The hippopotamus, scientifically known as Hippopotamus amphibius, is a large, semi-aquatic mammal native to sub-Saharan Africa. Often shortened to "hippo," the name "hippopotamus" comes from the Greek words for horse (hippo) and river (potamos), aptly earning them the nickname 'river horse'. These formidable creatures are considered the second largest land mammal, smaller only than elephants. Hippos are primarily herbivores, consuming vast quantities of food to sustain their large size, with adults typically weighing between 3,000 and 3,500 pounds.
Habitat and Lifestyle
Hippos inhabit rivers, lakes, and mangrove swamps, requiring sufficient water to submerge themselves while remaining close to grassy areas. They spend most of the day partially submerged in these waterways or foraging for food nearby. Common river hippopotamuses are native to sub-Saharan Africa and can be found in various African countries, with populations in both East and West Africa. Pygmy hippos, in contrast, are found in the tropical rainforests and swamps of four West African countries: Côte d'Ivoire, Guinea, Liberia, and Sierra Leone.
These semi-aquatic creatures live in and around lakes, rivers, and grassland swamps. Hippos must keep their skin wet with mud or water for most of the day to stay hydrated. While they don’t have sweat glands, they secrete a red substance called blood sweat. This helps protect their reddish-brown, thick yet sensitive skin from sunburn and keeps it moisturized. Despite their need to spend most of their time in the water, hippos can’t swim. Instead, they run or walk along river beds. When they’re rafting-napping while submerged in water-a subconscious reflex keeps them near the water’s surface.
Physical Characteristics
The average male hippo can weigh around 3,200 kilograms, while females usually weigh 30% less. They’re typically 3.5 meters long and 1.5 meters tall. Hippos have bulky, barrel-shaped bodies, short and stout legs, large heads, and enormously powerful jaws. Their jaws can open to 180 degrees and bite down with three times the strength of a lion. They’re armed with impressive, sharp teeth that can grow up to 50 centimeters long, used for eating and defense. The common hippopotamus has four webbed toes adapted to support them on land and in water. On the other hand, pygmy hippos tend to have less webbing and slightly longer legs. A hippo’s eyes and nostrils sit on top of its head, which helps it stay mostly submerged in water while breathing and keeping an eye on its surroundings. Although hippos can’t breathe underwater, they can hold their breath for up to five minutes.
Herbivorous Diet in the Wild
Hippopotamuses are primarily herbivores. In the wild, the common hippopotamus consumes a mostly herbivorous diet. Grasses, supplemented with nutrient-rich fruits, seem to make up the great majority of their food intake. They also enjoy small shoots and reeds emerging from the ground, but aquatic plants seem to form a surprisingly small percentage of their diet. Pygmy hippos eat a higher proportion of leaves and roots than grasses. A hippo meal will sometimes leave entire areas bereft of grass and shoots.
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The hippopotamus is a nocturnal feeder. It leaves the water with the coming dusk and settles in a grassy area near the waterbed, arriving via the same foraging path every day. These are known as “hippo paths,” and their grazing areas are called “hippo lawns.” Both areas are often unique to each group or individual. If food is particularly scarce, then it may end up traveling a few miles away from the water hole, but they prefer to stay as close to the sleeping area as possible. Their superb sense of smell helps them find their preferred foods. When grazing, the hippo’s highly muscular lips enable it to pull up food from the ground or tear off leaves from a plant. The hippo softens the food in its mouth without chewing to prevent any nutrient loss. While they do have incisor and canine teeth, these play a minimal role in feeding, except in cases where they eat meat; the teeth are instead used for defense. The hippo is well-adapted for an herbivorous diet. Their highly complex stomachs, divided into three distinct chambers, are specialized for digesting plant matter. However, unlike cattle, they do not regurgitate and chew the cud. All hippos are born with relatively sterile intestines and obtain bacteria from their mother’s feces. Hippos are very reliant on only a small number of food sources. They very rarely eat anything else outside of their normal feeding behavior.
Just before night begins, they leave the water to forage on land. A hippo will travel 3-5 km (1.9-3.1 mi) per night, eating around 40 kg (88 lb) of grass.
The Occasional Carnivorous Tendencies
Traditionally, it was thought that the hippo scavenges for meat only during exceptional times of food scarcity, because their stomachs are not adapted for meat consumption. However, a 2015 study (Dudley et al., 2015) spanning four continents found that this meat-eating behavior is not limited to scavenging carcasses. This carnivorous behavior is part of a broader pattern of meat-eating. The hippo has been known to attack and consume other animals and even steal meat from predators; wildebeests, zebras, and kudus seem to be among their more common kills. They have even been seen engaging in cases of cannibalism, consuming other hippos. The common hippo is certainly large and aggressive enough to take down just about any other animal, but hunting requires a huge expenditure of energy, which hippos would rather not expend.
Food Consumption
Regardless of their exact dietary composition, it’s estimated that they consume around 1% or more of their body weight each day. The common hippo typically weighs between 3,000 and 9,900 pounds, with most adults consuming about 30 to 150 pounds of food per day.
Diet in Captivity
Compared to their wild diet, captive hippos in zoos eat a wider range of foods, carefully selected and provided by nutritionists, trainers, and staff. At the San Diego Zoo, the hippos are fed a combination of herbivore pellets, alfalfa and Bermuda hay, lettuce and other mixed vegetables, and the occasional melon.
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One of the first things to get cut from the hippos’ daily diet was watermelon, their all-time favorite, due to its high sugar content. Genny and Button’s current diet consists of timothy hay, wild herbivore grain, hay enhancer grain, veggies, and greens like romaine, bok choy, swiss chard, kale, and collards. Despite being on a diet, Genny and Button typically eat around 30-35 lbs. And, while the hippos won’t turn their noses up at most foods, their diet has unveiled some of their least favorite eats.
Social Behavior and Territorial Marking
Hippos exhibit strong social bonds and typically live in groups with anywhere from 40 to 200 hippos, known as bloats, pods, or herds. They are territorial creatures and use their dung to mark their territory. Male hippos have been known to flick their dung around to assert dominance. This dung is actually important to their aquatic ecosystems, as it transfers vital nutrients from the land to the water, allowing insects, plants, and other microorganisms to flourish.
Hippos engage in "muck-spreading" which involves defecating while spinning their tails to distribute the faeces over a greater area. Muck-spreading occurs both on land and in water and its function is not well understood. It is unlikely to serve a territorial function, as the animals only establish territories in the water.
Ecological Role
The hippopotamus plays a crucial role in the ecosystem it inhabits in other ways, too. These herbivores graze on the vegetation surrounding wetlands, helping prevent overgrowth, which can clog water sources, disrupt the natural flow of water, and threaten the biodiversity of aquatic ecosystems. Healthy wetlands act as effective carbon sinks and remove excess carbon from the atmosphere. By maintaining the balance of these delicate ecosystems, hippos help mitigate the effects of climate change.
By defecating in the water, the animals also appear to pass on microbes from their gut, affecting the biogeochemical cycle.
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Conservation Status and Threats
The common hippopotamus has been listed as vulnerable on the IUCN Red List since 2006, meaning it is not endangered. Their population is currently stable, meaning it’s neither increasing nor decreasing. The pygmy hippopotamus is classified on the IUCN Red List as endangered and has also held that status since 2006. The pygmy hippopotamus population is decreasing, with less than 2,500 mature individuals currently remaining.
Hippos are facing several threats to their survival as a species. Global conservation efforts are critical to ensuring the well-being of the species and the ecosystems it inhabits. Hippos are also subject to unregulated hunting and poaching.
Hippo Paths
The hippo is a sedentary animal, wallowing for most of the day in the water or mud without moving much at all. Surprisingly, for a semi-aquatic animal, the hippo cannot swim or even float. Their bodies are simply too heavy. 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, keeping the land clear of vegetation and depressing the ground. Over prolonged periods, hippos can divert the paths of swamps and channels.
Locomotion
Despite being semiaquatic, an adult hippo is not a particularly good swimmer, nor can it float. It rarely enters deep water; when it does, the animal moves by bouncing off the bottom. An adult hippo surfaces every four to six minutes, while young need to breathe every two to three minutes. Hippos move on land by trotting, and limb movements do not change between speeds. They can reach an airborne stage (a stage when all limb are off the ground) when they move fast enough. Hippos are reported to reach 30 km/h (19 mph) but this has not been confirmed. They are incapable of jumping but can walk up steep banks.
Sleep Patterns
The hippopotamus sleeps with both hemispheres of the brain resting, as in all land mammals, and usually sleeps on land or in water with the nostrils exposed. Despite this, it can sleep while submerged, intermittently surfacing to breathe seemingly without waking.
Social Structure
Hippo pods fluctuate but can contain over 100 hippos. Although they lie close together, adults develop almost no social bonds. Males establish territories in water but not land, and these may range 250-500 m (820-1,640 ft) in lakes and 50-100 m (160-330 ft) in rivers. Territories are abandoned when the water dries up. The bull has breeding access to all the cows in his territory. Younger bachelors are allowed to stay as long as they defer to him. A younger male may challenge the old bull for control of the territory. Within the pods, the hippos tend to segregate by sex and status. Bachelor males lounge near other bachelors, females with other females, and the territorial male is on his own.
Vocalization
The most common hippo vocalisation is the "wheeze honk", which can travel over long distances in air. This call starts as a high-pitched squeal followed by a deeper, resonant call. The animals can recognise the calls of other individuals.
Reproduction and Life Cycle
Cows reach sexual maturity at five to six years of age and have a gestation period of eight months. A study of endocrine systems revealed cows may begin puberty at as early as three or four years. Bulls reach maturity at around 7.5 years. Both conceptions and births are highest during the wet season. Male hippos always have mobile spermatozoa and can breed year-round. After becoming pregnant, a female hippo will typically not begin ovulation again for 17 months. Hippos mate in the water, with the cow remaining under the surface, her head emerging periodically to draw breath. Cows give birth in seclusion and return within 10 to 14 days. Calves are born on land or shallow water weighing on average 50 kg (110 lb) and at an average length of around 127 cm (50 in). The female lies on her side when nursing, which can occur underwater or on land.
A hippo's lifespan is typically 40 to 50 years.
Predators and Parasites
Hippos coexist alongside a variety of large predators in their habitats. Nile crocodiles, lions, and spotted hyenas are known to prey on young hippos. Beyond these, adult hippos are not usually preyed upon by other animals due to their aggression and size. Cases where large lion prides have successfully preyed on adult hippos have been reported, but it is generally rare. Lions occasionally prey on adults at Gorongosa National Park and calves are sometimes taken at Virunga. Crocodiles are frequent targets of hippo aggression, probably because they often inhabit the same riparian habitats; crocodiles may be either aggressively displaced or killed by hippos, although they will avoid crocodiles larger than 3.5 m (11 ft). In turn, very large Nile crocodiles have been observed preying occasionally on calves, "half-grown" hippos, and possibly also adult female hippos.
Hippos occasionally visit cleaning stations in order to be cleaned of parasites by certain species of fishes. They signal their readiness for this service by opening their mouths wide. This is an example of mutualism, in which the hippo benefits from the cleaning while the fish receive food.
The parasitic monogenean flatworm Oculotrema hippopotami infests hippopotamus eyes, mainly the nictitating membrane.