I stood in front of a smoking grill at a campground in Florida with a pair of raw T-bone steaks. As I dropped them onto the grill, I had that eerie sense of being stared at. I turned around and saw a whitetail doe staring at me from only a few yards away. Habituated to human presence and living in a place where hunting was forbidden, she seemed very interested in what I was doing. When the steaks came off the grill I sat down to eat and found an aggressive snout that wanted a share. I carved off a bite of rare meat and held it out for her. This supposed herbivore greedily devoured bites of cow flesh. Realizing that nobody would ever believe me, I grabbed a camera and got it on video.

Any third-grader can tell you the difference between herbivores and carnivores. Herbivores eat plants and carnivores eat meat, and then there are a few oddball omnivores that eat both. The dichotomy of plant-eaters and meat-eaters has become more than a set of rules for animals—it helps shape our view of the world. But is nature really so clear-cut?

The more I looked around, the more exceptions I began to find. It turns out that deer of various species have long been observed eating the flesh of the dead. Scientists have recorded deer devouring dead fish that had drifted to shore, gobbling them up at a rate of up to eight per minute. Deer have often been spotted feeding on larger carrion—sometimes even on the guts of other dead deer.

Deer belong to a group of animals called ruminants, which have a special organ called a rumen for digesting tough plants. Cows are probably the best-known ruminants (and have been witnessed eating birds). Yet even animals from this highly plant-specialized group will eat meat when given the chance. The various species of duiker (imagine a very small antelope with a fat belly) in Africa often eat carrion and have also been observed hunting for small birds and frogs.

Other groups of herbivores have also been caught crossing the divide. Hippopotamuses have been filmed eating meat often enough that it can’t be dismissed as a fluke. In fact, they have even been witnessed in the act of cannibalism. Most reports of hippos eating meat involve the scavenging of animals killed by other causes. But sometimes hippos will hunt, kill, and eat prey. In 2002, there were news reports from Ethiopia’s Kaffa Province of hippos killing and eating livestock.

I’ve heard all sorts of apologies for this behavior. The animal simply wanted the salt, or they are iron-deficient or starving. But the whitetail doe that demanded my steak wasn’t simply licking off the Worcestershire sauce—she was swallowing whole chunks of meat. And this buck caught noshing a gut pile on camera looks pretty fat to me.

These animals can’t tell us why they are eating meat. We can only speculate about what the advantage is in this behavior. It is entirely possible that deer and hippos eat meat now and then simply because they can and they feel like it. Every single facet of animal behavior does not have an evolutionary explanation. But in the long run, there is always upheaval: Habitats disappear, climates change, food sources shift, and old niches in ecosystems disappear as new ones emerge. Highly-specialized animals are more likely to become evolutionary dead-ends than are generalists.

Generalists, such as raccoons and pigs, can survive in a variety of habitats and climates and can utilize many different food sources. Specialists, on the other hand, tend to exploit one niche very thoroughly and may be able to dominate it for as long as it exists. Koalas are a good example of a specialist species, feeding entirely on the leaves of eucalyptus trees. The koala’s strategy works only for as long as there are eucalyptus leaves to eat, whereas raccoons will probably be around long after koalas (currently listed as a threatened species) are extinct.

A herbivore that is willing to occasionally eat meat has the potential to either exploit an available niche as a predator, or to survive a brief period of starvation by eating meat when plants are scarce. For example, a prolonged drought may cause many herbivores to weaken and die, creating a scavenging opportunity for the survivors. Consuming meat from time to time may be a way for a species to maintain an option on carnivory, which may be the difference between extinction and survival in an emergency.

When we look at the evolutionary histories of some of these herbivores, we find even more omnivorous behavior in their past. Early forms of deer about 30 million years ago are thought to have eaten large amounts of grubs, insects, baby birds, eggs, and small mammals in addition to plant matter. Hippo ancestors split off from cetaceans about 60 million years ago—yes, whales and dolphins are their close kin—had teeth that suggest an omnivorous diet as well. These iconic herbivores have a lot of flesh-eating in their past.

Much like many college students I’ve known, the species we think of as herbivores may only be passing through a phase. They probably ate a lot of meat in the past, their descendants will eventually do so again, and meanwhile they indulge every so often when nobody seems to be paying attention. A flexible diet makes an animal more adaptable and more likely to survive than a fully-committed herbivore.

The hungry doe that I met at the campground certainly had no idea about any of this evolution business. She didn’t know about differing levels of protein, iron, or salt. She smelled a steak cooking, she wanted to eat it, and it wound up in her mouth. Come to think of it, this was pretty much the same reason why I started eating meat after growing up as a vegetarian.