Reputation: Slow and sluggish, these gentle herbivores spend their lives lolling about in muddy tropical waters, keeping to themselves.

Reality: Hippos are remarkably agile and aggressive. They occasionally kill or scavenge impala, kudu and buffalo. Sometimes they even kill humans. On nightly sojourns out of the water, hippos trek miles to nibble on tasty vegetation – including crops, making humans and hippos uneasy neighbours.

Believe what you see and read in fiction, and hippos are among the most gentle of Africa’s giants. They are depicted wearing tutus or going to great lengths to nurture friendships.

Tourists would do well to recognise and respond appropriately to these threat signs

To be fair, real-life hippos do look vaguely ridiculous with their lugubrious faces and swollen bellies propped up on four stubby legs. Looks can be deceiving, though.

“The majority of people from overseas are not aware of how dangerous these animals can be,” says Johan Eksteen, a South African conservation ecologist who has researched hippo-related issues for nearly three decades and now works for the Mpumalanga Tourism and Parks Agency.

Tourists see a hippo repeatedly yawn and assume the animal is happy and content. “They are a threat sign,” says Eksteen.

“They also make a sound like a laugh,” he says. It’s another threat sign, even though a “laughing” hippo might appear calm and comical.

Hippos also appear to have no qualms about occasionally eating each other

Tourists would do well to recognise and respond appropriately to these threat signs, because an unhappy hippo is more hazardous than it might look.

“I think the most common misconception about hippos is that people consider them to be pure herbivores,” says Leejiah Dorward, a PhD student at the University of Oxford who studies human-carnivore conflict in Tanzania.

Over the years in various parts of their range, 87 hippos have been observed eating impala, kudu, eland, wildebeest and buffalo. “There is a growing body of evidence that they not only eat meat but will actively predate other animals,” says Dorward.

Hippos also appear to have no qualms about occasionally eating each other. While working in Kruger National Park in South Africa, Dorward saw a hippo feeding on the decayed carcass of another hippo.

The most common misconception about hippos is that people consider them to be pure herbivores

“Unfortunately we did not know how the hippo being fed on had died as this may have shed more light on the behaviour,” he says.

Hippos sometimes attack humans too. “They don’t go out and hunt humans, but if you encounter them on the way back to the water and you are in between,” says Eksteen, “then you have to make a plan!”

In Niger in 2014, 13 people drowned when hippos overturned their boat. Similar incidents happen from time to time across their range.

Jessica Kahler studied human-wildlife conflict in Namibia for her Master’s research, looking particularly at how humans perceive the risk of various wild animals. She heard a few gory tales about hippos, including that of a husband and wife killed one night by a hippo lurking just outside their home.

The hippo is “a crazy design for something that’s got to eat grass”

Kahler herself had a scary hippo encounter. Staying in a small tent by the river to do her research, one night, some cattle must have been grazing down by the river, “and a hippo wasn’t very pleased”, she says. The hippo sent the herd of cattle running through her campsite.

She was torn between fleeing to the safety of her car, risking encountering the hippo in the dark on the way, or staying put in a flimsy tent, which a cow was trying to enter. She opted to stay in the tent, smacking the unwelcome cow on the nose to keep it at bay. In the morning, she says, “there were all these footprints everywhere”.

However, hippo guts are unsuited to digesting meat, says Dorward, so carnivory may only happen when hippos are deficient in a particular nutrient. Most of the time they nibble on plants – although arguably, the large animals are awkwardly suited to this diet too.

The hippo is “a crazy design for something that’s got to eat grass”, quips Eksteen.

At night hippos can cover relatively large distances in search of vegetation

Most grazers rely on sharp incisors to carefully nip the top off vegetation. Hippos do have lengthy incisors and canines but their purpose is combat, not grass mowing. These mammals have to clip grass with their lips, Eksteen explains.

By day, hippos loll around in water to cool themselves down, which might have contributed to their reputation for being relatively sluggish and sedentary. It’s difficult to imagine their tiny legs carrying such a heavy animal particularly far.

But in fact, at night hippos can cover relatively large distances in search of vegetation.

Where Eksteen has studied them near Kruger National Park, individuals travel an average distance of 0.8 miles (1.3km) to graze. Hippos move longer distances by winter’s end when they’ve depleted their munchable waterside greens.

So voracious are hippo appetites for tasty greens, and so far will they travel, that they maraud farmers’ cash crops when they can.

Hippos’ habit of dung spraying and stirring up the river bottom “enriches the rivers”

Though electric fences have had some success in keeping hungry hippos out, in some riverside habitats in South Africa, farmers have given up on vegetables and switched to citrus trees. It’s a shrewd move: hippos, not interested in the fruit but happy to graze between the trees, provide night-time security detail for citrus farmers, explains Eksteen.

Hippos might hold other benefits for humans.

“They don't have any sweat glands, they have mucous glands,” says Eksteen. Within minutes, the initially colourless acidic hippo slime they secrete changes to red-orange, then brown. It acts as hippo sun protectant – and its chemical composition has intrigued biochemists that have uncovered its dual role as a sunscreen and antimicrobial.

Will future skincare products mimic the chemical composition of hippo slime? It’s possible.

Hippos also play an important ecological role. Their habit of vigorous dung spraying and stirring up the river bottom “enriches the rivers” and benefits fish populations, says Eksteen.

Will hippos sink or swim as a species?

Their beneficial mud stirring happens because they really can’t swim. “They actually run along the bottom of rivers,” he says.

So despite the danger they can pose to people, hippos are important stewards of their habitats. It’s bad news, then, that they are considered vulnerable by the International Union for Conservation of Nature. Hippos are threatened by poaching, unregulated hunting and habitat loss.

The possibility for tension between humans and hippos is strong given that we can be direct competitors for both freshwater and crops.

Will hippos sink or swim as a species? That remains to be seen.