Scientists have got it wrong about hippopotamuses a lot over the years. Their name in ancient Greek translates to “river horse” yet modern science linked the animals to pigs. The most recent studies have found they are more closely related to whales.
They also don’t sweat blood as once thought, but excrete a red fluid that contains antibacterial sunscreen. Plus the stubby-legged rotund creatures have surprised biologists with running speeds of up to 19mph.
We assume that we know everything about these animals because of centuries of human study and observation but... there are things happening out there that do not meet with what we think we know
With a chambered stomach that works like a fermentation factory to break down plant matter, the animals are known to be herbivores. But is there about to be a disturbing U-turn on that knowledge too?
One detail that cannot be misunderstood is that hippos are huge – mature males weigh up to a whopping 3,200kg. They also have a well-documented reputation for aggression and engage in brutal battles over mates, slashing and biting with their incisor teeth, which can measure up to 40cm (1.3ft) in length.
Grisly encounters with unlucky locals and tour guides that got too close have led to hippos being named the most dangerous animals in Africa, allegedly responsible for more human deaths than lions.
Yet hippos only prey on grass. To maintain their impressive aspects, they consume around 40kg (88 lb) a night and maintain their fatty figures with a sedentary lifestyle, spending most of their time wallowing to cool off and digest their food.
But recently scientists have started to uncover some exceptional behaviour that could change our opinion of the animals once again, especially their seemingly limited diet. This month, PhD student Leejiah Dorward of Imperial College London published a paper in the African Journal of Ecology recording a rare experience.
“I was in the southern end of Kruger National Park in South Africa when crossing a river we saw two hippos in the water with a quite decayed, dead hippo surrounded by crocodiles. This was not initially that unique a sight until we saw one of the hippos feeding from the carcass,” he said.
Dorward was surprised to see a known vegetarian feeding not only on meat, but on the meat of its own species.
“It was not until I got back to the UK and did some reading around hippo diets that I realised how unreported this behaviour was,” he said.
In fact, Dorward’s record is only the second confirmed account of cannibalism by hippos in scientific literature. His is the latest piece in a puzzle Dr Joseph P Dudley of the University of Alaska, US, has been looking at for two decades.
I think the important thing is that this demonstrated how much we still have to learn about nature in general, and even some of the largest and most charismatic wildlife species on the planet
Dr Dudley made the first ever record of hippos eating meat at Hwange National Park, Zimbabwe in 1995. Since then he has been collecting evidence of hippos eating impala, baby elephants and even their own kind. While the cannibalistic incidents still number fewer than a dozen, they have occurred across the animals’ range, from South Africa to Uganda.
In the past, hippo expert Dr Keith Eltringham suggested that the animals are not predators but are driven to scavenge meat when food or particular nutrients are scarce. It’s true that the animals are now facing increasing pressure from humans that hunt them for their meat and ivory teeth, encroaching settlements and growing competition for freshwater. The International Union for the Conservation of Nature lists the species as vulnerable across its range after declines of up to 20% over the past two decades.
“If carnivory is driven by dietary deficiencies in hippos then they may become more dangerous during droughts or other times when their access to food is restricted and trying to understand if this is the case will be important,” said Leejiah Dorward who studies the interaction between poor communities and their environment.
Dr Dudley believes the hippos’ meat-eating behaviour is not on the rise but has simply been overlooked in the past.
“We assume that we know everything about these animals because of centuries of human study and observation but as this phenomenon demonstrates, there are things happening out there that do not meet with what we think we know,” said Dr Dudley.
“This [carnivory] is something that has been going on since there were hippos, it’s not new simply because human beings discovered it was happening only recently. This is the fault of our own ignorance, not the results of some evolutionary leap on the part of hippos in the last two decades.”
Dr Dudley and a group of colleagues are now working together to solve the latest hippopotamus mystery. He hopes that the boom in safari-loving tourists sharing photos and videos online, alongside traditional scientific observations, will provide more opportunities to understand the creatures with a history of contradiction.
“I think the important thing is that this demonstrated how much we still have to learn about nature in general, and even some of the largest and most charismatic wildlife species on the planet,” he said.
Follow BBC Earth on Twitter, Facebook and Instagram.