Charles Darwin was intrigued with so many evolutionary puzzles. One thing that vexed him was why so many domesticated animals, particularly dogs and livestock, tended to have drooping ears.

"Our domesticated quadrupeds are all descended, as far as is known, from species having erect ears," Darwin pointed out in "The Variation of Animals and Plants under Domestication." "Cats in China, horses in parts of Russia, sheep in Italy and elsewhere, the guinea-pig in Germany, goats and cattle in India, rabbits, pigs and dogs in all long-civilized countries."

Darwin noted that wild animals constantly use their ears like funnels to catch every passing sound. The only wild animal with non-erect ears, according to his research at the time, was the elephant.

"The incapacity to erect the ears," Darwin concluded, "is certainly in some manner the result of domestication."

When domestication happens

silver fox A Russian geneticist bred silver foxes, choosing each generation based on their friendly personalities. (Photo: Newfiewild/Shutterstock)

All sorts of things happen, Darwin noted, when animals become tame. It isn't just their ears that change. Domesticated animals tend to have shorter snouts, smaller jaws and smaller teeth, and their coats are lighter and sometimes splotchier.

He called the phenomenon domestication syndrome.

Darwin thought there had to be a reason for all those changes, even though there appeared to be no related link. For years, scientists offered theories, but none were readily accepted.

About a century later, in the late 1950s, Russian geneticist Dmitri Belyaev, began an experiment using silver foxes. He hypothesized that the changes in animals were a result of breeding selection based on behavioral traits.

Belyaev began breeding the foxes, choosing those that were calmest around people and less likely to bite. Then he bred their offspring, choosing the animals using the same criteria. In just a few generations, not only were the foxes friendly and domesticated, but many of them also had floppy ears. In addition, they had changes in their fur color, as well as their skulls, jaws and teeth.

It started with adrenalin

A new study published this week in the journal Genetics offers a theory as to why domestication had such an impact on a dog's ears, as well as other physical traits.

Led by Adam Wilkins of the Institute of Theoretical Biology in Berlin, the study theorizes that perhaps an early man noticed a wolf that was different from the others. He wasn't afraid of humans and maybe even joined him for leftovers and eventually became a companion.

This early wolf was possibly lacking an excess of adrenalin from the adrenal gland, which fuels the "fight or flight" response. The adrenal gland is formed by "neural crest cells." These cells also move to the different parts of an animal where these changes between wild and floppy-eared domestic animals are most obvious.

The researchers theorize that if the neural crest cells don't reach the ears, then they become somewhat deformed, or floppy. If cells cause problems with pigmentation, that explains patchy, instead of solid fur. If cells are weak when they arrive at the jaw or teeth, they might grow to become slightly smaller.

Surprises like floppy ears weren't anticipated, but were they a bad thing? ABC News asked Wilkins to find out.

"I think not," he said. "In the case of domesticated animals, most of them would not survive very well in the wild if they were released, but in captivity they do perfectly well and while the traits of the 'domestication syndrome' are technically defects, they do not seem to harm them."

Our dogs, for example, don't need to blend in with solid colored coats or have ears constantly on alert, looking for trouble. Plus it worked out quite well for humans.

"And for us, the domestication of animals was a major advance that made the development of our civilizations possible," Wilkins said, "or at least they contributed substantially to that."

Explaining your dog's ears

three dogs sitting Dogs still have all sorts of different ear shapes. (Photo: Grigorita Ko/Shutterstock)

Obviously, not all dogs' ears are floppy. Plenty of breeds, like Nordic breeds (Malamute, Siberian husky, Samoyed) and some terriers (Cairn, West Highland white) are known for their prick or upright ears.

As dog author and professor of psychology Stanley Coren, Ph.D. points out in Psychology Today, "Through selective breeding, human beings have modified the pointed prick ear shape of the wolf into a variety of different shapes. For example the French bulldog ... has large upright ears with the sharp tip altered into a smooth curve producing what dog people call blunt ears or rounded tip ears."

Coren goes on to illustrate many pointed and drooped ear types with names ranging from pendant to rose, button to folded, candle flame to hooded.

But all ears, belonging to all dogs, have one thing in common, Coren points out.

"Rest assured that regardless of their shape, most dogs like to be scratched lightly behind their ears, especially if you make loving sounds at the same time."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Why do dogs have floppy ears?
Charles Darwin knew that floppy ears in dogs had something to do with domestication.