Elaine Powers first became interested in studying coprophagy—the practice of eating feces—when she was teaching high school biology. Powers often told her students about rabbits, which excrete a special form of poop called a cecotrope that is meant to be eaten, as the rabbits cannot absorb all the nutrients in their food in the first go-round. Her students, perhaps unsurprisingly, demonstrated a distinct enthusiasm for hearing more about poop-eating.
After Powers retired from teaching, she wondered just how many animals ate their own poop. She had started an internship at the Smithsonian's National Zoo and Conservation Biology Institute, where she discovered just how frequently the zoo fields visitors' questions about animals eating poop. In one particular multi-species exhibit, a South American rodent routinely ate the food for and excrement of their monkey roommates. The monkey's food, and by extension, their poop, was high in vitamin D. The rodents quickly developed vitamin D toxicity, which mineralized their muscle tissues.
As Powers scoured the internet, she began to realize these stories were quite commonplace, both in captivity and in the wild. "Zoos do have to pay attention to it, as opposed to the usual thing they have to worry about, which is the monkey throwing poop at the people walking by," she said.
Powers fell in love with zoology in college, when she remembered thinking, "I want to study it, but all the interesting stuff has been studied." But when she searched the scientific literature on coprophagy, she came up empty. This absence was not totally surprising. "We're used to thinking of—legitimately!—to thinking of poop as disease transmission, not poop as food," Powers said. Our natural disgust of feces helps prevent the transmission of disease and parasites. But Powers's research makes it clear that animals make different trade-offs in their diets, eating poop not just as a last resort but also, occasionally, as a first choice.
Powers, along with Smithsonian researchers Sally Bornbusch and Erin Kendrick, recently published a review of all the vertebrates known to eat poop as a normal part of their diet (as opposed to a maladaptive behavior accompanying illness). One such revelation? Rats eat up to 40 percent of their own poop. The researchers' paper in the journal Animal Behaviour suggests that poop-eating should be considered another form of foraging. It also cites one of the most disgusting scientific papers I've ever read. So please kick back, relax, and enjoy my conversation with Powers about the life-changing power of eating poop.
This interview has been edited and condensed for clarity.
Is feces the same as poop? Or is feces an umbrella under which poop and other excrement exist?
We basically said we're not going to treat them as different. We're going to look at things that come out of the back end of a digestive system. We're calling them all feces, whether it's koala pap or or bird excrement, which is feces and urine combined a lot of time because they have a cloaca, or whether it's rabbits' cecotropes. We're going to call it all feces, and define it that way. Because what we want to know is who is eating what comes out of either their own or somebody else's digestive tract.
I had no idea that songbirds produce fecal sacs and a lot of the parents eat them. The people who study songbirds know it, but the people who study rabbits don't know it. Fish feces! That was the most amazing study. It was the Robertson study. He—and presumably some other people, he can't have done it all himself—documented about 6,000 fish feces on a coral reef and what happened to them. Basically, who ate them? He has this wonderful table of all these fish that eat feces, and this, almost, reverse food chain of who eats whose feces.
You got the fish people who might know that. You got the bird people who know about the songbirds eating fecal sacs. You got the the mammal people who are very familiar with the fact that capybaras and guinea pigs and rabbits and rats eat their own poop. But putting it all together by just saying we're going to treat it as feces, and look at who eats it, was really illuminating to me.
I understood that feces is what your body produces after you've extracted the nutrients from it. And yet there are still nutrients in the feces. Could you talk a little bit more about how that works?
It's as much as you can extract from it while it's in you. But it doesn't stay in you forever. Think of all the corn kernels that come out of human feces, for example. If you're a bear and you're eating berries, most of the seeds are going to go straight through you. It turns out there's a whole lot of mice that find that a perfectly handy way to get seeds. They'll forage in the bear feces, or they'll forage in a latrine.
River otters apparently have latrines that get visited by a fair number of other animals. Exactly what those animals are getting out of the poop isn't totally clear, because the river otters are presumably eating a lot of fish and things like that. So there is almost certainly essentially pieces of food in the poop that's sufficiently undigested that it's real food for somebody else. One of the examples that I loved, particularly of the description in the research paper, was a particular toad in South America [whose tadpoles] will all swim over if a capybara comes and poops at the edge of the water, to go eat the capybara poop.
In Danté Fenolio's paper on cave salamanders, the cave salamanders are eating bat guano. They're living in a place where they can eat invertebrates, but the food chain is basically based as much on bat guano as anything else. There's this transfer of nutrients from outside the cave to this dark environment. So they looked at the nutrient composition of bat guano, and we know it has lots of nitrogen and things in it, right? People use it as fertilizer for their plants. [The researchers] compared it to a McDonald's hamburger, and basically said their bat guano sample had twice the protein and two-thirds the calories of the hamburger. That was a wonderful description of how nutrient-full feces can be.
We touched on this with the rabbits' cecotropes, but I wanted to make sure I'm highlighting all of the different non-poop poops, or specialty poops, of species.
The fecal sac in the songbirds is the poop with a little urine in it. But it's packaged in mucus, so somebody described it as poop in a diaper that the bird can actually pick up and take out of the nest. There's a whole bird literature, it turns out, on whether these birds were observed to be eating them, as opposed to just flying away and dropping them someplace else. If it's about cleaning the nest, why doesn't everybody just fly away and drop it? It appears that at least in some species, they're getting nutrients out of eating it.
If you want a specialty poop, I think from the descriptions—we don't seem to know a whole lot about it because cutting into koalas is just not a thing—but koala pap is a beautiful example of true specialty poop. It, as best I can tell, is essentially a specialized cecotrope, meaning it appears to be material coming from the cecum that is microbe-heavy, as well as having some nutrients in it. But it's produced by koala moms when they have a baby that is going to be weaned, and it was one of the early, really powerful examples of transmitting gut microbes from mother to child. Because koalas eat eucalyptus leaves and they are toxic. You and I would get sick eating them. It's not just that they wouldn't do us any good. They'd be bad for us. Koalas have microbes that essentially detoxify them, and so the babies have got to acquire that. And they acquire it, at least in part by eating this.
How long did your research take?
Couple of years. I mean, we didn't have any funding for it.
That's a long time.
Yeah, it was the advantage of being semi-retired. If you're a mid-career professional, you can't do things for two years without funding.
It sounds like a labor of love.
Yeah, it definitely was. A love of coprophagy.
How were you able to distinguish between the sometimes blurry distinction of whether the poop is being eaten for nutrients or whether it's an example of maladaptation?
We were deliberately really looking hard at the literature on animals in wild environments, but we didn't exclude captive environments. For example, a lot of the studies of rats and mice and rabbits, laboratory animals, farm animals, those are in captive environments, but they're also the ones where people would go the furthest in, for example, showing that the animals would grow poorly if they didn't have access to feces. Or in the case of the salamander example, where he actually did isotope analyses of the salamanders and nutritional analyses of the bat guano and basically said, "When we first saw it, we thought it was just incidental." Which is another word of saying, not necessarily maladaptive, but maybe they just ate the bat guano because they were trying to eat insects in the bat guano, which is a perfectly reasonable hypothesis. You know, cows probably eat some rabbit poop incidentally because they happen to be eating the grass and there's rabbit poop in it. But then they actually studied it and said, "No, there's nutrients here, and it appears to be showing up in the tissue of the salamander."
The captive studies generally needed to be much more specifically showing that there was a nutrient component. In the wild studies, it was more, is it deliberate? Have they observed it? Have they described it really clearly? And does it seem like it looks like foraging? Some of them you really can't tell. Like some of the latrine studies, they could tell the animals were digging around in the poop, but they can't tell whether it's eating the bugs in the poop or the seeds in the poop. You might be an opossum and maybe you don't care whether it's the poop or the the insect. But it's hard to observe clearly.
Your paper acknowledges this historic state of being where lots of people know this is happening but no one is writing it up in the literature. Can you talk more about why this was known but not recorded for so long?
Some of it is just human observer bias, not even just disgust. We just don't think of poop as a resource until people started analyzing it. It was the 20th century before people even really understood that rabbits, which people have been keeping around for a long time, that when they're licking their butt, they're actually licking off the cecotrope.
Oh, so they eat it directly out of their butt?
Yes, they lick directly from their anuses. "Take directly from the anus" was one of my favorite lines that I got to write.
I have to say, two of my favorite examples were of deliberate food selection. One of them was sloths in Peru. It's the only research paper I've read in which the word disgusting is in the title ["Disgusting appetite: Two-toed sloths feeding in human latrines"]. These researchers are out there doing research on something entirely different, and they've set up a crude latrine that's basically set up on wooden slats so that so the feces and toilet paper and urine all fall through. They discovered that there was a sloth coming and sneaking into the latrine and eating things from the latrine. And it kept coming back. So they have a photograph of a sloth with this handful of human feces, with toilet paper and everything, just slobbering it up. I mean, that's foraging! I mean, it's coming there on purpose, multiple times. But it's not something people are looking for.
The other one that really got me was two separate studies, one in I think it was northeastern Africa and one in southern Africa. Both of them happen to be studying free-ranging dogs. They just run around in a pack in the neighborhood and scavenge and eat. In both studies, they were studying what these dogs ate. So these are dogs that have access to carcasses and human garbage and anything that they can get. And it turned out in both studies, up to roughly 20 percent of the food selected by the dogs, given an array of choices, was human feces. And, like, they got a lot of choice here. You wouldn't think poop would be high on the list, but it is. Human poop, apparently.
Something that has been hanging over my mind as we're talking is, when I lived in Seattle, my downstairs neighbor had two corgis, one of which was younger, and my roommate had a chihuahua. All the dogs would just run around in the backyard, and the baby corgi, Lily, loved eating poop so much that, as soon as she discovered that another dog was pooping, she would just run and open her mouth so that the poop went directly from their butt into her mouth. Like it never even touched the ground. And we would just, like, watch.
See those are the kind of anecdotes that get you saying there's got to be something going on there. Maybe it's part of the zoo problem. You don't know how much of it is, "Hey, I'm in a limited environment, and this is just stimulation. It's just fun, it's interesting. I don't really care, because I'm not scared of poop." And how much of it is, "Hey, I get something out of it." Was she getting the microbes that helped her got adjust to living in that community? Apparently in multi-dog households, the dogs are more likely to eat poop, and it's not just because they have more access to it. So maybe there's a social thing going on. I want my gut microbes to look like your gut microbes. I want to see if you've eaten something really good.
I realize we've been talking for an hour.
I love talking to people about animals eating poop. Like I said, most people don't really want to sit down over dinner and talk about animal poop.
