Carnivory in Hippos

Replica of the skull of the Hippopotamus, Hippopotamus amphibius, at Disney’s Animal Kingdom Park (via Wikipedia)

Hippos are one of those animals that are sometimes used to drive home the point that appearances can be deceiving; despite their fearsome dentition, they’re really herbivores. They may have bad attitudes and be a terror along African waterways, but at least they’re not carnivorous. At least, that’s the story.

It’s easy to see why the hippopotamus got it’s name, meaning “River Horse.” Taken at the Philadelphia Zoo, August 2006.

While the vast majority of the hippo’s food intake is short grass that grows along river banks, this doesn’t exempt them from eating meat every now and then. In 1996, J.P. Dudley published the paper “Record of carnivory, scavenging and predation for Hippopotamus amphibius in Hwange National Park, Zimbabwe,” followed two years later by “Reports of carnivory by the common hippo Hippopotamus amphibius.” I do not have access to either of these papers, but as far as I understand it, they are the best scientific accounts that we have that hippos do sometimes eat meat. A pdf file announcing the Dudley’s work, states that not only was carnivory witnessed among the hippos, but even cannibalism, the strange habits of the studied group perhaps contributing to an anthrax outbreak that ended up killing 300 hippos in the area.

Taken at the Philadelphia Zoo, August 2006.

More clues are offered up by a short summary from a 1999 issue of Natural History;

Although the hippopotamus’s usual fare is this habitual vegetarian can occasionally turn carnivore. Field biologist Joseph P. Dudley, formerly at Hwange National Park in Zimbabwe, observed a male hippo killing an impala ram that had swum through a pond to evade a wild dog. After eating some of the meat, the hippo returned to his herd.

A few minutes Later, ten more individuals from the group gathered at the floating carcass for a communal feast. Later a few of them climbed the pond’s banks to wrest another dead impala from wild dogs. “It seems almost incredible,” writes Dudley, “that carnivorous feeding behavior by hippos, even if of very infrequent occurrence, could have gone unreported for so long.” Perhaps, he opines, the behavior may have been attributable to “nutritional stress caused by severe drought conditions.”

Taken at the Philadelphia Zoo, August 2006.

Even last night I caught a few moments of a nature documentary showing hippos mouthing/playing with bloated carcasses of wildebeest that drowned in an attempt to cross a river during their annual migration between the Serengeti National Park and Masai Mara National Park. The segment was short, but overall it didn’t seem that the hippos were very interested in consuming the dead antelope as much as mouthing it a bit before moving on. Likewise, hippos in captivity have been known to “play” with/harass other animals in their enclosures, especially birds. At Adventure Aquarium in New Jersey, many Turacos ended up dying of stress when they would fall into the hippo enclosure, the mammals taking the birds into their mouths and lashing them about, keepers frantically trying to rescue the birds with a dip net. Darren Naish has also posted anecdotal evidence of carnivory in captive hippos before, as well.

As I said before, I don’t have access to the scientific papers, but it seems that many of the hippo predation/scavenging events occur in years of drought, where large groups of the animals are brought together in one area when grasses and other “normal” foods are scarce. This could be why hippos in the documentary I mentioned above only “mouthed” the carasses of the wildebeest; they may have had plenty of food so there was no need to eat something that’s not a regular part of their diet. Still, as has been mentioned by Dudley, hippos often eat at night, and their nocturnal eating habits have generally kept us in the dark as to what they might be munching on other than grass.

But what about cannibalism and anthrax? A 2004 article in NewScientist suggests that the spread of anthrax in hippos was actually helped by a particularly harsh year where carnivory and cannibalism came apparent. The problem probably started in water contaminated with anthrax or grass containing anthrax spores. The problem with the hypothesis that hippos were initially infected by anthrax-contaminated water, however, is that the park in which the outbreak occurred is said to have plenty of flowing water, not the stagnant pools typical of an environment that harbors anthrax.

Regardless of how it got into the local ecology and transmitted to hippos, however, a particularly harsh or lean year could have easily caused rapid transmission in a population, even if only a small group were infected. If resources were scarce and hippos (known for their aggressiveness) were pushed closer together in the remaining areas of water and food, there would be much more fighting, and hippos leave tremendous gashes (prone to infections) on each other during these fights. This would make the animals more susceptible to infection, and because hippos appear to eat meat more when other food is scarce, the infected hippos would be more likely to be consumed, infecting even more hippos and leading to a runaway outbreak.

Looking at the dentition of hippos, part of the reason why I wouldn’t necessarily expect carnivory to be common is that their teeth seem to be more adapted to be weapons than efficient cutting/slicing teeth. Hippos may have a good amount of power behind their bite, and the teeth in the front of their jaw are certainly powerful when it comes to puncturing, but it would seem to me that they would have to puncture carcasses with their massive front teeth and then try to rip off bits to then swallow whole; they lack the more efficient piercing and cutting teeth of carnivores like dogs and big cats. The apparently get by well enough without it when times get hard, but it doesn’t seem like they’re particularly well-adapted to carnivory. The following clip gives us a brief glimpse of a hippo scavenging from a buffalo carcass, although it doesn’t seem particularly interested in consuming much or fighting the lions for the carcass;

While it may be easy to write off such behaviors as aberrant or abnormal, they are very important to our understanding of ecology. Many people have a somewhat Disney-esque idea of what nature is like, with the “great Circle of Life” allowing each animal to have their own niche and live in harmony with each other; carnivores only eat other animals, herbivores only eat plants, and mammalian babies are cared for by both the “mommy and daddy” until they grow up. The truth of the matter is that ecological interactions are far more complex than we realize, sometimes revealing a “darker” side of nature than some people care to acknowledge (but again, this is putting a moral judgment on nature, which does not operate by a moral code or “Law of the Jungle”). There is cooperation in nature, but there is also fierce competition, and when ecologies start to become strained, whole new dimensions of behavior that are otherwise hidden or only barely visible come into play in an attempt to allow for the survival of the individual animal. Why should we expect otherwise?

Hippo Skeleton. Via Wikipedia.