For several years, scientists have been recording changes in the behaviours of some animals that indicate that they may be going through a process of self-domestication. But why are they doing it?
By Curtis Abraham
When we think of domestication, it’s often in terms of man-made selection – the shrewd selective breeding processes deployed by humans that created our modern farm animals, pets and beasts of burden. But research now indicates that domestication can also occur via natural selection in the wild and in urban environments. Some scientists believe that this ‘self-domestication’ is a survival strategy – one that can occur when an animal’s environment favours reduced aggression compared to that of a recent ancestor, in response to an imminent or perceived threat.
Recent studies suggest that urban species in particular are self-domesticating. A small number of wildlife species inhabiting urban areas in the USA, including coyotes (more on which below), are showing traits of self-domestication. Researchers have also identified several wild primate species in Africa that might have undergone, or are perhaps in the midst of undergoing, the self-domestication process.
What is domestication?
In the past, scientists tended to exclude domestic animals from serious cognitive or behavioural research. It was thought that domestication made animals dull and stupid since they no longer had to make a living in the wild. As science writer and historian Jared Diamond incorrectly surmised, brains presumably got smaller during the domestication process because brains were a ‘waste of energy in the barnyard’.
Today, experts agree that reduced reactive aggression, which leads to tameness or docility, is the single unifying trait that kickstarts the domestication process. (The Greek philosopher Aristotle, writing in his book The History of Animals in around 350 BCE, was perhaps the first to observe that domestic animals, compared to their ancestors in the wild, were much tamer, ‘easy-tempered’ and therefore ‘easily domesticated’.) Tameness – which is simply an animal’s tolerance of humans in close proximity – isn’t the end of the process. Full domestication must be ingrained into the animal’s genetic make-up, ensuring that friendliness towards humans is passed from one generation to the next. However, it is a start. ‘As animals come to associate with humans, without fear, they do indeed start becoming domesticated,’ says Adam Wilkins, a geneticist and evolutionary biologist at Humboldt University in Berlin.
Domestication also creates a phenomenon known as domestication syndrome, which is characterised by a series of odd, seemingly unrelated biological traits that have no obvious evolutionary advantage – what experts call non-adaptive traits. In addition to their friendliness, they also tend to show a variety of other features, including youthful faces (paedomorphism), with smaller jaws and floppier ears; patches of white fur (depigmentation); unique coat colours and patterns; reduced brain size and body mass; and reduced sexual dimorphism (physical difference between the sexes). Not all domesticated animals have all of these non-adaptive traits, but an assortment of them emerges in each domesticated species.
Research into this process is ongoing, but some scientists argue that the underlying link between these features could be the group of embryonic stem cells called the ‘neural crest’. Researchers have observed that neural crest deficiencies can cause depigmentation in some areas of the skin, malformed ear cartilage, tooth anomalies and jaw-development changes, all of which are seen in cases of domestication syndrome.
Charles Darwin also observed these biological and behavioural oddities in domestic animals. Attempting to develop a general theory of heredity, Darwin published an entire volume on The Variation of Animals and Plants Under Domestication, and used domestication to beautifully illustrate the main principles of his evolutionary theory by emphasising parallels between the domestication of animals and plants by artificial selection and evolution by natural selection.
More recently, Russian scientists have been able to trigger domestication syndrome in long-term controlled experiments by breeding successive generations of silver foxes (Vulpes vulpes) for reduced aggression. The experiment, which began during the late 1950s, was the brainchild of Soviet scientist Dimitry Belyaev. It continues today under his protégé, Ludmilla Trut.
Domestication and conservation
The phenomenon of animals exhibiting aspects of self-domestication could have profound conservation implications in both urban and wild habitats. Several of these animals are classified as Endangered on the Red List of Threatened Species and are listed on Appendix I of the Convention for International Trade in Endangered Species. The Western chimpanzee, for example, has suffered an 80 per cent population decline in the past 25 years as a result of habitat destruction, poaching and disease. It’s the most at risk of extinction of the four known subspecies of chimpanzee. The Zanzibar red colobus monkey, another African primate species thought to have undergone self-domestication, is highly sensitive to hunting and habitat destruction, and has been referred to as probably the most threatened taxonomic group of primates in Africa.
According to experts, those animals that can adapt quickly to urbanisation will reap the benefits of increased food availability and reduced predation from larger animals. However, the downside is that they will also be exposed to dangers such as pesticides, vehicular traffic and human abuse (in some locations, coyotes are considered to be vermin and are hunted ruthlessly).
For wildlife that still inhabits the wild, national and international recognition that these species are transforming right before our eyes could lead to additional conservation efforts such as sustainable ecotourism and further academic research activities (both of which are potential foreign currency earners for host countries). On the other hand, however, they could also become targets for wildlife trafficking because of their uniqueness.
According to Richard W Wrangham, a professor at Harvard University’s Department of Human Evolutionary Biology, Belyaev’s experiment had a purely economic and practical purpose. Soviet farmers wanted to make further profits from exports of silver foxes by creating an additional breeding season. ‘Belyaev knew that domesticated animals bred more than their wild ancestors,’ says Wrangham, ‘so he decided to domesticate silver foxes from scratch, hoping that that would similarly increase their breeding cycle and document the outcomes. Not only did successive generations of foxes become friendlier towards humans as less-aggressive pups were selected and bred, but the experiment also produced the peculiar non-adaptive traits seen in domesticated animals.’
Urban domestication
Researchers have documented biological changes in many urban species. For example, animals as diverse as house finches (Haemorhous mexicanus), the common bat (Pipistrellus kuhlii), the white-footed mouse (Peromyscus leucopus), the common wall lizard (Podarcis muralis) and many others have shown changes to their body structure as a result of living close to humans in urban areas. However, investigators studying other urbanised fauna have documented changes not only to the animals’ morphology but also to their physiology, psychology and behaviour. In other words, the domestication syndrome.
In 2020, Kevin Parsons, a senior lecturer at the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow, led a team of researchers investigating red foxes (the same species as Belyaev’s foxes) living in England’s rural forests and urban spaces in order to see if urbanisation had had any effect on their biological evolution. According to records, areas such as London, Birmingham and Bristol have been populated by red foxes for well over a century.
The researchers examined the National Museum of Scotland’s fox-skull collection. An estimated 1,500 skulls had been collected from 1971 to 1973 in London and the neighbouring countryside during a fox-culling campaign. All were marked with their locations – rural or urban. Parsons photographed 57 female and 54 male skulls and identified key features.
He and his colleagues did indeed note changes to the urban red foxes that were absent in forest-dwelling red foxes. In particular, urban individuals tended to have shorter and wider muzzles (domestic dogs also diversified into different breeds based on changes to their skulls, particularly snout lengthening and shortening). These alterations were extensive and related to muscle-attachment sites. Nature was perhaps giving these city-dwellers a more robust bite to deal with increased food availability, such as discarded bones. The researchers also noted that these animals had smaller braincases, reduced brain size and reduced sexual dimorphism – all of which are components of domestication syndrome.
According to archaeological evidence, red foxes have long inhabited areas populated by humans in various parts of the world, including Alaska’s Kodiak Archipelago, where they’ve lived near or with people for at least 7,500 years, since humans first colonised the land. However, despite their close proximity to people, these foxes never became domesticated. Why not? The answer is complex and not entirely understood. ‘There are lots of examples of how urbanisation is affecting evolution in animals,’ says Kevin Parsons ‘but not so many that can match up with domestication – since so few animals have been domesticated out of the millions of different species.’
Duke University evolutionary anthropologist Brian Hare and his student James Brooks used camera traps in their study of how coyotes (Canis latrans) in the US state of North Carolina are adapting to urbanised environments. Coyotes have been very successful in adapting to human activities and have flourished in both rural and urban areas.
The pair analysed the reactions to camera traps of 636 coyotes in 578 sequences at 575 sites across 67 counties from June 2017 to June 2019 (camera traps are an unknown human object to the coyotes and so could be considered to be a threat). Based on these observations, they concluded that the likelihood of a coyote approaching the camera increased with human housing density, indicating that urban coyotes are experiencing selection for boldness and a lack of reactive aggression by becoming more attracted to objects associated with humans. Remarkably, when the researchers compared the self-control levels of some 36 different species, coyotes were not only better than dogs and wolves; they were the only other animal as good as great apes.
Hare and Brooks also found evidence that urbanisation was altering the coyotes’ physical appearance. These were mainly changes to the colour of the animals’ coats. Out of the 636 individual coyotes they observed, seven had abnormal colouration, ranging from black to light-coloured to brindled colouration. They also observed one individual with a white tail-tip and pointed out that sighting of unusual colouration might be an underestimate due to the fact that some colour variants would be indistinguishable in night-time black-and-white photos. Other atypical physical traits such as blue eyes, white chest patches, dog-like hide colours and white feet were also documented from other coyote studies and in the scientific literature.
Aspects of self-domestication are also appearing in several deer species in the USA. In the Florida Keys, there is a population of deer known as Key deer (Odocoileus virginianus clavium) that is native to the islands. These deer, which live close to urban areas, have become less fearful, bigger, more social and more fertile than those that don’t have regular contact with humans.
In other urban areas in the USA, people have seen common white-tailed deer (Odocoileus virginianus) with unusual colouring such as splotchy or albino coats. Researchers have also collected anecdotes of ‘deformities’ in piebald and albino deer, including short legs, shorter lower mandibles and longer tails – all the kind of changes that are typically associated with domestication syndrome.
And, it isn’t only mammals that are apparently going through these sorts of changes. According to investigations carried out by Jesko Partecke of the Max Planck Institute of Animal Behavior and colleagues, urban blackbirds in Europe are less aggressive than their rural relatives, exhibit higher breeding densities and a longer breeding season. They also live longer and have lower levels of corticosterone, a stress hormone, than their rural relatives.
Wild domestication
According to some biologists, rapid human population growth and increasing urbanisation might be the engines that are driving these emerging traits of self-domestication. Currently, more than half of the Earth’s human population is urbanised, and a future isn’t far away in which three-quarters are city-dwellers.
According to Brian Hare, there were already hints of this in Belyaev’s fox experiments. ‘Unlike other models that suggest domestication can occur only in rare species that are useful to humans, Belyaev’s work predicts that our increasing population density will be enough to drive the next great self-domestication event through natural selection,’ he says. ‘Because urban species must live close to humans, the selection pressure will be on friendly, non-aggressive individuals, simulating the conditions for Belyaev’s experiment.’ Some experts argue that a similar scenario unfolded during the last Ice Age, which ended about 11,700 years ago and helped to pave the way for the domestication of wild wolves into domestic dogs by early hunter-gatherers.
Nevertheless, in recent years, evidence has also emerged of self-domestication in the wild, with the bonobo, the sister species to chimpanzees, the poster species for the phenomenon. Comparative studies of bonobo behaviour, social structure and physiology with that of chimpanzees have convinced scientists that selection against reactive aggression will produce self-domestication even in the wild and in the absence of humans. (Chimps, for example, show a high propensity for reactive aggression; bonobos do not. Bonobos have juvenile-like skulls, while those of chimps are fully mature in structure.)
An examination of some wild primates in Africa and its nearby islands is providing some tantalising clues that these creatures might have experienced, or are undergoing, self-domestication in the wild. One stark example is the Western chimpanzee. This subspecies of chimp exhibits the main characteristic of being self-domesticated: reduced aggression in many aspects of its social structure, including relations with individuals from outside an immediate group. For example, female West African chimpanzees are quite gregarious and often support one another in conflicts with males, resulting in a more gender-balanced hierarchy than that of the rigidly patriarchal East African chimpanzee. Male West African chimpanzees are generally respectful of females and do not forcibly confiscate food from them, which may at least partly stem from the gregarious females forming alliances.
In addition, these female chimps have been observed hunting and accompanying males on territorial patrols, playing a more important role in social dynamics than other chimpanzee subspecies. While it was traditionally accepted that only female chimpanzees immigrate and males remain in their natal troop for life, Western chimpanzees uniquely exhibit female and male immigration between groups, suggesting males are less territorial and more willing to accept unfamiliar males.
Another candidate for self-domestication in the wild is the Kinda baboon (Papio kindae), an unusually docile species that lives in the miombo woodlands of Angola, Zambia, the Democratic Republic of the Congo and possibly western Tanzania. Males join new groups and may rise in rank, but unlike other baboon species, violent confrontations are mostly avoided. In addition, Kinda baboons are also characterised by a shorter snout and pinkish skin surrounding their eyes (perhaps depigmentation due to neural crest deficiencies). Kinda baboons have smaller teeth and a reduced cranial size compared to other baboons. Infants are usually born with white hair rather than the black typical of other baboons. Sexual dimorphism in the Kinda is more moderate than in any other baboon species.
Drawing conclusions
Nevertheless, despite the intriguing evidence being collected by researchers around the world, drawing meaningful conclusions about self-domestication is difficult. It might not be enough to simply point out the biological and behavioural end results of the process. Finding conclusive proof of self-domestication requires peering back into the animals’ evolutionary past. ‘Even if self-domestication has been common, we might not be able to find evidence for it,’ says Richard W Wrangham. ‘For most species, the necessary evidence will be elusive because the behaviour of the (normally extinct) ancestor is a crucial part of the equation, but is extremely difficult to ascertain.’
Knowledge about a recent ancestor of the species suspected of having been self-domesticated is necessary in order to demonstrate that it was more aggressive than today’s populations. But if the ancestor is extinct, very often we will have no fossils of it, and in any case, behaviour rarely fossilises. More often than not, we can’t know whether reactive aggression has decreased.
Scientists are also divided about whether or not the mechanism that caused domestication in our farm animals is the same as the one that is setting some wildlife on the path to self-domestication.
‘Although this pattern [urbanisation causing traits in wildlife similar to modern domesticated species] could be described as self-domestication, this does by no means mean that urban animals are on their way to being domesticated,’ says Madeleine Geiger of the Palaeontological Institute and Museum at the Universitat of Zurich, Switzerland. ‘They probably never will be. This just means that the underlying mechanisms behind both phenomena [urbanisation and domestication] are similar.’
But Wrangham, who also argues that we humans are a self-domesticated species in his 2019 book The Goodness Paradox, goes a step further. He argues that the underlying mechanism that causes these various changes to urbanised and domesticated animals is one and the same. ‘It seems to me that the case for a meaningful connection between features of the domestication syndrome occurring in the farm fox experiments and in urbanised species is very strong,’ he says. ‘If that idea turns out to be wrong, the questions about why we see domestication syndrome traits in urbanised animals will, of course, still be fascinating!’
