It’s not just birds that should fear deer. They’ve also been spotted eating small mammals, fish, and insects. Though we might call deer herbivores, they don’t turn down an easy meal, even if that meal is meat. Why might they be abandoning their vegetable diet for a mixed one? There are a number of likely explanations.

Deer and other hoofed animals have long been known to eat bone and antler to add calcium to their diet. It could be that they’re eating live animals for a similar reason. When red deer eat seabird chicks they avoid the meat and eat only the bones. However, it’s overly simplistic to say that deer only eat meat for the calcium. Eating other animals would also give them a boost in proteins and fat, resources especially needed during periods of antler growth.

If you think that deer are the only herbivores with this sort of “aberrant” behavior, think again. The more research that is done on the subject of carnivorous herbivores, the more common it becomes. Cows will accidentally slurp up tadpoles at watering holes, but they’ll also very deliberately gobble bird chicks. Some have even been filmed eating adult chickens! Duiker (small antelopes) eat carrion and frogs. Hippopotamuses are some of the worst offenders. They’ve been documented eating impala, young elephants, and even engaging in cannibalism.

It’s very unlikely that deer, hippos, and cows only recently decided to engage in recreational carnivory. Rather, this is something that has always been going on, but that we have only recently acknowledged. It is easy to portray nature in black and white terms, calling one animal a meat-eater and one a plant-eater. It becomes much more difficult when shades of gray come into play.