Why Some Dogs Have Floppy Ears and Wolves Don't
Domestication accidentally created floppy eared dogs
Posted Jan 31, 2018
"It all started when my son, who is in his first year at university, observed that our English Cocker Spaniel has floppy ears and wolves don't." The man speaking with me was a casual acquaintance from our Faculty of Engineering. We were attending a university reception and he had pulled me aside to ask me the question that his son had posed to him. As we stood there sipping lukewarm coffee he continued, "He was really puzzled since he had read that dogs were domesticated from wolves, although there was some evidence that jackals, coyotes, African wild dogs, and a whole bunch of other wild canines may have ultimately contributed their genes to the modern dog. My son's problem is that when he went on an Internet search to look at all of the wild canines he found that every one of them has upright, pricked, ears. So the fact that there are so many breeds of dogs which have floppy ears simply made no sense to him. Do you have any idea what is going on here?"
It turns out that this is not a new question, nor is it trivial. The evolutionary theorist, Charles Darwin, mentioned this issue in 1859, in his incredibly influential book "On the Origin of Species". He noted that of all of the wild animals that he knew of, only the elephant has floppy ears. Yet according to him "Not a single domestic animal can be named which has not in some country drooping ears…"
This issue was bothersome enough to Darwin so that nearly a decade later, in 1868, he wrote "The Variation in Animals and Plants under Domestication" which is a massive volume (over 800 pages) which looks at the differences between domestic animals and wild animals. One of the reasons why this book is so remarkable is because Darwin was writing it nearly three decades before the work of Gregor Mendel was rediscovered and the science of genetics was launched.
What Darwin found was that domestication involved a whole suite of changes both in the behavior and the physiology of animals. The actual process of domestication involves selectively breeding animals so that they will be tamer and more manageable. What we are looking for in domestication is an animal with reduced stress and fear around humans, and a more stable, manageable, and easy-going temperament. Wild animals have a very sensitive "fight or flight" response, which we now know is mostly due to hormonal secretions from the adrenal glands and responsiveness of the sympathetic nervous system. So domestication tries to reduce this reaction since the fight response will make the animal aggressive and the flight response will make the animal unmanageable and untrainable.
Darwin observed that along with the desirable behavior changes in tameness came a number of changes in the animal's physiology and body shape. Specifically the muzzles of domestic animals are shorter, so the jaws on a dog are smaller than that of a wolf. The teeth of domestic animals are also smaller and may be fewer in number. White patches appear on the fur of many domestic animals which are not found in their wild counterparts. There may also be a reduction in brain size. And then of course there are those floppy ears… In recent years this constellation of changes (the intended behavioral changes and the unintended physical by-products) have been labeled the "Domestication Syndrome".
The interesting scientific question is how did all of these changes come about since there are changes in bone structure, fur pigmentation, musculature, and neural structure involved. No single convincing answer was available until a few years ago when an exciting paper was published in the journal Genetics. The team of researchers was headed by Adam Wilkins who is currently at the Institute of Theoretical Biology at Humboldt University in Berlin, Germany.
This new theory runs something like this. Wild canines tended to hang around the settlements of primitive humans because, although humans were good hunters, they were also extravagant and sloppy and they would leave the unused portions of killed animals out near the edge of their villages. This provided a convenient and safe source of food for wild canines like wolves. The animals which benefited from this the most were those with less adrenaline and a reduced fight or flight response. Too much fearfulness would mean that the animals would run away at the least disturbance by humans and thus not get a good food supply. Too much aggressiveness would cause the humans to not tolerate the animal's presence, and they would drive it away or kill it. Ultimately it was at those wild canines which had less responsive adrenal glands who would be chosen for domestication since they would be more likely to be tame and would be happy to remain peaceably close to people.
Now here is where the fascinating science comes into play. The adrenal gland is formed by a group of stem cells. Stem cells are part of the developing embryo, and they have an almost magical ability to morph into different kinds of cells, depending upon the location in the body which they find themselves. The particular stem cells involved in forming the adrenal glands are called "neural crest cells". The researchers argue that the domestication process actually selects for animals which have a slight genetic defect which causes a modest reduction in the number or activity level of the neural crest cells. This means that both the aggressive and stress related behaviors which are triggered by hormonal secretions from the adrenal glands are reduced, which is the desired outcome of domestication.
Now remember that these stem cells are effectively "shape shifters". They are also involved in producing the bones in the face, teeth, pigment cells, nerves, and muscular connective tissue. So if domestication has selected for animals which have weaker neural crest cells, then some of them will arrive at the jaw in a weak condition and the jaw will be smaller. Some cells might not be strong enough to trigger vigorous pigment production which could result in the white patches on the fur of domestic animals. And here, finally, comes the answer to our main question, since if all of the cells required don't reach the ears, the ears will be slightly deformed, having weaker connective tissue which means that they won't maintain their upright condition and we end up with a floppy eared dog.
For a wild animal floppy ears are not a good thing since the ear flap hanging down will cover the entrance to the auditory canal. In other words that floppy ear blocks some of the incoming sound making the ability to detect faint noises more difficult. However for a domestic dog, those floppy ears simply appear to be "cute" and something for us to fondle when we are showering affection on our pets. Perhaps the only difficulty which floppy ears cause for some domestic dogs is the fact that, if they are long enough, they may end up drooping into their water or food bowl and causing a bit of a mess.
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Adam S. Wilkins, Richard W. Wrangham and W. Tecumseh Fitch (2014). The “Domestication Syndrome” in Mammals: A Unified Explanation Based on Neural Crest Cell Behavior and Genetics. Genetics,197 (3), 795-808; https://doi.org/10.1534/genetics.114.165423