With so many nice things to eat in the world, why would any animal look at the menu and order a number two?

For Saturday's Weird Animal Question of the Week, we're answering Manuela Castro, who asked via Facebook, "Why do animals eat their own feces?"

Coprophagia is the term for an animal eating excrement—both their own and that of others. Dung beetles, rabbits, chimps, and domestic dogs are among animals that are members of the dung diners' club. Most of them eat feces because it contains some undigested food—and thus vital nutrients—that would otherwise go to waste.

"I think lagomorphs [the group that includes rabbits] are the most interesting feces eaters," Kevin Hunt, an anthropologist at Indiana University in Bloomington, says via email.

The small guts of these high-metabolism animals don't break down everything the first time, "so they send it back a second time. They have two kinds of feces, the kind they eat and the kind they don't."

The kind they eat are called cecotropes—sometimes called "night feces"—nutrient-packed pellets that the rabbits eat fresh. (See "What's the Difference Between Rabbits and Hares?")

Chimpanzees don't regularly eat their poop, but like rabbits, the most likely reason is some of the seeds they eat "are only partly digested, and they eat them again to harvest those nutrients, said Hunt, who has studied coprophagy in chimpanzees. (Read "It's Just a Little Pre-digested; It's Still Good, It's Still Good.")

Dung beetles also take advantage of raw materials in animal excrement, usually deposited by plant-eating mammals. Baby beetles, for instance, munch on solids, while adults have specialized mouthparts to slurp up the dung's liquidy parts, according to the San Diego Zoo.

Depending on the species, the dung doubles as housing or nursery—a place for the insects to lay their eggs.

Poopy Chow

Wild animals are one thing, but when a person sees their dog having a scat snack, you can hear the "OMGs!" from blocks away.

Not to worry: It's natural and not harmful at all, even if the poop belongs to another species, says Nicholas Dodman, a veterinary behaviorist at Tufts University and chief scientific officer of the Center for Canine Behavior Studies. "Poop is poop," he says.

Dogs may be attracted to undigested extras in scat, but the behavior may also simply be a habit learned from their mothers.

Nursing females will lick their puppies in the area of the perineal region, under the tail, which stimulates small secretions of urine and feces. These are immediately cleaned up by the mother "the only way she knows how."

"She doesn't have opposable thumbs and tissues. She can't pick it up and throw it out, so she just licks it up and disposes of it that way," Dodman says. It doesn't make mom sick, he says, noting that puppy feces contains "friendly bacteria."

Puppies might mimic their mom's behavior for up to a year, and sometimes the habit persists into adulthood. (How much do you know about man's best friend? Take National Geographic's dog quiz.)

It's not a bad thing, Dodman says, but if they feel strongly, dog owners can take steps to curtail this behavior. These include changing the dog's diet, providing more opportunities for exercise, and scooping poop immediately.

Just hope they don't think it's a doggie bag.