Hippos are huge animals with fearsome tusks and aggressive natures, but they mainly eat plants. Sometimes they attack people and can tangle with crocodiles, sure, but they’re not predators or carnivores. Right?

But a closer look reveals that hippos aren't so herbivorous after all, says biologist Joseph Dudley. Despite their grass-heavy diets and all the adaptations that make them great grazers, hippos have been known to eat their fair share of meat. There are scattered reports by scientists and amateur observers (note: NSFW gory pictures) of hippos attacking, killing, and eating other animals, stealing kills from predators, and scavenging carcasses, including those of other hippos. In a new paper published in the journal Mammal Review, Dudley and his fellow researchers argue that these incidents aren’t as unusual as they seem or isolated to a few animals or populations. They say there’s a pattern of carnivorous behavior in hippo populations across the animal’s entire range—and that behavior has consequences for hippos.

Evolution has outfitted hippos and other big herbivores for a plant-based diet, and their guts and the microbes that live within them are adapted for fermenting and digesting lots of plant material. That doesn’t mean these herbivores animals can’t add meat to their menu, though. Many can and do. Antelope, deer, and cattle have been known to feed on carrion, birds’ eggs, birds, small mammals, and fish. What might hold most of these animals back from more frequent carnivory, Dudley suggests, isn’t their digestive physiology, but “biomechanical limitations” in securing and ingesting meat. In other words, they aren’t built for taking down prey or biting into flesh. The hippo is another story.

THOSE WHO CAN, DO

“Due to its large body size and unusual mouth and dental configurations, the hippo may represent an extreme case in which the predation and scavenging of large mammals by an ungulate species is not constrained by biomechanical factors,” Dudley and his team write. Not only can hippos kill and eat other big animals more easily than other herbivores, the researchers say, the fact that they’re territorial and highly aggressive may facilitate carnivory, putting them in situations where they kill other animals and can get themselves something to eat.

And eat they do. Since Dudley made the first scientific record of carnivory in hippos in 1996, other cases of hippo carnivory and even cannibalism have also been documented. Dudley lists instances where wild hippos have fed on impalas, elephants, kudus, wildebeest, zebras, and other hippos that they either killed themselves or were killed by other predators. Events like these have been seen both during times when carnivory may be a last resort (e.g. droughts when food is scarce), and when it was merely a convenient opportunity, like a mass drowning of wildebeest crossing a river. There are also reports of captive hippos in zoos killing and eating their neighbors, including tapirs, wallabies, flamingoes and pygmy hippos.

“Our scientific records, coupled with those of other investigators and observers, demonstrate that the phenomenon of carnivory by hippos is not restricted to particular individuals or local populations but is an inherent characteristic of the behavioral ecology of hippos,” the team writes.

EATING MEAT WHILE WE SLEEP

If that’s the case, then why did it take so long for anyone to figure it out? Part of the blame can fall on conflicting schedules. Hippos are mostly active at night, which means their meals, meat or otherwise, usually go unseen by humans. Their carnivorous ways, Dudley thinks, have simply been overlooked.

They may also explain why hippos are so susceptible to anthrax and experience higher mortality rates during outbreaks, the researchers say. Hippos, they think, are doubly exposed to the disease because they ingest and inhale bacterial spores on plants and in the soil like other herbivores, and also consume them when feeding on contaminated carcasses. Cannibalism during outbreaks exacerbates the problem.

That carnivory might make these outbreaks worse in hippo populations has implications for controlling the disease and protecting both animals and humans. During anthrax outbreaks among wildlife, many human illnesses occur because of contaminated “bush meat.” During a 2011 outbreak in Zambia, for example, 511 human cases of anthrax and at least five deaths were linked to people handling and consuming meat from infected hippos. Burying or burning suspected infected animal carcasses is a standard practice during anthrax outbreaks, and the researchers think that this may be especially effective in hippo habitats because it takes infected meat off the menu for both humans and hippos.