Everyone Poops. Some Animals Eat It. Why?

Consuming feces can benefit not only the health and microbiomes of some animals, but also their environments

That looks nutritious. humonia / iStock

“Coprophagia” isn’t the sort of word that rolls off the tongue. That’s fitting. After all, the act it describes—consuming excrement—is frankly disgusting. Yet, more often than not, when animals engage in this behavior, they’re not trying not to repulse us—but to communicate something vital about their health and biology.

If you’re a pet owner, your main context for coprophagia is probably canine. Whether or not you’ve experienced it yourself, you’ve likely heard stories about otherwise good-natured pups that inexplicably decide to chow down on their own feces or raid the cat’s litter box. It’s the kind of behavior that can inspire loving pet owners to rush to the vet on the assumption that something’s wrong with their beloved animals—but the situation doesn’t always indicate illness. In fact, even when coprophagia does suggest that there’s something wrong with a dog, they’re often engaging in it because they’re trying to make things right, not because they’re fundamentally broken.

To be sure, in some cases it may be an issue of problematic training: In puppy mills, for example, nightmarish conditions involving overcrowded and underfed dogs may learn to confuse excrement and kibble. But in other situations, even adult dogs may turn to coprophagia in order to correct pancreatic insufficiencies, which can limit their ability to produce insulin and other enzymes, or in an attempt to rebalance their gastrointestinal systems, says Karen Becker, a veterinarian who has written on the topic. “In veterinary medicine we often call coprophagia a behavioral problem, but we need to ask what they’re communicating,” Becker told me over the phone.

When dogs eat poop, Becker holds, it’s not because they enjoy the taste. To the contrary, “They are craving something that’s in the feces,” often something that’s missing in their existing diet. It’s an issue that comes up especially when they’re only eating processed dried foods, which may leave them without critical digestive enzymes, according to Becker. The vast majority of dogs won’t eat feces that are more than two days old, suggesting that they prefer the fresh stuff for its microbial punch, she says, especially when they’re attempting to regenerate their own gut flora.

In this respect, dogs are hardly alone in finding something beneficial in coprophagia. Garret Suen, a professor of bacteriology at the University of Wisconsin-Madison points out that it’s widespread throughout the animal kingdom. Mice, for example, are known to eat their own feces in laboratory conditions (and in other environments as well), a fact that may actually complicate studies that we perform on them.

Other rodents are also known for eating poop, both in captivity and in the wild. Capybaras, for example, are notorious for the practice, as are guinea pigs. Becker cites the latter species as a particularly compelling example of a domestic pet that may eat poop in order to stay healthy, not because it’s sick. “Guinea pigs, they say, can reuse their poo up to fifty times in an hour. It happens so fast that you might not know that it’s going on,” she says.

I noticed something similar while I was at the National Zoo in Washington, D.C., reporting a story on cassowaries, the world’s most dangerous bird. Shortly after the zoo’s cassowary had finished its meal of fruit, the bird pooped. Then it turned around and started digging through the excrement, pulling out partially digested chunks of fruit from the pile. Though this sight initially added to the strangeness of a very strange bird, it turned out to have a perfectly reasonable explanation. Despite their size, the bird’s handler told me, these flightless birds have relatively short digestive tracts, meaning that much of what emerges out of their backsides is still edible and nutrient-rich. It was really just trying to get the most out of its lunch.

(Dogs, as it happens, occasionally do something similar, Becker claims. When they “wolf down” their meals too quickly, they’ll sometimes regurgitate their food before they’ve digested it. If they subsequently attempt to eat this vomit-like junk, it’s really just that they’re trying to keep from missing out on the good stuff they really need. When they’re actually sick, they’ll be much less likely to eat their vomit, according to Becker.)

In cassowaries, coprophagia may be beneficial not only for the health of the individual, but for the health of its overall environment. “Their gut treatment is gentle and fast, so they’re not killing seeds in the gut,” says Andrew Mack, a conservation researcher and author of the book Searching for Pekpek: Cassowaries and Conservation in the New Guinea Rainforest. Accordingly, Mack found while studying the dwarf cassowary in Paupa New Guinea, eating and excreting allows these remarkable birds to distribute fertilized seeds more widely throughout the rainforest.

Poop-eating can benefit offspring, too: Cassowary chicks tend to root around in their fathers’ feces for chunks of undigested fruit. In fact, eating parental poop can also key for many young animals. Rabbits rely heavily on their parents’ poop as they begin to develop their own thriving microbiomes. “Before I was a veterinarian, I was a wildlife rehabilitator, and neonatal bunnies are some of the hardest animals to raise because mothers will feed their feces to the babies,” Becker tells me. “If you don’t have healthy feces to inoculate their digestive tracts, it’s very hard.”

The same is true for panda cubs, which need to consume their parents’ feces in order to develop necessary microbes. We’re not always sure what those microbes are actually doing for the animals: “The jury is still out on whether [pandas’] gut biomes are digesting cellulose,” says Suen, who’s conducted research of his own on panda poop. Still, many zoologists say that infantile coprophagia is critical for many animals. For pandas, as with rabbits, eating their parents’ poop is “just a way for the young to gain access to those microbes that they would not have access to otherwise,” Suen says.

When I asked Suen how such behaviors emerged in the first place, he acknowledged that we’re not really sure. He did, however, lay out a striking possible explanation: “Often, these microbes become very specialized and adapted. They become wholly dependent on their host,” he says. If they only live in specific species, the microbes gain “access to food, protection from predators, and so on.” That level of adaptation effectively compels the young animals reliant on these microbes to acquire them from adults, since they can’t find them elsewhere in nature.

In other words, it might be that bacteria have inclined some animals toward coprophagia, creating a symbiotic loop where these higher animals can only thrive if they eat the excrement of their own kind. Such examples show that coprophagia can follow from a healthy—if complex—relationship between animals and their environments. Indeed, something similar may be true even for our dogs, however off-putting their behaviors may seem. As Becker puts it, “Animals are sometimes making decisions to consume poo for reasons that we haven’t always identified. It can be a kind of self-medication.”

Humans, however, typically require no nutrients from their own feces. So readers, please: Don’t try this at home.

Get the latest Science stories in your inbox.