Hippopotamuses are among the largest living land mammals, being only smaller than elephants and some rhinoceroses. Mean adult weight is around 1,500 kg (3,300 lb) and 1,300 kg (2,900 lb) for males and females respectively, very large males can reach 2,000 kg (4,400 lb) and an exceptional male weighing almost 2,700 kg (6,000 lb) has been reported. Male hippos appear to continue growing throughout their lives while females reach maximum weight at around age 25. Different from all other large land mammals, hippos are of semiaquatic habits, spending the day in lakes and rivers.
The eyes, ears, and nostrils of hippos are placed high on the roof of their skulls. This allows these organs to remain above the surface while the rest of the body submerges. Their barrel-shaped bodies have graviportal skeletal structures, adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river. Hippopotamuses have small legs (relative to other megafauna) because the water in which they live reduces the weight burden. Though they are bulky animals, hippopotamuses can gallop at 30 km/h (19 mph) on land but normally trot.
They are incapable of jumping but do climb up steep banks. Despite being semiaquatic and having webbed feet, an adult hippo is not a particularly good swimmer nor can it float. It is rarely found in deep water; when it is, the animal moves by porpoise-like leaps from the bottom. The testes of the males descend only partially and a scrotum is not present. Hippopotamus teeth sharpen themselves as they grind together. The lower canines and lower incisors are enlarged, especially in males, and grow continuously. The incisors can reach 40 cm (1.3 ft), while the canines reach up to 50 cm (1.6 ft). The canines and incisors are used for combat and play no role in feeding. Hippos rely on their broad horny lips to grasp and pull grasses which are then ground by the molars.:259, 263 The hippo is considered to be a pseudoruminant; it has a complex three-chambered stomach but does not "chew cud". Unlike most other semiaquatic animals, the hippopotamus has very little hair.
The skin is 6 cm (2 in) thick, providing it great protection against conspecifics and predators. By contrast, its subcutaneous fat layer is thin. The animals' upper parts are purplish-gray to blue-black, while the under parts and areas around the eyes and ears can be brownish-pink. Their skin secretes a natural sunscreen substance which is red-coloured. The secretion is sometimes referred to as "blood sweat", but is neither blood nor sweat. This secretion is initially colourless and turns red-orange within minutes, eventually becoming brown. Two distinct pigments have been identified in the secretions, one red (hipposudoric acid) and one orange (norhipposudoric acid).
The two pigments are highly acidic compounds. Both pigments inhibit the growth of disease-causing bacteria; as well, the light absorption of both pigments peaks in the ultraviolet range, creating a sunscreen effect. All hippos, even those with different diets, secrete the pigments, so it does not appear that food is the source of the pigments. Instead, the animals may synthesize the pigments from precursors such as the amino acid tyrosine. Nevertheless, this natural sunscreen cannot prevent the animal's skin from cracking if it stays out of water too long.
Like almost any herbivore, they consume other plants if presented with them, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants. Hippos are born with sterile intestines, and require bacteria obtained from their mothers' feces to digest vegetation. Hippos have (rarely) been filmed eating carrion, usually close to the water. There are other reports of meat-eating, and even cannibalism and predation. The stomach anatomy of a hippo is not suited to carnivory, and meat-eating is likely caused by aberrant behaviour or nutritional stress.
Hippo defecation creates allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function. A 2015 study suggests that hippo dung provides nutrients from terrestrial material for fish and aquatic invertebrates. Because of their size and their habit of taking the same paths to feed, hippos can have a significant impact on the land across which they walk, both by keeping the land clear of vegetation and depressing the ground. Over prolonged periods, hippos can divert the paths of swamps and channels. Adult hippos move at speeds up to 8 km/h (5 mph) in water; typically resurfacing to breathe every three to five minutes. The young have to breathe every two to three minutes. The process of surfacing and breathing is automatic. A hippo sleeping underwater rises and breathes without waking. A hippo closes its nostrils when it submerges into the water.
As with fish and turtles on a coral reef, hippos occasionally visit cleaning stations and signal, by opening their mouths wide, their readiness for being cleaned of parasites by certain species of fishes. This is an example of mutualism in which the hippo benefits from the cleaning, while the fish receive food. Studying the interaction of male and female hippopotamuses has long been complicated because hippos are not sexually dimorphic; thus females and young males are almost indistinguishable in the field. Although hippos lie close to each other, they do not seem to form social bonds except between mothers and daughters, and they are not social animals. The reason they huddle close together is unknown.
Hippopotamuses are territorial only in water, where a bull presides over a small stretch of river, on average 250 m (270 yd) in length, and containing 10 females. The largest pods can contain over 100 hippos. Younger bachelors are allowed in a bull's stretch, as long as they behave submissively toward the bull. The territories of hippos exist to establish mating rights. Within the pods, the hippos tend to segregate by gender. Bachelors lounge near other bachelors, females with other females, and the bull on his own.
When hippos emerge from the water to graze, they do so individually. Hippopotamuses appear to communicate vocally, through grunts and bellows, and they may practice echolocation, but the purpose of these vocalizations is currently unknown. Hippos have the unique ability to hold their heads partially above the water and send out a cry that travels through both water and air; individuals respond above and under water.