In the grander scheme of things, hippos belong to the order Artiodactyla, most of the other members of which, such as deer and cattle, are cloven-footed ruminants. Hippos are neither. Like their fellow artiodactyls, the pigs, they are not ruminants, and they have four functional toes. Unlike pigs, however, the toes of hippos are all more or less equal in size and weight-bearing (in pigs, especially warthogs, the two side toes may be used to help steady the animal on slippery or uneven surfaces, but bear little if any of its weight during regular walking).
For these reasons, among others, it was long assumed that pigs were the closest living relatives of hippos. One of the big surprises, when we began to develop genetic means of tracing animal family trees in the late '80s and early '90s, was that the closest living relatives of hippos turned out to be cetaceans - dolphins, whales, and the like. Which sort of makes sense in retrospect, given that they're both large, water-loving, mostly hairless animals, but really wasn't obvious at the time; the similarities looked like a case of parallel evolution.
Indeed, there are rather more anatomical similarities between hippos and pigs than there are between hippos and dolphins. Even leaving aside such obvious features as blow-holes and flippers, the two kinds of animal eat very different foods. Artiodactyls (as traditionally conceived) are one of the great groups of herbivorous animals. Deer, cattle, giraffes, antelopes, camels... they all feed primarily on plant matter. Which makes sense, especially for very large animals like hippos. That larger size protects against most predators, and there are always going to be more plants around than prey animals, so you don't need to rely on a relatively infrequent, and hard-to-catch, food supply to build up the bulk.
For whales, of course, that's not an option, since there are no large plants in most parts of the sea, so switching to something like fish or krill is about your only option. For hippos, in shallow, fresh-water environments, that's no issue at all, and they had no need to undergo the drastic shift in diet that their relatives apparently had to.
Hippos are generally considered to be grazing animals, with the largest portion of their diet consisting of grass, which is perhaps surprising, given that they'd have to get out of the river to eat it. Their digestive system is adapted for this diet, with a large, three-chambered stomach that ferments plant matter to extract the maximum amount of nutrition, even though it isn't adapted to the extent that is seen in ruminants, and is anatomically rather different.
However, it is also widely recognised that they don't restrict themselves to grass, especially during those seasons when fresh, verdant grass is in short supply. In fact, modern analyses have shown that hippos supplement their grassy diet with a number of other plants, such as sedges, herbs, and assorted freshwater plants. Based on these chemical and tooth-wear studies, it may be more accurate to say that, while they prefer grass if they can get it, hippos are, in practice, mixed feeders, with a diet somewhere between that of true grazers (such as buffalo or zebras) and pure browsers (such as giraffes).
But, either way, hippos are herbivores. Except... what does that really mean? In practice, few animals have a 100% restricted diet. Hippos will, inevitably, eat the odd insect or other invertebrate while scooping up shovel-loads of grass or water plants. That's essentially impossible to avoid, but it's also a tiny and negligible part of their diet. What's less well-known though, is that hippos also eat meat.
We are not, of course, talking here about hippos stalking antelope across the plains and luring them to their doom. For the most part, the hippos are scavenging. They come across a carcass of an animal that has died of natural causes, or that has, perhaps, been attacked by a crocodile that never finished its meal, then, seeing the supply of ready food, don't allow it to go to waste. They don't have the sort of teeth that are ideal for stripping meat off the bone, but, with their large size and great strength, they are perfectly capable of eating quite large chunks of flesh.
But hippos are also aggressive animals, particularly in defence of their territories. They will fight one another, especially during the dry season when suitable places to swim become scarce, forcing them together into smaller spaces. And they also fight other animals that see as intruders, and, given their sheer size, the other animal often comes off worse when they do so. Attacks by hippos on humans, for example, are a real danger if you're wandering around in the wrong place. And, while the reason for the attack is likely not often hunger, once the victim is dead, it is not as unusual as one might think for the hippo to eat its victim. From the wild, we know of cases in which hippos have killed and eaten a range of different kinds of antelope, along with zebras, and even buffalo.
In fact, while it seems to be rare, there are even reported instances of cannibalism, likely after a territorial fight went a bit too far.
Of course, none this is a large portion of their diet. The digestive system of hippos is, as I noted above, quite clearly adapted for digesting plant matter, and contains bacteria that help to break down tough vegetation, just as those of cows and horses do. You might think that this would mean that they really wouldn't be very good at extracting nutrition from meat, but this doesn't seem to be the case. (It's perhaps worth pointing out that some whales also have multi-chambered stomachs like those of hippos, and, of course, they don't eat grass at all).
In fact, while hippos have an obvious advantage in terms of their ability to kill things, they are not unique among ungulates in adding a little meat to their diet from time to time. Deer, for example, have been seen killing and eating the young of ground-nesting birds, as have sheep and cattle. And many herbivores do eat the placenta of their young immediately after birth, although nobody seems to know whether or not hippos are among them. Meat, if you can get it, is generally easier to extract nutrition from than plants, which is why carnivores don't need the complex digestive systems that herbivores do. Indeed, farmers took advantage of this for a long time by feeding cattle with meat-and-bone meal, which served as a useful source of protein to help their growth.
Which... um... didn't end so well.
That was for reasons entirely unrelated to the ability of the animals to eat meat products, but that doesn't necessarily mean that it's irrelevant, either. A recent review of carnivory among hippos asked whether or not occasional meat-eating might contribute to disease outbreaks among the animals. There isn't any BSE wild in Africa, at least that we know of, but a number of other livestock diseases do exist. As it happens, hippos are among the few artiodactyls that are immune to foot-and-mouth disease, and before it ceased to be a problem, it also helped that they were unusually resistant to rinderpest.
But anthrax outbreaks can kill literally thousands of hippos at a time. A major outbreak in 2005 is notable for affecting very few animals other than hippos, indicating that they were likely doing something that the other animals around them weren't. We also know, from the history of the outbreak, that it spread downriver. We don't know if this was really due to infected carcasses drifting downstream and being eaten by hungry hippos - although, according to the review, there is apparently one report of a hippo doing exactly that.
If so, the fact that hippos eat at least some meat, and that this isn't some weird aberration caused by desperation or pre-existing illness may prove more than a curious piece of trivia.
[Photo by "Gusjer", from Wikimedia Commons.]
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