Hippos Kill More People Than Lions. Nobody Warns You.

In 2004, a team of Japanese researchers led by Kimiko Hashimoto at Kyoto Pharmaceutical University published an analysis of the reddish oily secretion that coats a hippopotamus’s skin. For years, the substance had been described in popular accounts as blood sweat, a vivid phrase that implied the animal was literally sweating blood through its pores. The reality, as Hashimoto’s team found, is stranger and more useful than that description suggests.

The secretion contains two previously unknown pigmented compounds, which the researchers named hipposudoric acid and norhipposudoric acid. The first is red. The second is orange. Neither is blood, and neither is sweat. The hippo has no sweat glands. What it produces instead is a specialized skin secretion that functions simultaneously as a sunscreen, an antimicrobial agent, and possibly a moisturizer, all without any of the chemical inputs that human cosmetic products require.

Hipposudoric acid absorbs ultraviolet light across a broad spectrum, providing sun protection to an animal that spends significant time out of water during the day. The antimicrobial properties were confirmed in laboratory tests showing that both compounds inhibit the growth of pathogenic bacteria. For an animal that spends its days in water shared with thousands of other hippos, in conditions that would be ideal for bacterial infection, this built-in antimicrobial coating is not a luxury. It is a survival mechanism.

The secretion starts colorless and turns red within minutes of exposure to air, which is why observers have historically described hippos as sweating blood. The color change is a chemical reaction, not a sign of injury. A hippo emerging from water and standing in sunlight is not bleeding. It is applying sunscreen.

The Most Dangerous Animal in Africa

Hippopotamus amphibius kills more people in Africa each year than lions, leopards, and crocodiles combined, by most estimates. This statistic surprises people who associate hippos with the rotund, slow-moving animals of zoo enclosures. Wild hippos are neither slow nor predictable.

Adult males can weigh up to 4,500 kilograms and run at speeds of up to 30 kilometers per hour over short distances, faster than most humans. The canine teeth, which can reach 50 centimeters in length, are used in territorial fights between males that are genuinely violent, producing wounds that would be fatal in most other animals. The jaw can open to 150 degrees and generates enough force to bite a small boat in half.

The danger to humans comes primarily from territorial behavior near water. Hippos are highly territorial in aquatic environments and will charge boats, canoes, and people who enter or approach their stretch of river without warning. The charges are not bluffs. Researcher Keith Eltringham, who studied hippo behavior and ecology extensively and wrote the definitive monograph on the species, documented that hippos show little of the warning behavior that other large African mammals display before attacking. The attack often comes with minimal prior signaling.

On land, hippos are dangerous primarily when encountered between water and their grazing areas. They feed on grass at night, moving up to 10 kilometers from water, and a hippo on its return route to the river will not detour around a human obstacle. It will go through it.

Social Life in the Water

Hippo social structure is organized around water. Groups of 10 to 30 individuals, called bloats, share a stretch of river or lake under the dominance of a single adult male who controls mating access to the females in his territory. Subordinate males are tolerated within the territory as long as they do not challenge the dominant male or attempt to mate.

The social interactions within a bloat are complex and have been studied in detail by researcher Shirley Strum and colleagues working in East African lake systems. Females form stable associations with each other and with their offspring, maintaining bonds across years. Calves are born in water and can swim before they can walk effectively on land. Nursing occurs underwater, with the calf surfacing to breathe every 30 to 40 seconds.

Communication between hippos involves a range of vocalizations including the characteristic wheeze-honk that carries across water for several kilometers, as well as infrasound components that travel through both water and ground. A hippo calling from the middle of a river is simultaneously broadcasting through the air and through the water, reaching individuals in both media.

A Species Under Pressure

Hippopotamus amphibius is listed as Vulnerable on the IUCN Red List, with populations declining across most of their range. The primary threats are hunting for ivory, since hippo canine teeth are a significant source of ivory following restrictions on elephant ivory trade, and habitat loss as rivers and lakes are modified for agriculture and human settlement.

The Democratic Republic of Congo, which holds some of the largest remaining hippo populations, has seen dramatic declines during periods of civil conflict, when wildlife protection collapses and bushmeat hunting increases. Population surveys conducted after periods of conflict have found reductions of 90% or more in some areas.

An animal that produces its own sunscreen, runs faster than most humans, and communicates simultaneously through air and water is losing ground to ivory hunters and habitat loss. The blood sweat story gets the attention. The conservation crisis mostly doesn’t.