Aah, hippos. Africa’s chubby water cows. They’re big, they’re dangerous and they have incisors that can reach lengths of some 50 centimetres (20 inches). But these mega-mammals may have a few secrets that we're only just starting to unravel.

While taking guests out on safari in South Africa’s Pilanesberg Game Reserve in February 2013, field guide Solomzi Radebe from Mankwe GAMETRACKERS came across something a little odd: a meat-munching hippo. 

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A hippo joins the lion pride at their kill. Image © Solomzi Radebe/Mankwe GAMETRACKERS

Guests were watching a lion pride tucking into a freshly killed wildebeest when a curious hippo ambled over to take a spot at the carcass. “The lions did act uneasy with snarls and growls at the hippo,” explains Tarryn Rae from Mankwe GAMETRACKERS. “But at the end of the day they probably understand that one can hurt the other just as severely.”

Hippos are traditionally regarded as strictly herbivorous. They can munch up to 40kg (88lb) of grass every evening to help maintain their fatty figures, before retreating to the safety and coolness of the water to digest their meal. However, sightings such as this have raised doubts about the hippos' dietary preferences, and suggest that they may crave something a little more meaty from time to time.

“This isn’t the first or last time that these lions shared their meal with the hippo family,” Rae explains. “At the end of last year at that same dam the lions were on a zebra kill close to the water and the hippos decided to join in on the feast, with a lot of growling and snarling … most of the guides have seen hippos eating meat at one point or another.”

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Nom nom nom ... wildebeest. Image © Solomzi Radebe/Mankwe GAMETRACKERS

So just how unusual is this behaviour? Hippos hardly have the equipment to digest a steak – their anatomy would suggest that they are more suited to a diet consisting of plant matter. Much like cows, they have chambered stomachs that work like fermentation factories, breaking down grass and extracting the necessary nutrients. And yet, there is more than one record of hippos eating meat.

In 1995, Dr Joseph P. Dudley of the University of Alaska recorded the first instance of hippos eating meat in Hwange National Park in Zimbabwe. He’s been collecting evidence ever since. The latest development in the hippo carnivore conundrum comes from PhD student Leejiah Dorward of Imperial College London who published a paper early this year detailing what is believed to be only the second confirmed account of hippo cannibalism (zombie hippos!).

But why tuck into rotting carpaccio when there’s plenty of salad to go around? Hippo expert Dr Keith Eltringham suggests that the species will be driven to scavenge meat only when their usual food is scarce (during a drought, for example). Dorward supports the theory: “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.”

Despite an increase in reports of meat-eating hippos, Dr Dudley doesn't think that the behaviour is on the rise, but rather that it has been overlooked in the past. “This 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,” Dudley told BBC Earth.

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The lioness suffocating her prey before the rest of the pride (and their chubby dinner guest) arrived at the party. Image © Solomzi Radebe/Mankwe GAMETRACKERS