Dogs are so much a part of American life - valued members of or even substitutes for human families - that it can be easy to forget they are still animals with teeth and the ability to use them if instinct demands it.
Add the lack of an owner and steady meals, and dogs can quickly begin to resemble their wolf ancestors, teaming up in packs for hunting and protection. They may look like pets, but behave like predators.
That is what investigators believe Sherry and Lothar Schweder encountered along a country road in Georgia a week ago. They say a pack of wild dogs killed Sherry Schweder as she took an evening walk and fatally mauled her husband when he went to look for her. Authorities euthanized more than a dozen dogs they suspect were involved.
Very seldom do dogs kill people in the United States: At least 20 Americans have died so far this year from dog attacks, a handful fewer than have died from lightning strikes.
But it is not unusual for dogs to use their teeth on people. Dogs bite about 4.5 million people in the U.S. each year, and nearly 900,000 of those, about half of them children, require medical care, the Centers for Disease Control says. More than 31,000 Americans needed reconstructive surgery after dogs attacked them in 2006, center figures show.
James Serpell, director of the University of Pennsylvania's Center for the Interaction of Animals and Society in Philadelphia, said the number of dog-bite cases is remarkably low, considering how closely humans and dogs live together.
"Frankly it's a triumph that dogs don't bite us more often than they do," Serpell said. "Any dog will bite if you provoke it sufficiently, or if it's in pain, it will defend itself. Potentially any dog will show predatory behavior, especially if it's very, very hungry."
Still, the statistics prove plenty of dogs bite the hand that feeds them. The CDC says adults with two or more dogs at home are five times more likely to get bitten than people with dog-free households. What gives?
Predatory behavior - hunting for food - is just one of many reasons canines, whether wild or housedogs, may attack people, experts say.
Spaying or neutering might help some dogs in some cases, such as reducing aggression toward other dogs, but won't make much difference in predatory behavior such as pack attacks on humans, said Dr. Wayne Hunthausen, a veterinarian and director of animal behavior consultations at Westwood Animal Hospital in Westwood, Kan.
Dog bites also may be caused by medical problems; possessiveness over food and toys; fear aggression resulting from genetics or poor socialization; redirected aggression, such as when a dog fights with another dog and goes after a person who intervenes and protection of territory from a stranger.
"Just because the dog sees you as a source of food doesn't necessarily mean there's any respect there," said Hunthausen, a past president of the American Veterinary Society of Animal Behavior.
In Georgia, a man living in the area where the Schweders were killed said he had fed the dogs and never had a problem with them. He didn't believe they had killed the couple.
Dr. Bonnie Beaver, a former president of the American Veterinary Medical Association who headed a task force on dog attacks, said it didn't surprise her that the dogs hadn't attacked that man but would go after the couple.
Many dogs, including packs, are fine with people feeding them, but if someone strange enters their territory or stares at them, it's a different scenario, she said. She also said it wouldn't have been surprising if the pack had turned on the man that fed them.
"I can put food in a wolf's pen and put it in, and put it in, and put it in, and the wrong thing happens, and I'm toast," Beaver said.
Dogs were domesticated tens of thousands of years ago. In some ways they are far removed from wolves - principally in their willingness to cozy up to humans. In other ways, they seem surprisingly close to their ancestors.
A strong predatory instinct was key in the wild, and the more important a trait is to a species' survival, the harder it is to breed out, Hunthausen said.
In some breeds - ratting dogs such as terriers or hunting and herding dogs - the prey drive was fostered or tweaked through breeding to achieve behaviors humans wanted. That strong predatory drive is one reason some dogs chase joggers, bicyclists and even cars.
"The faster something moves, usually the more likely it is to release predatory behavior," Hunthausen said.
Pit bulls, historically bred for fighting, are the breed that seems to most frequently make headlines for aggression. Earlier this month, two pit bulls killed their owner's brother and a mixed-breed pug in a Leesburg, Va., home.
Hunthausen's instructions for children who encounter strange dogs are similar to those suggested by grizzly bear experts: Stand still like a tree or curl up and stay still like a rock; don't move or make eye contact. Unlike grizzly bear-related guidance, he said if he met with a pack of wild dogs and had a chance to get up a tree or to another safe spot, he would probably take his chances and run for it.
Research shows in general, how many wolf-like traits a dog has is related to how little or how much a dog looks like a wolf, said Beaver, a veterinarian and professor at Texas A&M University's Department of Small Animal Medicine and Surgery.
For example, Siberian huskies and Alaskan malamutes are nearer to wolves genetically than Chihuahuas or toy poodles are, Beaver said.
In general, domestic dogs are closer to wolf cubs than to adult wolves, she said. Like wolf cubs, dogs bark a lot and are dependent on a leader, Beaver said.
Interestingly, one way domesticated dogs differ from wolves is in their response to food rewards, the University of Pennsylvania's Serpell said. While humans can use food to motivate dogs to follow commands, wolves, though smart, tend not to see the point, he said.
If humans kicked dogs out en masse, some could live off the land and hunt in packs, Serpell said. Future generations would probably start looking something like dingoes, he said, referring to domestic dogs-gone-wild in Australia.
"I think most of them would die, actually, if humans weren't around," Serpell said. "They just would not be able to cope."