*New anatomical and DNA evidence on the relationship between Artiodactyla (even-toed ungulates) and Cetacea (whales and dolphins) recently led to a merging of the two orders into a new group, Cetartiodactyla (Montgelard, 1997; reviewed in Kulemzina, 2009). As of October 2012, experts had not agreed on whether to define Cetartiodactyla as an official taxonomic order that would replace Artiodactyla and Cetacea. Some continue to list hippopotamuses in the order Artiodactyla (Franklin, 2011) or use the term Cetartiodactyla without defining it as an order (IUCN, 2008).
Taxonomy, Phylogeny, and Historical Records
Hippopotamus: Hippos from Greek for horse, potamos from Greek for "river or rushing water"
Choeropsis: Meaning from the Greek: "having the appearance of a young pig" (Parker 2010)
Common name (for Common or River Hippo)
Common Hippopotamus, River Hippopotamus, or Nile Hippopotamus; In Afrikaan: seekoei (sea cow).
Height: Common Hippo 150-165 cm (4.9-5.4 ft) Pygmy Hippo
about half the height of Common Hippo Weight:Common Hippo males up to 1,475 kg (3253 lb);
Females 1,360 kg (655-2,344 kg). Pygmy Hippo about a sixth the weight of the larger Common Hippo Length:Common Hippo males 300-505 (9.8-16.6 ft), females 290-430 cm (9.5-14 ft) Pygmy Hippo males 157 cm ( in ) , females
up to 150 cm ( in) Tail:Common Hippo 28-35 cm (11-14 in)
flattened laterally and bristled Pygmy Hippo
16 cm (6.3 in) General
Measurements are subject to much regional variation.
Comparison of the two species:
Pygmy Hippo is smaller overall
Pygmy Hippo's head proportionally smaller and rounder
Pygmy Hippo's neck is proportionally longer
Pygmy Hippos skeleton more lightly built
Pygmy Hippo's back slopes forward, Common Hippo's is straight
Eyes of Pygmy Hippos less bulging, more on side of head (less aquatic in habits)
Muzzle is noticeably bigger in males and jowl area more
"Tusks" are twice as long in males
Body is barrel shaped, legs short
Brain case small
Jaws hinged far to the back, allowing enormous gape of 150° (humans have only 45°)
stomach; no caecum or gallbladder
Females have 2 mammae.
functional toes with wide splay
Nail-like hooves on ends of toes
Aquatic Adaptations of H. amphibius:
Head is adapted for submersion:
Eyes slightly periscoped and
aligned on top of head along with ears and nostrils
Nostrils close before submersion.
Ears fold into recessed area to close underwater
Jaw and ear similarities to cetaceans that are helpful in underwater hearing
(Barklow 1997) (Vaughan et al 2011)
Lower jaw bone (dentary) has flattened dish shaped area
Middle ear is suspended by ligaments
Fatty tissue connects middle ear to the dentary
Water-born sounds in dolphins travel from jaw bone to fat tissue to middle ear bones
Hippos may be hearing underwater sounds when lower jaw is below water.
Bones are dense (osteosclerotic) as seen in many aquatic and semiaquatic mammals
Partial webbing on feet may aid in stabilization while bottom walking
Pygmy Hippo feet little or no webbing
Many muscle groups and tendons in hind limbs are different from those of other artiodactyls (Fisher et al 2010)
Flexor and extensor muscles allow control and splaying of side digits of foot
Other artiodactyls: reduced lateral digits that don't bear weight and can't be splayed
For hippos, being able to splay digits is a good adaptation for walking in water on soft river bottoms
Other distinguishing features of hippo limb muscles are for powerful propulsion through water, but not swimming.
Muscles present represent "primitive" condition in artiodactyls; reflect ancient divergence of hippos from other artiodactyls
Additional aquatic adaptations of feet probably not seen, since hippos also need to walk on land.
Well-developed muscles for constricting airways in the lung's bronchioles also found in whales. (Cowan et al 1967)
Hippos display tusk-like canines sharpened
by wear against their upper teeth. To the front of
a pygmy hippo jaw are a single pair of incisors.
Common hippos with their wider jaws
have two or three pairs of incisors.
Canines and incisors enormously enlarged and continuously growing
Tusk-like canines are used for fighting
Male canines can grow to 50 cm
(1.5 ft) and are usually twice the length of female's
Kept sharpened by constant vertical wear against the shorter upper
Incisors used for digging
Jaws capable of 150 degree gape
(slightly less in pygmy hippo)
molars in back for chewing
Dental formulas differ in two species:
Larger Common Hippo has 2-3 pairs of
incisors; very wide mouth
Smaller Pygmy Hippo, 1 pair incisors; mouth less wide
Common Hippo Purple-gray or slate brown
Lower surfaces and skin around eyes and ears tend to be brownish pink
Pygmy Hippo Greenish black above, grey on sides, greyish-white below
Scantily covered with fine hairs except for short bristles on head, back and tail
No scent or sweat glands, but mucous glands secrete a thick, oily red
fluid "blood sweat"
(Galasso & Pichierri 2009)
Dries like lacquer and serves to
protect the thin epidermis against water loss, sunburn and infection
Hippos suffer many bite wounds from fighting, but red pigment's ("hipposudoric acid") antibiotic properties inhibit pathogenic bacteria
Epidermis (outer horny layer) is uniformly thin
The endodermis varies
from 5-6 cm on back and rump to < 1cm (.4 in) on head and belly
Albino hippos are common. (Pitman 1962)
BEHAVIOR & ECOLOGY
(Coughlin & Fish 2009) (Eltringham 1999, 2001) (Estes 1991)
(Kingdon 1979) (Klingel 1990, 1995) (Lang et al 1990) (Mosepele et al 2009) (Oliver 1993) (Roberts 1951)
Pygmy hippos often spend time resting and feeding on land.
Their behavior in the wild is still largely unknown.
Daily Pattern (Common Hippo)
Herd leaves water shortly before dark, following the same
branching well-trampled paths for 3-5 km (1.9-3.1 mi)
Grazing is a solitary activity (except for mothers with calves)
Usually graze approximately 5 hours except when threatened
Will also sleep on land at night, on sandy ground inside thickets (Klingel 1995)
Spends day submerged in deep water with only eyes and nostrils above
surface or basks in sun on sandbars adjacent to water.
climb on backs of mothers in water to sun/rest
If disturbed, entire group submerges, resurfacing after several minutes
with only the flat top of the head above water.
If water dries up or there is a shortage of food, long migrations
40-60 km (24-30 miles) may occur.
Pygmy Hippos spend day resting near moist or wet places, but also on dry ground
(Lang et al 1990)
Change sleeping place once or twice a week.
General (Common Hippo)
Gregarious (but not social; no permanent bonds between adults) and amphibious
Groups are changeable combinations rather
than fixed units
Basic social unit is mother and her young
Cows may be followed by up to 4 successive
Females will also tolerate unrelated young and subadult
Herd is generally 10-15 but can range from 2-50 (and reach up to 150)
Average density in lakes: 7 hippos/100 m shoreline
Average density in
rivers: 33 hippos/100 m shoreline
Pygmy Hippo is less gregarious; usually found solitary or in pairs;
like Common Hippos, also a nocturnal feeder on land
Little is known about behavior in wild; dens reported at base of erosion-eroded trees on river banks
Aggression (Common Hippo)
Males begin jaw-to-jaw sparring and "yawning" contests at
Aggressive behavior is most frequent in dry season when conditions
most crowded and breeding most conspicuous
Territorial bulls may attack and even kill calves
Mothers counterattack bulls, often successfully because they approach from the side
(don't follow the male's
ritualistic head-on battle procedure)
Fighting includes tusk clashing, rearing and pushing with the lower
jaws, slashing and biting
Bulky shape prohibits agility; contests consist of animals locking teeth and wrestling using
weight and strength (similar to deer locking antlers)
Territorial Behavior (Common Hippo)
Males are territorial only in water, not at night on land
Both males and females have similar daytime home ranges - about 200 m ( ft) along a shoreline
Territories defended for mating rights, not food
Males defend their immediate vicinity
Size of area and degree of
intolerance vary with local conditions (crowding, drought) and perhaps
female sexual cycles
Mature bulls control 50-100 m (55-109 yards) sections of river or 250-500 m (273-547 yards) of
lakeshore as exclusive mating territories
Grazing pastures not part of territory - free access for
Dung middens found along hippo paths leading into and out of
Constantly renewed at night as the bulls go to
pasture and during the day when they emerge and defecate on the heaps
Middens may reach as high as the hippo's anus
Ritualized dung-showering occurs between males at the borders of their
Two bulls turn sideways, bodies parallel, heads in
opposite directions and a distance of several feet between them.
Tail swings rapidly back and forth while their excrement (mixed with
urine) is showered for several feet in all directions.
Displays / Visual signals
Mother/calf bond is extremely close.
Mother licks, nuzzles, scrapes
calf with lower incisors
Discipline achieved by nudging or biting
Calf responds by prostrating itself; prostration gesture continues
into adulthood as sign of submission within herd.
Yawning, water-scooping and head-shaking,
Rearing, lunging, chasing
Roaring grunting, explosive wheezing.
Facing aggressor with open mouth
Urinating with slow tail-wagging,
Explosive exhalation of breath indicates alarm
Vocalization Common Hippo (Barklow 1994, 1997)
(Klingel 1990) (Vaughan et al 2011)
80% of hippo vocalizations are made underwater, in studies in Tanzania's Ruaha National Park
Complex bellows, shrieks and grunts are made both in and out of water
The exhaled bellow of a
dominant bull often triggers a deafening chorus from other dominant hippos
Sound carries at least a mile over the noise of the river
When a hippo resting at the surface
bellows, nostrils flare and sound is hummed through nose
and nasopharynx to the air
Contact call is a deep, choppy "o-o-o-o" often ending in a
High-pitched squeals made during threatening frontal
Produce at least three categories of sounds underwater
(Barklow 1995, 1997)
Rarely audible on surface; expel little air when made
High pitched tonal whines that are somewhat similar to humpback whale song notes
Pulsed croak between calves and sub-adults
Click-like sounds (but no evidence yet for echolocation)
Relatively silent on land; use exhaled breath to express threat and
Keen sense of smell.
Likely that individuals recognize each other by scent
Dung middens do not mark territories on land, as formerly assumed; function of middens not clear to researchers
Ritualized dung spraying
who may deliberately defecate on subordinate animals in
Subordinate males often spray faeces in the face of dominant males
Large males may also defecate in water without another male being near
Syringe-like vomeronasal organ functions underwater drawing a sample of
urine in water through ducts leading from the mouth
Almost all male
ungulates sample a female's urine to test for possible estrus hormonal
levels prior to courtship
Unique skin (thin epidermis, no sweat glands) loses water at several
times the rate of other mammals.
Rapid dehydration and overheating are
Water or mud is a secondary means of cooling in hot dry weather;
sunlight used for warming.
Locomotion (Coughlin & Fish 2009)
When agitated can charge at over 30 km/hr.
Able to climb steep banks if footing is secure.
Sits on haunches before lying down; rise using front legs first
(similar to pigs)
Their weight aided by very dense bones allows them to travel on the bottom of
Underwater gait similar to a gallop and a trot.
Do not really swim; move forward when in water by punting off the river bottom
Other animals that can bottom-walk include nine-banded armadillos and water chevrotain
Bottom walking may have been also used by ancestors of whales (Thewissen et al. 2007)
Mature hippos can remain under water for 5 minutes.
is 104 seconds.
A 2-month calf can remain under water for about 30
Environmental Modification ("hydro engineering")
(Eltringham 1999, 2001) (Mosepele et al 2009)
Okavango Delta in Botswana owes its topography to hippo's movements along rivers, across land
Hippos help keep main channels open; also create side channels leading to islands
Hippo trails serve important role as drainage channels during floods
On land, hippo gullies may grow to 20 m (65.6 ft) deep that fill with water during rains
Other species like elephants and buffaloes create land paths; only hippo trails go through lowland waters
In Okavango Delta, a diverse fish fauna owes its habitat to hippos' habits.
Interspecies Interaction: (Deeble & Stone 2001) (Kingdon 1979)
(Olivier and Laurie 1974)
Most defecation is directly into the water causing a massive buildup of
Support fish, insects, fish-eating birds and human
population that relies on fish for essential protein.
Egyptian geese, cormorants, cattle egrets (hunt ticks and insects),
even turtles, rest on hippo backs and heads.
Common Sandpipers forage for aquatic organisms from hippos' backs
Two species of ox peckers feed on tissue in wounds on hippo skin
African Pied Wagtails sit on hippos' backs, chase insects above hippos
African Jacanas observed a national park in Central Northern Republic
Spend up to 5 hr/day alternately foraging on lake's edge
and on hippos' backs
Take parasites from hippo skin (mutualism)
Also remove flesh around wounds which seems to retard healing and distress hippos (semi-parasitism)
4 species of fish clean hippos in Mzima Springs of Kenya's Tsavo
West National Park (Deeble & Stone 2001)
Fish feed on vegetable matter/excreta voided into water by
A carp (Labeo) uses wide rasping mouth to clean hippo's hide
Garra clean wounds
Barbus cleans cracks in bottom of feet
Cichlids clean hippo tail bristles
Hippos visit sites where fish gather and "invite" cleaning behavior
Graze in the same areas as buffalo, waterbuck, puku, various antelopes
Enemies: both hyenas and lions take hippo calves; lions occasionally
Hippo usually escapes enemies by entering water
Hippos and Nile Crocodiles occupy the same water and land habitats; hippos are dominant. (Cott 1975)
(BBC Wildlife 2009)
Hippos may push aside a crocodile basking on land or knock it into the water
A female hippo with a calf or others in the herd will drive out all crocodiles from their pool of water
Hippos kill crocodiles if they stray too close to calves.
As of 2004, 290 captive-born Pygmy Hippos in 135 zoos. (Hlavacek et al. 2005)
Solitary Pygmy Hippos in captivity may experience undue stress when pairs of females or male/female pairs housed together.
POPULATION AND CONSERVATION STATUS (Lewison & Oliver 2008a,b ) (Nowak 1991) (Oliver 1993) (Kingdon,1979)
IUCN Status: Pygmy Hippo (Lewison & Oliver 2008a)
Presently (2008) the population trend is decreasing; with no regional conservation plans, "viability should be considered extremely poor."
IUCN Status: Common Hippo (Lewison & Oliver 2008b)
2006 Vulnerable A4cd Ver. 3.1
1996 Lower Risk/least concern
CITES Status: Pygmy Hippo Appendix I (no trade allowed)
CITES Status: Common or River Hippo Appendix II (trade must be regulated)
1968-present Protected by 1968 African Convention on the Conservation of Nature and Natural Resources, but protection not enforced.
1990-2010: a 7-20% reduction in Common Hippo populations
2010-2040 a 30% reduction predicted in populations if current trends continue
125,000 to 148,000 Common Hippos estimated across their range
(Lewison & Oliver 2008)
Most numerous in: Ethiopia, Kenya, South Africa, Sudan, Tanzania,
Uganda, Zambia and Zimbabwe. Largest populations found in Zambia.
in Democratic Republic of Congo
In East Africa' Democratic Republic of Congo 30,000
Tens of thousands in Ethiopia, Sudan and Tanzania
Several thousand in Kenya and Uganda
In all of East Africa, perhaps 70,000
About 7,000 in West Africa, in numerous small groups; fragmented populations at risk
2,000-3,000 Pygmy Hippos remaining in Sierra Leone, Republic of Guinea, Ivory Coast, Liberia
( IUCN Action Plan estimate, Oliver 1993))
This estimate may have been too high
Population estimate only available in 1993 for Sierra Leone: 80-100 individuals
Two key areas for Pygmy Hippo habitat and conservation:
Sapo National Park in eastern Liberia - 1318 sq km (509 sq mi)
Tai National Park in western Ivory Coast (Côte d'Ivoire)
Threats to survival
Not in any danger of extinction but potentially vulnerable because of
Loss of grazing habitat
Desertification of some parts of Africa
Many groups in west Africa have less than 50 animals
Farming and human settlement
Logging which leads not only to habitat destruction but is often tied to poaching when forests become more accessible
Wars in Sierra Leone, Guinea and Liberia
Limited enforcement of existing protections in Sierra Leone, Guinea, Ivory Coast
Oil pollution in estuaries by the sea further threaten Pygmy Hippo populations. (Oliver 1993)
Conservation status and viability of this species considered poor. (Lewison & Oliver 2008 in IUCN assessment)
Major threat: Humans
Increasing human population, timber and fish industries
because of destructiveness to crops and attacks on fishermen
Hunted for fish bait in some areas
Hunted for meat in some areas.
Hams are palatable but carcasses
spoil quickly in hot weather
Meat is avoided in some areas
(Zambians believe it causes leprosy)
Muslim communities consider
it too closely related to the pig
Main trophy product is tusks (softer and easier to carve than the
true ivory of elephant tusks)
Thick hide is cut into strips and rubbed with fat to form a whip
known in South Africa as the "sjambok"
IUCN/SSC Hippo Specialist Group: Website designed to promote conservation and research for hippos and their habitat. Includes free downloadable information packets and hippo maps for grade school youth and conservation groups; see also an extensive bibliography for hippos.
EDGE: Evolutionarily Distinct & Globally Endangered: Website maintained by the Zoological Society of London for little-known animals or ones currently receiving little conservation attention. Focus is on species that "represent a sinificant amount of unique evolutionary history" Pygmy Hippos are listed. Site also has a geographical search feature that pinpoints where individual species can be found.