their meat is said to be of excellent quality, like that of a wild
boar; unlike those of the common hippo, the pygmy hippo's teeth have no
value.
[11] The effects of West Africa's civil strife on the pygmy hippopotamus are unknown, but unlikely to be positive.
[1] The pygmy hippopotamus can be killed by
leopards,
pythons and
crocodiles. How often this occurs is unknown.
[10]
C. liberiensis was identified as one of the top-10 "focal species" in 2007 by the
Evolutionarily Distinct and Globally Endangered (EDGE) project.
[24] Some populations inhabit protected areas, such as the
Gola Forest Reserve in Sierra Leone.
[25]
Basel Zoo in Switzerland holds the international
studbook
and coordinates the entire captive pygmy hippo population that freely
breeds in zoos around the world. Between 1970 and 1991 the population of
pygmy hippos born in captivity more than doubled. The survival of the
species in zoos is more certain than the survival of the species in the
wild.
[15][22] In captivity, the pygmy hippo lives from 42 to 55 years, longer than in the wild.
[10] Since 1919, only 41 percent of pygmy hippos born in zoos have been male.
[23]
History and folklore
Pair at the Mount Kenya Wildlife Conservancy
While the common hippopotamus was known to Europeans since
classical antiquity,
the pygmy hippopotamus was unknown outside its range in West Africa
until the 19th century. Due to their nocturnal, forested existence, they
were poorly known within their range as well. In Liberia the animal was
traditionally known as a
water cow.
[11]
Early field reports of the animal misidentified it as a
wild hog. Several skulls of the species were sent to the American natural scientist
Samuel G. Morton, during his residency in
Monrovia,
Liberia.
Morton first described the species in 1843. The first complete
specimens were collected as part of a comprehensive investigation of
Liberian fauna in the 1870s and 1880s by Dr.
Johann Büttikofer. The specimens were taken to the
Natural History Museum in
Leiden, The
Netherlands.
[11]
The first pygmy hippo was brought to Europe in 1873 after being
captured in Sierra Leone by a member of the British Colonial Service but
died shortly after arrival. Pygmy hippos were successfully introduced
to Europe in 1911. They were first shipped to Germany and then to the
Bronx Zoo in
New York City where they also thrived.
[10][11]
In 1927,
Harvey Firestone of
Firestone Tires presented
Billy the pygmy hippo to U.S. President
Calvin Coolidge. Coolidge donated Billy to the
National Zoo in Washington, D.C. According to the zoo, Billy is a common ancestor to most pygmy hippos in U.S. zoos today.
[22][26]
Several folktales have been collected about the pygmy hippopotamus.
One tale says that pygmy hippos carry a shining diamond in their mouths
to help travel through thick forests at night; by day the pygmy hippo
has a secret hiding place for the diamond, but if a hunter catches a
pygmy hippo at night the diamond can be taken. Villagers sometimes
believed that baby pygmy hippos do not nurse but rather lick secretions
off the skin of the mother.
[11]
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