Though dogs are so close genetically to wolves that many taxonomists consider them to be a subspecies, most people wouldn't let a wolf lick their hand as readily as a Shih Tzu. When animals are domesticated, as the dog was, their traits change; an artificial selection occurs over many generations, which, in the case of the dog, probably happened through unconscious selection bias among ancient humans and their canid hangers-on.
Other animals, too, saw similar phenotype changes through the process of domestication. When wild boars were first domesticated in areas of both modern Turkey and China, the farmers who bred them preferred animals with less fur, more meat and a tamer disposition. Similarly, a famous 2020 study on wild foxes in urban areas found that the city-dwelling creatures had distinctly different physical traits, such as "a noticeably shortened wider snout with a reduced maxillary [jaw] region" and with "a braincase appeared to be smaller in the urban habitat" — which suggested they were becoming semi-domesticated.
Indeed, there are so many physical traits that humans observe as distinguishing domesticated animals from wild ones that the process is sometimes called "domestication syndrome." As famously laid out by Russian zoologist Dmitry Belyayev in the 1960s and 1970s during his "silver fox experiments," domestication syndrome posits that there are certain common physical traits that emerge among most species as they get domesticated by humans.
"If there isn't a single trait that is common to all these different definitions [of domesticated], what is there?" Larson told Salon.
Yet what if there is no pattern between which traits correspond with domestication? What if those alterations are in fact species-specific, and arguments to the contrary are simply scientists succumbing to apophenia (a tendency to wrongly discern patterns in unrelated things)?
Some researchers believe this may be the case — which, in turn, means we have been thinking about domestication all wrong.
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According to Dr. Greger Larson — an archaeology professor at the University of Oxford — advocates of domestication syndrome simply have not provided enough evidence to support their theory. In 2020, he and several other scientists wrote a paper for the journal Trends in Ecology & Evolution called "The History of Farm Foxes Undermines the Animal Domestication Syndrome" that made precisely this point. It started by deconstructing Belyayev's famous experiment with farm foxes, the one that seemed to prove domestication theory. Belyayev found that after multiple generations of controlled breeding silver foxes on Prince Edward Island, the animals began to develop traits associated with domesticity: Docility, floppy ears, spotted coats and curled tails.
Yet there are two problems with concluding that this proves domestication theory, as the 2020 paper pointed out: First, it ignores how Belyayev used foxes bought from a fur farm, and who therefore may have already had preselected traits. Additionally, an analysis of the different domesticated animals does not show any consistent patterns in terms of evolutionary traits.
"These are the general categories of things that people have used to distinguish a domesticated animal from a wild animal," Larson told Salon. He had pulled out a chart with a list of nine domesticated animals: Dogs, cats, goats, pigs, rabbits, rats, mice, foxes, and the original Russian farm-foxes. These were then cross-referenced with traits associated with domestication such as changes in their skeletons, coats, ears, tails, brain sizes and seasonalities. When placed on a grid, it became patently obvious that no patterns existed. For example, on some occasions both types of foxes along with dogs, cats and goats experienced increased variation in their coat coloring due to domestication — but that did not happen all the time, and it was by far the most prevalent "yes" category for showing supposed signs of "domestication syndrome." Quite often there was simply not enough data, and when it came to traits like skeletal changes and tail evolution, no meaningful trends or patterns existed at all.
"If there isn't a single trait that is common to all these different definitions, what is there?" Larson told Salon. "If you don't have a single characteristic amongst what, 30 separate characteristics here, that is the same across 10 different, separate definitions of it, what the hell is it? Everybody assumes they know what it is, but as soon as you start looking at it, it just vanishes before your fingertips."
Kathryn Lord, a postdoctoral associate in the Karlsson Lab who works at the Broad Institute of MIT and Harvard (and whom Larson described as "brilliant"), was instrumental in co-authoring the 2020 paper. She argued that their research undermined Belyayev's thesis, even though his experiment is still impressive.
"While [Belyayev]'s did indeed successfully select for increased tameability in his foxes, all of the traits that supposedly came along with that selection previously existed in the population from Prince Edwards Island decades before the experiment," Lord told Salon by email. "Therefore, [Belyayev]'s experiment, while still fantastically interesting for changes in tameability, does not provide support for the idea of the domestication syndrome."
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After all, Lord pointed out, there is no getting around the facts from their own paper: "We found that none of the traits show up across the 7 very common domestic mammals we looked into," Lord wrote to Salon. "In most cases where domestication syndrome traits were reported they were appearing in specific modern breeds. The problem with this is that modern animal breeds only came into existence in the 19th century and are the result of selection on already domestic animals."
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So can science definitively determine whether domestication syndrome is a real thing?
Larson explained that the community must first come to a more concrete understanding of what it would actually look like. For now, it is more difficult to study domestication because the benchmarks can move around. Larson compared this to his experience working in a very different industry — music.
"There is increasing evidence to suggest that many domestic animals simply adapted to an environment we created and that we may have rarely, if ever, had a direct hand in the process."
"I was kind of a consultant on the Pearl Jam film 'Pearl Jam Twenty' that came out in 2011," Larson recalled. "There was a group of us then who were meeting, and one of the guys suggested that maybe what we should do is try and figure out how many live shows they'd actually played because it appeared to all of us said they might be getting close to a thousand live shows." That seemed like a cool idea — until the experts realized that "live show" had a rather fluid definition.
"Very much like the domestication syndrome, you've got to define it," Larson recalled. "Well, what constitutes a live show? Did the three songs on "Saturday Night Live" count? What about when they only played in front of small audiences? If they appeared on a radio with only two members of the band, did that count?
"It's the same thing with how you define domestication syndrome," Larson told Salon. "It definitely depends on what you're looking for and what you're counting. So all we are saying in that paper is that everybody has just assumed that it's a real thing, but nobody's actually gone through and tried to define it. If everybody had first said, 'Look, it's a thing, now we require an explanation to describe the thing,' that can work, but nobody ever actually tested whether or not this thing exists. So if the thing doesn't exist, why would you ever expect a single unifying cause for it?"
Even the one trait that seemed to be most prevalent among all different types of domesticated animals — their tameability — becomes murkier when places in a scientific context.
"Increased tameabillity may be a common theme across domestic animals," Lord wrote, but "the definition of domestication often includes increased tameability so it is a bit of a circularity problem. We will only know if increased tameability is really a thing across domestic animals if we agree upon a definition of domestication that doesn't require it." Similarly, when trying to determine how animals are changed by humans, "many definitions require human control precluding our ability to study how much humans were actively involved in the process. There is increasing evidence to suggest that many domestic animals simply adapted to an environment we created and that we may have rarely if ever had a direct (let alone intentional) hand in the process."