What makes an animal 'feral’?

“Feral” is a cloudy term, one most of us have heard in connection with animals that aren’t domestic but aren’t wild either. So what exactly is a feral animal?

It's one that lives in the wild “but was descended from domesticated stock,” says Kayce Bell, assistant curator of mammalogy at the Natural History Museum of Los Angeles. Their time being domesticated, which can entail generations of being selectively bred and cared for by humans, usually as pets or livestock, is “the only thing that distinguishes them from wild creatures, she says.

The expression “feral” is sometimes seen as pejorative, as if an animal has failed at or rejected domesticity. Feral animals are also sometimes called wild, “because they are they are technically wild, free-living animals,” Bell says, and there is debate about what definitions apply to what animals.

Domestic cats are a good example. A feral cat is a domestic cat not born in human custody and adept at living outdoors, while stray cats once had human care and aren’t used to living in the wild. Community cats are stray or feral, and may have a person who feeds them, but not an owner. Domestic cats have a human home but may spend time outdoors.

Whatever you call them, cats that go outdoors kill an estimated one to four billion birds a year in the U.S. That’s why the American Society for the Protection of Animals advocates trapping, neutering, and releasing free-roaming cats as a way of controlling their populations. (Read more about the challenges of managing outdoor cats.)

Pigeons and parrots

The numerous pigeons that flock cities worldwide are feral, descended from rock doves that have been kept by humans for 10,000 years. People historically used the birds as food, racing animals, and messengers; 32 even received medals for their military service in World War II.

The domesticated rock doves that escaped or were released found cities very comfortable, especially because buildings, with all their ledges and nooks, are similar to the cliff faces their wild ancestors lived in, Bell says.

Pigeons are considered a nuisance species because they deface property with their copious droppings and can spread parasites and disease to other birds, though such transmission is considered low risk. They can also be beneficial to the ecosystem, particularly as prey for the once-endangered peregrine falcon.

Various species of feral parrots and parakeets have also taken up residence across the U.S., particularly in California.

What does domesticated mean?

To further the confusion, we go to Florida.

The Sunshine State is home to loads of invasive species, including a ballooning population of Burmese pythons in the Everglades, most of which are descendants of pet reptiles that escaped or were released into the wild as long ago as the 1900s, says Jacquelyn Guzy, an ecologist with the U.S. Geological Survey and lead author of a new report from the USGS about Florida’s serpentine transplants.

Despite their pet ancestors, though, Guzy says Burmese pythons in Florida are not considered feral.

“A species can be both feral and invasive, such as feral pigs,” Guzy says via email. Beginning in the 1500s, European explorers and settlers brought wild boar, Sus scrofa, to the New World many different times, either as livestock or as animals to be hunted for food or sport. The U.S. government now refers to them as feral swine. (Read how feral hogs are running wild in the U.S., and spreading disease.)

Invasive species are non-native animals or plants, sometimes introduced by people, that cause environmental, economic, or property damage. Though Burmese pythons have wreaked ecological havoc, Guzy “would not consider Burmese pythons feral,” because they weren’t ever really domesticated. 

Domestication is a “process of years and years of artificial selection by humans,” instead of natural selection that occurs in the wild, says Kate Schoenecker, a wildlife biologist for the USGS. As with dogs, chickens, or cows, people breed animals to influence the way they look and behave.

Domesticated animals and people often share a mutual relationship, Guzy adds. If you provide backyard chickens food and shelter, for example, you get free eggs in return.

“Snakes in general neither need nor seek that relationship from humans,” she says.

'Born wild is wild'

Horses and burros on U.S. federal lands are protected under the Wild Free-Roaming Horses and Burros Act of 1971. They’re called wild because that phrasing is used in the legislative text, not because they are, says Schoenecker, a horse lover herself. 

As descendants of domestic animals brought here by European explorers hundreds of years ago, the term feral applies, she says. (Read more about the controversies surrounding wild horses.)

Not everyone agrees.

“Born wild is wild,” says Suzanne Roy, executive director of the American Wild Horse Campaign, a nonprofit that works to change some current horse-management practices, including aerial roundups on public lands. “They go back generations.”

Complicating matters, equids did evolve in North America, about 59 to 34 million years ago, before moving across the Bering Land Bridge into Asia. North American equids went extinct in North America in the Paleolithic era, along with saber-toothed cats and mammoths.

In 2021, a fossil study showed evidence of genetic flow between extinct North American and Eursian horses, meaning the animals moved back and forth over the land bridge and interbred for hundreds of thousands of years. In the 15th century, European settlers began to bring domesticated horses back to North America.

For that reason, some scientists use the term “reintroduced” when referring to U.S. free-roaming horses.

Now that is wild.