How Tame Is Your Pet as Judged by These Qualities?

Story at-a-glance -

  • Some dogs come with perky, pointed ears, others have floppy ears and several other animals do, too, such as sheep, elephants and rabbits — but why?
  • Charles Darwin, author of “On the Origin of Species,” in 1868 wrote a second volume that asked the question of why some pets and livestock have “drooping” ears while others do not
  • Oddly enough, tame animals tend to have shorter snouts, and their fur is lighter-colored and often patchy or multicolored, but even Darwin couldn’t get to the bottom of the connection between behavior and appearance
  • Russian geneticist Dmitry Belyayev and Gregor Mendel, a Moravian monk, carried on with related research involving silver foxes and the common pea plant to further explore “domestication syndrome”
  • A number of interesting hypotheses have emerged in recent years that make it clear that domestication exacts changes in animal behavior and suggest how neural crest cells are involved

By Dr. Karen Shaw Becker

One of the most interesting and, yes, loveable things about dogs is the way they come in all sorts of sizes, colors and styles, and that means types of tails, face shapes, fur and perhaps most interesting of all, ears. Some dogs come with perky, pointed ears designed so the inner workings are quite visible, yet others have floppy ears, a few species surprisingly long. Other types of animals have them, too — certain species of sheep, elephants, pigs, rabbits, cats and cattle, for instance — depending on the variety.

Charles Darwin, who published his controversial book known as “On the Origin of Species” in 1868, also penned a lesser-known but interesting volume, “The Variation of Animals and Plants Under Domestication,” which addressed the question of why some pets and livestock have “drooping” ears, particularly in domesticated species. According to NPR, Darwin wrote:

“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 … The incapacity to erect the ears is certainly in some manner the result of domestication.”1

Tame animals also tend to have shorter snouts, and their fur is lighter-colored and often patchy or multicolored. It kicked off a steady stream of studies based on the theory of domestication syndrome. Thousands of years ago, our ancestors encountered both wild and potentially tame animals, which often came in handy for a number of functions, from protection to hunting to companionship.

When domesticated animals were deliberately bred to get more such animals, sometimes odd characteristics showed up. Try as he might, though, even Darwin couldn’t get to the bottom of the connection between behavior and appearance. It’s been scrutinized for decades. But more recently, scientists have settled on a remarkable hypothesis.

A Century Later, Geneticists Find Correlations

Interestingly, an experiment conducted in the 1930s by Russian geneticist Dmitry Belyayev, who concealed his real purpose of genetic studies to make it look more like study in animal physiology (since geneticists were being executed or sent to labor camps2), indicated that Darwin was correct (at least in part). NPR follows Belyayev’s study, not on dogs or wolves but silver foxes:

“Belyayev took 130 foxes from fur farms and started a breeding program. He only picked the tamest foxes — those that seemed less jumpy around humans, and less likely to bite — as parents. When their pups were grown, he'd pick the tamest ones to breed again. In just a few dozen generations, Belyayev's foxes were tame. And, lo and behold, their ears were distinctly floppier. Just as Darwin suspected, selecting for a change in behavior led to an unexpected change in appearance.”3

Further provoking thought into the changes domestication brings about in animals is the observation that drooping ears never seem to be seen in wild animals except in the case of elephants. In addition, tamed animals also undergo transformations in their characteristic behaviors; they seem not only willing but eager to be in the company of humans.

Gregor Mendel, an Augustinian monk from the Margraviate of Moravia (in the Czech Republic), raised and experimented principals of hybridization on about 5,000 pea plants for about eight years. In 1865, before any knowledge of genes existed, he proposed three “principals of inheritance” to explain how genetic traits are transmitted. Nature explains the basis of what’s called Mendelian, or the modern science of genetics:

“Traits are passed down in families in different patterns. Pedigrees can illustrate these patterns by following the history of specific characteristics, or phenotypes, as they appear in a family. The inheritance pattern of this characteristic is considered dominant, because it is observable in every generation. Thus, every individual who carries the genetic code for this characteristic will show evidence of the characteristic.

In contrast … [there’s] a different pattern of inheritance, in which a characteristic disappears in one generation, only to reappear in a subsequent one. This pattern of inheritance, in which the parents do not show the phenotype but some of the children do, is considered recessive.”4

Traits Involved in Domestication Syndrome

A study on domestication syndrome published in Genetics in 2014 notes that Darwin’s theory of heredity, particularly the fact that domesticated mammals have an observable set of heritable traits not seen in their wild predecessors (also seen in domesticated fish and birds), has been making scientists scratch their heads for more than 140 years.

The study proposes that “neural crest cell deficits” (described as “the multipotent stem cells that arise in vertebrate embryos from the dorsal part of the neural tube”) occur during embryonic development, sometimes as a direct result of such deficiencies; other times as indirect consequences. The theory was devised by embryologist and anatomist William His Sr.,5 and emerged the same year Darwin’s second book was published.

It notes a “gap” in Darwin’s initial evolution theory in regard to how heredity works. His subsequent treatise has resulted in a list of traits, besides floppy ears, in several changes, some of them only modern scientists would recognize:

Increased docility and tameness

Coat color changes

Reductions in tooth size

Changes in craniofacial morphology

Alterations in tail form

More frequent and nonseasonal estrus cycles

Alterations in adrenocorticotropic hormone levels

Changed concentrations of several neurotransmitters

Prolongations in juvenile behavior

Reductions in both total brain size and of particular brain regions

The diversity represented in this list of changes is recognized as a “major puzzle” and “one of the oldest problems in genetics,” the study acknowledges, but may have something to do with the process of how domestication occurred.

A Closer Look at Neural Crest Cells

A new hypothesis addresses the first thing most scientists look at when examining animal behavior in regard to domestication, such as lack of fearful aggression toward humans. Further, such a reduction in acute fear and long-term stress is a prerequisite to successful breeding in captivity, theories discussed in light of both Darwin’s,6 Belyayev’s and Lyudmila N. Trut’s research. The Genetics study explains:

“We trace the mechanistic basis of tameness to reduced size and function of the adrenal glands, which play a central role in the physiology of both fear and stress responses. Adrenal hypofunction and reduced stress hormone levels are well documented in domesticated species and have been induced experimentally by selection for tameness during experimental domestication of foxes and rats.”7

However, the authors conjectured there must be more to it than adrenal function, at which point they began examining neural crest cells, aka NCC, described as “the vertebrate-specific class of stem cells that first appear during early embryogenesis at the dorsal edge (‘crest’) of the neural tube and then migrate ventrally throughout the body …”

In short, NCCs encompass virtually every particle of the brain, from the sympathetic ganglia to the craniofacial regions, but without involving the central nervous system. The common foundation, the study continues, is that all affected tissues are neural crest derivatives or at least influenced by them.

Genetics discusses “preexisting genetic variants” to explain tameness, as well as genes involved in how NCCs manifest themselves in tissues, but there’s no genetic evidence that one particular gene is responsible for domestication as a result of mutations.

There’s also the possibility that the “fight-or-flight” instinct that belongs to species from earthworms to humans may also play a part in brain changes, involving the HPA axis: the hypothalamic-pituitary-adrenal system, the hormonal system, such as epinephrine-slash-adrenalin. Genetics goes on to explain:

“The peripherally based fear/stress system appears to be down-regulated in domesticated mammals and provides a clear link between tameness and the neural crest, specifically via adrenal hypofunction and reduced adrenal size. In experimentally domesticated foxes and rats, the adrenal glands are smaller than intheir unselected counterparts. In domesticated foxes the adrenals are already significantly reduced in utero …”8

The Takeaway: What We Don’t Know We Keep Studying

Needless to say, the study of how traits like floppy ears (as well as reduced muzzles and teeth in domesticated animals) and the lack of them in wild animals, including animals that are closely related, such as wolves compared to domesticated dogs, is a fascinating one. Scientists continue to follow a number of tracks to get to the bottom of not just why some dogs have floppy ears, but what it might mean in regard to developmental biology, neurobiology and genetics.

The trait of curly tails is an example of the many conundrums related to the topic. You can be sure that scientists studying not only Darwin’s suppositions but those of many others who have introduced theories, gathered solid evidence and offered threads of scientific possibilities to build on will continue. In the process, we’ll move toward a better understanding of heredity, epigenetics and human as well as animal behavior.