Hippos 'Sweat Blood,' Sleep Underwater and Bite Harder than a Lion

Hippos are among the most iconic animals in Africa and hold the distinction of being the third-largest land mammal living on Earth today after elephants and white rhinos. On top of their huge size, these animals have numerous fascinating behaviors and characteristics that have enabled them to adapt to their native environment over the course of millions of years.

This semi-aquatic animal (scientific name: Hippopotamus amphibius) is found across several parts of sub-Saharan Africa, inhabiting wetlands, rivers, lakes and swamps, among other similar habitats.

Biggest hippo ever

These giants tend to weigh between around 2,800 and 7,000 pounds as adults—similar to a car—and grow to lengths of between around 7 and 16 feet, according to the University of Michigan Museum of Zoology's Animal Diversity Web online database.

Some hippos grow even larger though—the biggest individual ever recorded was a captive individual kept in Germany that weighed almost 10,000 pounds.

While these bulky, barrel-shaped creatures might appear to be clumsy and slow-moving at first glance, they are surprisingly agile on land, capable of moving at speeds of nearly 20 miles per hour.

Hippos spend most of the daylight hours wallowing in water and they are well-adapted to the aquatic environment—for example, their eyes, ears and nostrils are positioned in such a way on the top of their head that they can see, hear and breathe while the majority of their body is submerged.

Hippos have a membrane that covers their eyes, which protects them while enabling the animal to see underwater. They can even close their nostrils and fold their ears to prevent water from entering their bodies when they are completely submerged.

Hippos can hold their breath for long periods of time—five minutes or even longer—and can even sleep underwater, automatically surfacing to take breaths when necessary without waking up.

Despite all these traits that have enabled them to adapt well to semi-aquatic life, hippos cannot technically swim and their bodies are too dense to float. To move while in the water, these giants essentially leap off the bottom of the body of water they are in before engaging in a galloping motion using their short legs and webbed feet.

Hippos don't sweat

You may be surprised to learn that hippos, which live up to 50 years in the wild, are actually closely related to whales. The two animal groups are each others' closest living relatives sharing a common ancestor that lived more than 50 million years ago.

Like whales, hippos also lack sweat glands and rely on water to keep cool. But they do produce a thick, oily later of a red, pigmented fluid that used to be referred to as "blood sweat" because it was once thought to be a mixture of these two components. Now, scientists know this fluid is made up of different acids and acts as a natural sunscreen while also potentially preventing the growth of potentially harmful bacteria.

Hippos are social animals and tend to live in flexible groups of 10 to 30 individuals, although some groups can reach 200 individuals in size. These animals are polygynous, meaning that the male bulls mate with several females in the herd.

These groups tend to have a dominant male at the top of their social hierarchy who is able to mate with any of the females. Sex among hippos usually involves the male forcing the female into submission while ensuring that her head is underwater—scientists do not yet know why this is necessary.

Male hippos compete for dominance and can be very territorial. They have been seen using their paddle-like tails to help spread excrement in order to mark their territories.

Fights between rival hippos sometimes occur and can be deadly in the case of a particularly aggressive battle. These animals can open their giant mouths 150 degrees, revealing their enormous, sharp incisors and canine tusks that they can use as weapons in any confrontation.

Hippos use a variety of sounds—including snorts, grumbles and even subsonic vocalizations—that can be heard both above land and underwater. Some of these sounds can be extremely loud, reaching levels of around 115 decibels, which is comparable to noise levels at a rock concert.

Hippos are generally herbivorous, wading out of the water after the sun goes down in search of food—which primarily consists of small shoots, grasses and reeds, among other types of plants.

Following well-trodden "hippo paths," these animals graze for several hours every night, consuming a huge amount of food—nearly 90 pounds. On rare occasions, hippos have been seen munching on dead animals, but this is not usually part of their diet and there may be particular reasons why they engage in such behavior—for example, they may be lacking in certain nutrients.

While they sometimes have to travel several miles to find food, hippos are generally quite inactive, living a mostly sedentary lifestyle when not eating, which helps them to conserve energy.

Lions, hyenas and crocodiles sometimes prey on young hippos, but due to their huge size, the adults have no known predators—apart from humans that is.

Hippos and human conflict

Hippos have a reputation for being dangerous to humans, and they are thought to be among the most deadly land mammals.

The number of people killed by hippos every year is not known given that no data repository exists on human injury or fatality due to hippo conflict. But some estimates indicate that the figure might be between 500 to 3,000 fatalities annually.

People often believe hippos kill more people than any other mammal but this may not necessarily be true, according to Lochran Traill, PhD, an ecologist and conservation scientist from the University of Leeds in the United Kingdom, who has past experience working in southern Africa.

"Context matters, and in some places like Mozambique, more people are killed by elephant and crocodiles than hippo," Traill told Newsweek. "What does seem to be true though is that if a person is attacked by a hippo, their likelihood of survival is lower than if they were attacked by lion or elephant. So it is true that hippo are very dangerous."

While hippos are generally quite docile, they can be territorial and have been known to act aggressively toward humans in certain situations. Hippos usually attack only if they perceive a threat—for example, when someone stands between them and the water they spend their time in. Mothers may also attack to fend off perceived threats to their young, while males are very protective over females and calves in the herd.

"Generally, incidents occur accidentally on land," Traill said. "People may bump into hippo at night, and hippo respond to the perceived danger by attacking people. In the water, hippo may feel threatened by people in boats and attack them."

If they do decide to attack, hippos pose a huge threat to humans. Traill said these animals are "very powerful" while also being surprisingly quick and agile. They also have an incredibly strong bite strength.

"Their bite strength is 12,600 kPa, which is around three times greater than that of a lion," Traill said. "That is why they are so dangerous —they cause substantial trauma to the human body through crushing force, whether this be through biting or by trampling."

One study published in the journal Oxford Medical Case Reports, found that the probability of being killed by a hippo in any given attack ranges from around 29 percent to 87 percent. The study, which documented 11 patients Burundi who suffered hippo bite injuries, found a high incidence of wound infections, amputations and permanent disability, as well as other complications, among the victims.

"The hippopotamus, with his ferocious jaw force, unique mouth size and sharp teeth, can easily bisect a human body in a single bite," the authors of the study wrote.

The chance of hippo-related injuries increases as humans encroach on the natural habitat of these animals—a trend that is becoming increasingly common as people build new settlements, increase agricultural production and construct new infrastructure.

"Hippo do not actively seek out people to harm or injure," Traill said. "Basically the issue is that people and hippo occupy the same area and compete for resources, and so conflict occurs."

Hippo populations have experienced a substantial reduction in their former range across Africa, which has principally been caused by two factors: habitat loss and over-exploitation, according to Traill.

"Over-exploitation includes the killing of hippo, mostly done illegally," she said. "This can occur for a few reasons; these being retributive killing of hippo following hippo human conflict, the killing of hippo for meat, and the killing of hippo for illicit trade in hippo ivory, or skin sales. Of those, the killing of animals for meat, and through conflict are the most serious."

While hippos are protected across most of Africa, illegal killings do still occur and park authorities may still shoot problem animals. Hippo are also legally hunted in some places, although the number of fatalities resulting from this practise is relatively low.

The number of hippos killed by humans is not well-known because, again, there is no centralized database and incidents may go unreported, according to Traill. But the African Wildlife Foundation estimates that hundreds are shot each year. Other research has shown that for every person killed by a hippo, 2.7 hippos are killed on average in retaliation.

"That's the highest ratio there is in southern Africa—the retaliation rate is lower for lion or elephant. I imagine these figures hold across much of Africa where there is ongoing conflict between people and hippo," Traill said.

The range of the hippo once extended across much of Africa, from the southern tip of the continent to the Nile Delta, but now it is mostly restricted to protected areas. While common hippos are not endangered, they are classified as "Vulnerable" on the International Union for Conservation of Nature's Red List of Threatened Species given that populations have been declining in recent years.

According to Traill, there are a number of measures that may be able to mitigate conflict between humans and hippos, including educating local communities about the value of hippos within their own culture and the need to conserve the animals.

"Other effective measures include the development of buffer zones along rivers, which are protected and prohibit livestock grazing, fishing or cropping," Traill said. "Also, simple barriers, such as low-lying walls or strong cables or fences around crops will help prevent raiding by hippo."

Other measures could include compensating people for damage done by hippo. Relocating the animals has also been trialled in certain situations, but is expensive, and the new habitat needs to be adequate.

"Finally, there is opportunity for community-based conservation schemes that allow the people affected by conflict the opportunity to derive income from hippo, such as through photographic tourism," Traill said.

"None of these mitigation approaches are without challenges. In the end, it is likely that the best physical approach may be the setting aside of further protected areas for hippo."

References

Moustafa M Haddara, Jean Berchmans Haberisoni, Miguel Trelles, Jean-Paul Gohou, Kwizera Christella, Lynette Dominguez, Engy Ali, Hippopotamus bite morbidity: a report of 11 cases from Burundi, Oxford Medical Case Reports, Volume 2020, Issue 8, August 2020, omaa061, https://doi.org/10.1093/omcr/omaa061

Dunham, K., Ghiurghi, A., Cumbi, R., & Urbano, F. (2010). Human–wildlife conflict in Mozambique: A national perspective, with emphasis on wildlife attacks on humans. Oryx, 44(2), 185-193. http://dx.doi.org/10.1017/S003060530999086X