So Hart and his co-authors hoped their two surveys, which were completed by nearly 3000 dog owners, might yield some useful data on the problem. The researchers found no evidence tying coprophagy to age, dietary differences or compulsive behaviours 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 faeces 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 faeces 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 faeces 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 labelled "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 faeces.
"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 faeces potentially attractive as a second hand 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 faeces. 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."
