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|Hippopotami are one of the largest extant mammals in the world. Hippos are considered megafauna, but unlike all other African megafauna, hippos have adapted for a semi-aquatic life in freshwater lakes and rivers.|
Because of their enormous size, hippopotami are difficult to weigh in the wild. Most estimates of the weight come from culling operations that were carried out in the 1960s. The average weights for adult males ranged between 1500–1800 kg (3,300–4,000 lbs). Females are smaller than their male counterparts, with average weights measuring between 1300–1,500 kg (2,900–3,300 lbs). Older males can get much larger, reaching at least 3,200 kg (7,100 lbs). Male hippos appear to continue growing throughout their lives; females reach a maximum weight at around age 25.
Hippos average 3.5 meters (11 ft) long, 1.5 meters (5 ft) tall at the shoulder. The range of hippopotamus sizes overlaps with the range of the White Rhinoceros; use of different metrics makes it unclear which is the largest land animal after elephants. Even though they are bulky animals, hippopotami can run faster than a human on land. Estimates of their running speed vary from 30 km/h (18 mph) to 40 km/h (25 mph), or even 50 km/h (30 mph). The hippo can maintain these higher speeds for only a few hundred meters or yards.
A bull hippo out of water during daylight, Ngorongoro Crater, Tanzania.
A hippo's lifespan is typically 40 to 50 years. Donna the Hippo, 56, is the oldest known hippo in captivity. She lives at the Mesker Park Zoo in Evansville, Indiana.
A drawing of a hippopotamus skeleton.
The eyes, ears, and nostrils of hippos are placed high on the roof of the skull. This allows them to be in the water with most of their body submerged in the waters and mud of tropical rivers to stay cool and prevent sunburn. Their general anatomical structure is an adaptation to their riparian lifestyle. Their skeletal structure is graviportal, adapted to carrying the animals' enormous weight. hippopotami have legs that are small, relative to other megafauna, because the water in which they live reduces the weight burden. Like other aquatic mammals, the hippopotamus has very little hair.
Their skin is 4 centimeters (1.5 in) thick, and accounts for 25% of their weight. For additional protection from the sun, their skin secretes a natural sunscreen substance which is red-colored. The secretion is sometimes referred to as "blood sweat," but is neither blood nor sweat. This secretion is initially colorless and turns red-orange within minutes, eventually becoming brown. Two distinct pigments have been identified in the secretions, one red and one orange. The two pigments are highly acidic compounds. They are known as red pigment hipposudoric acid and orange pigment norhipposudoric acid. The red pigment was found to inhibit the growth of disease-causing bacteria, lending credence to the theory that the secretion has an antibiotic effect. 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.
Hippopotamus amphibius was widespread in North Africa and Europe before the last glaciation event, and it can live in colder climates provided the water does not freeze during winter. The species was common in Egypt's Nile region until historic times but has since been extirpated. Pliny the Elder writes that, in his time, the best location in Egypt for capturing this animal was in the Saite nome; the animal could still be found along the Damietta branch after the Arab Conquest in 639. Hippos are still found in the rivers and lakes of Uganda, Sudan, Somalia, Kenya, northern Democratic Republic of the Congo and Ethiopia, west through Ghana to Gambia, and also in Southern Africa (Botswana, Republic of South Africa, Zimbabwe, Zambia). A separate population exists in Tanzania and Mozambique.
The Hippopotamus Hunt (1617), by Peter Paul Rubens.
Evidence through genetic analysis suggests that common hippos in Africa experienced a marked population expansion during or after the Pleistocene Epoch, attributed to an increase in water bodies at the end of the era. These findings have important conservation implications as Hippo populations across the continent are currently threatened by loss of access to fresh water. Hippos are also subject to unregulated or illegal poaching. In addition to addressing these common threats, the genetic diversity of hippos would need to be preserved to ensure the safety of the species. In May 2006 the hippopotamus was identified as a vulnerable species on the IUCN Red List drawn up by the World Conservation Union (IUCN), with an estimated population of between 125,000 and 150,000 hippos, a decline of between 7 percent and 20 percent since the IUCN's 1996 study.
The hippo population declined most dramatically in the Democratic Republic of the Congo. The population in Virunga National Park had dropped to 800 or 900 individuals from around 29,000 in the mid 1970s, raising concerns about the viability of that population. The decline is attributed to the disruptions caused by the Second Congo War. Poachers are believed to be former Hutu rebels, poorly paid Congolese soldiers, and local militia groups. Reasons for poaching include the belief that hippos are unintelligent, that they are a harm to society, and also for money. The sale of hippo meat is illegal, but black-market sales are difficult for WWF officers to track.
An open mouth signals that the Hippo feels threatened.
Hippos spend most of their days wallowing in the water or the mud, with the other members of their pod. The water serves to keep their body temperature cool, and to keep their skin from drying out. With the exception of eating, most of hippopotami's lives—from childbirth, fighting with other hippos, and reproduction—occurs in the water.
Hippos leave the water at dusk and travel inland, sometimes up to 8 kilometers (5 mi), to graze on short grass, their main source of food. They spend four to five hours grazing and can consume 68 kilograms (150 lb) of grass each night. Like almost any herbivore, they will consume many other plants if presented with them, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants. 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 behavior or nutritional stress.
The diet of hippos consists mostly of terrestrial grasses, but they spend most of their time in the water. Most of their defecation occurs in the water, creating allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function. Because of their size and their habit of taking the same paths to feed, hippos can have a significant impact on the land they walk across, both by keeping the land clear of vegetation and depressing the ground. Over prolonged periods hippos can divert the paths of swamps and channels.
A submerged hippo at the San Diego Zoo. Adult hippos typically resurface to breathe every 3–5 minutes.
Adult hippos are not generally buoyant. When in deep water, they usually propel themselves by leaps, pushing off from the bottom. They move at speeds up to 8 km/h (5 mph) in water. Young hippos are buoyant and more often move by swimming, propelling themselves with kicks of their back legs. Adult hippos typically resurface to breathe every 4–6 minutes. The young have to breathe every two to three minutes. The process of surfacing and breathing is automatic, and even a hippo sleeping underwater will rise and breathe without waking. A hippo closes its nostrils when it submerges. One hippo calf survived after being pushed out to sea during the tsunami caused by the 2004 Indian Ocean earthquake and was rescued on a nearby sandy reef.
Studying the interaction of male and female hippopotami has long been complicated by the fact that hippos are not sexually dimorphic and thus females and young males are almost indistinguishable in the field. Although hippos like to lie in close proximity to each other, they do not seem to form social bonds except between mothers and daughters, and are not social animals. The reason they huddle in close proximity is unknown.
It is difficult to identify the gender of hippos in the field, because all researchers can usually see are their backs, like with this pod in Tanzania.
Hippopotami are territorial only in water, where a bull presides over a small stretch of river, on average 250 meters in length, and containing ten females. The largest pods can contain up to 100 hippos. Other 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 will 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.
Hippopotami appear to communicate verbally, through grunts and bellows, but the purpose of these vocalizations is unknown. Hippos have the unique ability to hold their head partially above the water and send out a cry that travels through both water and air; hippos above and under water will respond.
Female hippos reach sexual maturity at 5 to 6 years of age and have a gestation period of 8 months. A study of endocrine systems revealed that female hippopotami may begin puberty as early as 3 or 4 years of age. Males reach maturity at around 7.5 years.
A study of hippopotamus reproductive behavior in Uganda showed that peak conceptions occurred during the end of the wet season in the summer, and peak births occurred toward the beginning of the wet season in late winter. This is because of the female's oestrous cycle; as with most large mammals, male hippopotamus spermatozoa is active year round. Studies of hippos in Zambia and South Africa also showed evidence of births occurring at the start of the wet season. After becoming pregnant, a female hippopotamus will typically not begin ovulation again for 17 months.
Hippos can be rather dangerous to humans, as this sign from Kruger National Park notes.
Mating occurs in the water with the female submerged for most of the encounter, her head emerging periodically to draw breath. Hippos are one of the few mammals that give birth under water, along with Cetaceans and Sirenians (manatees and dugongs). Baby hippos are born underwater at a weight between 25 and 45 kg (60–110 lb) and an average length of around 127 cm (50 in) and must swim to the surface to take their first breath. A mother typically gives birth to only one hippo, although twins occur at an unknown ratio. The young often rest on their mothers' backs when in water that is too deep for them, and they swim underwater to suckle. They also will suckle on land when the mother leaves the water. Weaning starts between six and eight months after birth and most calves are fully weaned after a year.
Hippos are considered K-strategists, meaning that they favor quality over quantity in their reproduction. K-selection is the norm for large animals that produce few young at each birth.
Adult hippos are hostile toward crocodiles, which often live in the same pools and rivers as hippos. This is especially so when hippo calves are around. Hippos have been known to be aggressive towards humans, and it is often claimed that hippos are the deadliest animal in Africa; however, according to Smithsonian Magazine, while the animal is very dangerous, reliable statistics for this are unavailable.
To mark territory, hippos spin their tails while defecating to distribute their excrement over the greatest possible area. Hippos also urinate backwards (are retromingent), likely for the same reason.
Hippos rarely kill each other, even in territorial challenges. Usually a territorial bull and a challenging bachelor will stop fighting when it is clear that one hippo is stronger. When hippos become overpopulated, or when a habitat starts to shrink, bulls will sometimes attempt to kill infants; sometimes female hippos will kill the bulls to protect their infants, but neither behavior is common under normal conditions.
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