Hippopotamus (Hippopotamus amphibius) & Pygmy Hippopotamus (Choerpsis liberiensis) Fact Sheet: Physical Characteristics

Body measurements

Notes
*Weight measurements here from Klingel (2013). Kingdon (1979), p. 256 states maximum weight of male hippo as 3,200 kg (7,100 lb) and female as 2,344 kg (5,168 lb). Other sources stating that hippos can weight up to 4,500 kg (9,900 lb) appears to reference Bere (1959) (and possibly other sources) but may not be accurate.

**Average, minimum, and maximum weights may vary by region and population.

***From Kingdon (1979)

General

• Body 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)
• Some sexual dimorphism
  • Muzzle is noticeably bigger in males and jowl area more developed.
  • "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°)
• Multi-chambered non-ruminating stomach; no caecum or gallbladder
• Females have 2 mammae
• Four functional toes with wide splay
• Nail-like hooves on ends of toes

Skin

• 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
• Skin 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
• Endodermis varies from 5-6 cm on back and rump to < 1cm (.4 in) on head and belly
• Albino hippos are common (Pitman 1962)

Teeth

• 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 canines
• Incisors used for digging
• Jaws capable of 150 degree gape (slightly less in pygmy hippo)
• Simple low-crowned 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

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)

The pygmy hippo spends more time on land than the common hippo.

Their heads are also rounder, more narrow, and their bodies are proportionately longer.

Image credit: © San Diego Zoo Wildlife Alliance. All rights reserved.

Estes (1991)
Eltringham (1999)
Kingdon (1979, 1997)
Nowak (1991)