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Why do dogs eat poop? New research suggests an ancient answer.

A new survey finds that lots of dogs eat dog feces, and researchers suggest this might be a tendency inherited from parasite-averse wolves. (iStock)

Does your pooch eat poo? If it’s any consolation, he’s in extremely good company.

In a new paper, veterinary researchers at University of California at Davis who surveyed thousands of dog owners found 16 percent of pups consume other canines’ feces “frequently,” having been spotted doing it more than six times by their owners. The vast majority prefer their poop to be fresh, or no more than one to two days since deposit.

But why? Google this topic, and you’ll be greeted with pages upon pages of articles offering confident explanations about stress or enzyme deficiencies. But when Benjamin Hart, a veterinarian who directs the Center for Animal Behavior at Davis, took a look at the scientific literature on poop-eating, or coprophagy, he found few answers.

“For every person you ask about this, you get a different opinion. Because they’re guessing, whether they’re veterinarians or experts in behavior,” said Hart, the lead author on the paper, which was published in the journal Veterinary Medicine and Science. “You don’t want to say to a client, ‘I don’t know why they do it.’ “ 

And those experts get asked about it a lot, because for many dog owners this is a foul and intractable problem. While gulping down turds at the dog park is typically not dangerous to the pooch, Hart said the mere idea — not to mention the resulting potty breath — is so revolting to some owners that they’re willing to give their poop-eaters away. “People can be quite tolerant of even aggressive dogs,” he said. “But put this one out there, and they’re really intolerant.”

So Hart and his co-authors hoped their two surveys, which were completed by nearly 3,000 dog owners, might yield some useful data on the problem. The researchers found no evidence tying coprophagy to age, dietary differences or compulsive behaviors such as tail-chasing. Frequent stool-eaters were also just as easily house-trained as other dogs, which ruled out the idea that they were simply more comfortable with poop than peers with more refined tastes.

But more than 80 percent of the coprophagic dogs were reported to favor feces no more than two days old. Hart believes this taste for freshness suggests a cause that goes back more than 15,000 years — to dogs’ wolf ancestors.

Wolves typically defecate away from their dens, in part because feces contain intestinal parasite eggs. But if, say, a sick or lame wolf did its business at home, the waste wouldn’t necessarily be dangerous immediately. Parasite eggs usually don’t hatch into infectious larvae for a few days, Hart said.

“So how do you get rid of it? They don’t have pooper-scoopers,” Hart said of wolves. “If they eat it right away, it’s safe to eat. They won’t get infected by parasites.”

That led Hart to proffer a new theory: That today’s poop-eating dogs still carry around this wolfy instinct, even though the feces of modern-day pets tend to be parasite-free thanks to preventive treatments. The case isn’t closed, he said, but “it’s a logical explanation.”

James Serpell, a professor at the University of Pennsylvania veterinary school and editor of a recent textbook called “The Domestic Dog: Its Evolution, Behavior and Interactions With People,” said he found that idea “plausible.”

But Serpell noted that the survey also found that coprophagic dogs were far more likely to be labeled “greedy eaters” by their owners, which he said might suggest dietary motivations. Previous research, Serpell said, has found that free-roaming dogs in developing countries, who must scavenge to fill their bellies, eat quite a lot of human feces.

“Given its historical survival value, this common village dog behavior may still be fairly widespread in the modern canine population,” Serpell said. “Modern dogs and cats are fed diets that are relatively rich in fats and protein, not all of which may be completely digested, making their feces potentially attractive as a secondhand food source.”

Clive Wynne, director of the Canine Science Collaboratory at Arizona State University, echoed that.

“The niche that dogs occupy is essentially one of making a living on people’s leavings — and that isn’t just our leftovers from dinner, but what we put down the toilet, too. So it’s only from our human perspective that coprophagy seems strange,” he said. Wynne said the survey also showed that stool-eaters had a greater tendency to eat dirt and cat feces. That, he said, “is consistent with the idea that the behavior is motivated by taste and dietary desires.”

But Wynne said the survey yielded another important finding. It asked owners about their efforts to stop dogs from eating poop — whether with behavior modification, such as lacing stools with pepper or rewarding dogs that obeyed when told “leave it alone,” or by using any of 11 popular commercial products. All failed miserably. Owners’ reported success with store-bought products, many of which are tablets that make dogs’ own poop taste terrible, ranged from 0 to 2 percent.

This isn’t a very inspiring finding for desperate dog owners, Hart acknowledged. But he offered a ray of hope. Available products haven’t faced clinical trials, he said, so he and his colleagues are going to develop their own.

“We’re going to be looking at some clinical trials on treatments that are different enough that they’ll stand a chance of working,” Hart said. “We’re going to put our heads to this.”

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