Hippopotamus Anatomy: A Complete Guide to How Hippos Are Built

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Last Updated on August 20, 2026 by Jorge Bastos

The hippopotamus is built to live in two worlds simultaneously. During the day it must stay submerged — or at least semi-submerged — to protect its skin, regulate its temperature, and maintain the social bonds of the pod. At night it must walk several kilometres on land to graze. Almost every anatomical feature of the hippo reflects this dual requirement: a body optimised to spend half its life in water and half on African grassland.

Size and Proportions

The common hippopotamus is the third-largest land mammal on Earth. Adult males typically weigh between 1,500 and 3,200 kg (3,300 to 7,000 lbs), with exceptional individuals recorded at over 4,000 kg. Adult females are substantially smaller, typically 1,400 to 1,700 kg — a significant difference that reflects the species’ highly competitive male-dominated social structure.

The body is described as barrel-shaped, which understates how distinctive the proportions actually are. The trunk is enormous relative to the legs, the head is enormous relative to the trunk, and the legs are short enough that the belly of a large bull comes close to the ground. This combination of massive torso and relatively short limbs creates an animal that looks ungainly on land but moves with surprising efficiency in water, where its body mass is supported by buoyancy.

Hippos stand approximately 1.5 to 1.6 m (5 to 5.2 ft) at the shoulder and reach up to 5 m (16.4 ft) in length from nose to tail.

Skin: The Most Remarkable Hide in the Animal Kingdom

Hippo skin is hairless, thin at the epidermis but extraordinarily thick overall — reaching up to 5 cm (2 inches) on the sides and back. This thickness makes it one of the most resistant natural skin structures of any land mammal, capable of absorbing significant physical trauma during the territorial fights that are a constant feature of hippo social life.

Despite this toughness, hippo skin is paradoxically vulnerable. Without water or its own protective secretion, it dries, cracks, and becomes susceptible to UV damage within hours. This is why hippos cannot simply leave the water whenever they choose — their skin requires constant management.

The solution is one of the most extraordinary biological adaptations in the animal kingdom: a pink-to-reddish fluid secreted by specialised glands that open directly onto the skin surface. This fluid — containing hipposudoric acid and norhipposudoric acid — functions simultaneously as a natural sunscreen, an antibiotic, and a moisturiser. It is neither blood nor sweat, despite the popular nickname “blood sweat.” It emerges clear from the skin and turns reddish only on contact with air.

We cover the full science of this secretion in our hippo blood sweat guide.

The Head: Built for Two Environments

The hippo’s head is disproportionately large — in adult males, the skull alone can account for a significant fraction of total body weight, with male skulls being approximately 44% heavier than female skulls of the same body size. This sexual dimorphism in skull mass reflects the role of the head as the primary weapon in male-versus-male combat.

Eyes, Ears, and Nostrils

The placement of the three primary sensory openings is the clearest anatomical signature of a semi-aquatic animal: all three — eyes, ears, and nostrils — sit on the top and upper sides of the head, allowing a hippo to see, hear, and breathe while the rest of its body is completely submerged.

The eyes are small relative to the skull and positioned high on the sides of the head, giving the hippo good vision both above and across the water surface. A transparent membrane can cover the eyes underwater for protection while still allowing limited vision.

The nostrils close completely when the hippo submerges, sealed by muscular action that prevents water entry. The ears also fold flat when the animal goes underwater. Both reflexes are involuntary — they happen automatically, even during sleep.

ippos can sleep underwater and have a surfacing reflex so deeply hardwired that they rise to breathe, exhale, and descend again without waking. This reflex is present from birth — newborn calves can nurse underwater because the same reflexes that seal the nostrils during submersion also prevent water from entering while feeding.

The Mouth and Teeth

The hippopotamus has the widest gape of any land mammal. The jaw can open to approximately 150 to 180 degrees — roughly four times the maximum angle of a human jaw — creating a mouth opening of up to 1.2 m (4 feet) across when fully extended.

The teeth are the hippo’s primary weapon and one of its most striking anatomical features. There are 36 teeth in total:

• Canine teeth — the visible tusks — can reach up to 50 cm (20 inches) in length in adult males. These are not used for eating. They are weapons, used exclusively in combat between males, and they are kept sharp through continuous self-honing as upper and lower canines grind against each other.
• Incisors can reach up to 40 cm in length.
• Molars, at the back of the jaw, do the actual work of processing food.

The hippo’s bite force is approximately 1,800 PSI — nearly three times that of a lion, and sufficient to sever a large crocodile’s body in a single closure. We cover hippo teeth and bite force in full in our dedicated guide.

Legs and Feet: Walking on Tiptoe

Despite appearances, hippos are digitigrade — they walk on their toes rather than flat-footed. Each foot has four toes, each tipped with a small hoof, and the toes spread slightly under the animal’s weight to distribute it across a wider area. This arrangement, combined with extremely powerful leg muscles, is what allows a 2,000 kg animal to accelerate to 30 km/h (19 mph) from a standing start.

The feet have a small amount of webbing between the toes — enough to aid movement in shallow water but insufficient for true swimming. Hippos do not swim; they walk and bound along riverbeds, pushing off the bottom to rise and breathe before sinking again.

The leg bones are unusually dense for a mammal — heavier than would be expected for an animal of this size, which contributes to the hippo’s lack of buoyancy and its ability to stay submerged without effort.

Digestion: Almost a Ruminant

The hippopotamus is a pseudo-ruminant — it has a multi-chambered stomach capable of fermenting plant material in a manner similar to cattle, sheep, and deer, but it does not chew cud. Food is processed through fermentation chambers rather than being regurgitated and re-chewed as in true ruminants.

This digestive system is well-suited to a herbivore that grazes primarily on short grass at night. An adult hippo consumes approximately 35 to 40 kg of grass per night — modest for an animal of its size, which is partly explained by the energy-efficient fermentation system and the energy savings from spending the day immobile in water.

Male vs Female: Anatomical Differences

Beyond overall body mass, the most significant anatomical difference between male and female hippos is in the skull. Male skulls are considerably heavier, more robustly built, and carry longer canine teeth — all reflecting the role of the head in male-versus-male territorial combat. Older males frequently show extensive facial scarring from decades of combat, along with worn or broken canines.

The difference in body mass is also significant: a large bull can weigh roughly twice as much as an average cow, giving males a decisive advantage in territorial disputes.

The Pygmy Hippo: A Different Anatomy

The pygmy hippopotamus (Choeropsis liberiensis) shares the family but differs anatomically in ways that reflect its forest lifestyle rather than its river one:

• The eyes are positioned on the sides of the head rather than the top — an adaptation for navigating forest floor rather than looking across water
• The legs are proportionally longer, suited to walking on forest terrain
• The nostrils are positioned lower on the muzzle
• The toes are less webbed and more spread, providing grip on forest soil
• The body is proportionally more streamlined

Everything about the pygmy hippo’s anatomy is modified for terrestrial forest life rather than the amphibious river life of its larger cousin. Our complete pygmy hippo guide covers these differences in detail.

Frequently Asked Questions

• How thick is hippo skin? Hippo skin reaches approximately 5 cm (2 inches) in thickness on the sides and back — making it one of the thickest natural skin structures of any land mammal. Despite this thickness, the outer layer (epidermis) is thin and requires constant moisture and the hippo’s natural secretion to remain healthy.
• Why do hippos have their eyes on top of their head? The elevated position of the eyes, ears, and nostrils allows hippos to remain largely submerged while still being able to see, hear, and breathe. This is a classic adaptation to semi-aquatic life, shared by other amphibious animals including crocodiles and some species of frog.
• How many teeth does a hippo have? 36 teeth — 4 canines (the tusks), 8 incisors, 12 premolars, and 12 molars. The canines can reach 50 cm in length and are used purely for combat, not for feeding.
• Can hippos actually run? Yes — and faster than most people expect. Adult hippos can reach approximately 30 km/h (19 mph) on flat ground for short distances. See our full guide on how fast hippos can run.
• What do hippos use their teeth for if they only eat grass? The large canine teeth and incisors are weapons, not eating tools. Hippos eat using their wide, muscular lips to pull grass from the ground, and their molars to chew it. The structure of the front teeth actually prevents lateral jaw movement, making the hippo a less efficient chewer than many other grazers.

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