In this spooky time of year, we find ourselves confronted with arrays of fearsome animals. Spiders decorate doorways. Rubber rats appear in windows. Bats become bolder as the sun sets earlier. These are the sorts of animals that scare us. However, there are other animals with frightful habits hiding in plain sight. The classification of herbivore vs carnivore is one of the earliest biology lessons we’re taught in school. Herbivores eat plants, carnivores eat meat, and omnivores eat some of each. However, these definitions are much more permeable than we once thought

When you’re looking out for werewolves and other flesh-eating beasties this Halloween, maybe you should keep an eye on your local deer, too…

When you hear the word “predator,” white-tailed deer probably don’t spring to mind. The discovery has been a surprise to scientists too. Researchers in North Dakota set up a series of nest cams over songbird nests. They saw a lot of predation, including squirrels, foxes, badgers, and weasels. Squirrels took the most nestlings and eggs. Foxes and weasels took relatively few. Somewhere in the middle range of hatchling killers was deer, plucking them out of the nest and devouring them alive.

Caution: Video content may be disturbing.

They saw repeated instances of white-tailed deer snatching hatchlings from the nest. It would be easy to write off the carnivorous habits of these supposedly vegan animals as a fluke, but evidence suggests otherwise. Scientists who set up mist nets sometimes find that white-tailed deer have made a buffet out of the trapped birds. In the video above, a family is alarmed to see a white-tailed deer eating a small songbird. Though a deer might not be able to catch an able-bodied adult bird, picking off the young, injured, or trapped is well within the range of their abilities.

Trail cameras have spotted deer scavenging rabbit carcasses.

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.

Caution: Video content may be disturbing.

Horses, one of our best allies in the animal world, are not immune from this hunger. They too eat young and injured birds, but also ducks, geese, and rabbits. Some cultures even encourage this herbivore omnivory. In Iceland, it’s common practice to feed salted fish to horses throughout winter to give them the protein and salt boost they need to weather the cold. In traditional Tibetan culture, horses are fed a mixture of millet and sheep’s blood. Plants are hard to digest and require a lot of gastrointestinal specialization. Comparatively speaking, digesting meat is easy.

Horses’ penchant for casual meat-eating may explain a few recent mysteries. Many different global cultures eat horse meat. One horse slaughterhouse in Connecticut was repeatedly closed down over the course of ten years due to trichinosis infection. Trichinosis is caused by the Trichinella roundworm and is generally spread by eating contaminated meat. It had been suggested that these horses might be getting trichinosis from eating garbage with meat residue, but maybe they’re eating infected rats instead.

Caution: Video content may be disturbing.

Fortunately for us, these deadly herbivores don’t seem keen on adding human flesh to their diet. There isn’t any evidence to suggest that a deer or a horse that eats other animals is any more aggressive to humans to one that doesn’t. There have been reports of the occasional maddened elephant eating people, such as the account of an elephant named Chang supposedly eating a worker at the Zurrich Zoo in the 1940s, but such cases have been documented too poorly to know for certain.

What is much clearer is that if you are a baby bird, you have a lot to be worried about. As more carnivorous herbivores are unmasked, like sheep and elk, it becomes apparent that this is a very common behavior. It’s a wonderful and eerie world we live in, that animals that we’ve lived beside for millennia can still surprise us.

- Kate Dzikiewicz, Paul Griswold Howes Fellow

(Need more spooky science? Check out: How Corn Created Dracula, The Deadly Companion of Dogs (And Bats), Why Are Black Cats Black?, Spooky Skeletons and the Flesh-Eating Beetles )