A small number of urban wildlife species are showing features of what some scientists call the Self-Domestication Syndrome (SDS), domestication via natural selection in the wild. Rapid human population growth and increasing urbanization might be the engine behind it, say experts.
In addition, the friendliness and docility that is the hallmark of the domestication process, might also have profound conservation implications to the survival of some urban wildlife. Animals that can quickly adapt their lifestyle to what scientists call human modified environments will benefit from an increase in food availability and reduced predation from larger animals that comes with living close to humans.
“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, Germany, “developing both those behaviors [such as less fearfulness, less reactive aggression, more 'tameness' in general] and some of the traits that we have called part of the 'domestication syndrome.”
Researchers have documented biological changes in many urban species. For example, animals as diverse as House finches, the Common bat, the White-footed mouse, the Common Wall lizard, and a number of other wildlife have all shown changes to their morphology (body structure) as a result of living close to us humans in urban areas.
However, scientists studying different urbanized fauna observed that these animals are not only undergoing morphological changes but also changes affecting their physiology, behavior, neurology and psychology-what experts call the Domestication Syndrome (DS).
Both natural selection in nature and artificial selection, which created our modern barnyard, share one key feature: reduced reactive aggression that leads to docility and friendliness (reactive aggression is aggression in response to a threat or frustrating event. Decreased reactive aggression is the single unifying factor of the domestication process.
(Biochemical changes, especially to the animal’s central nervous system, accompany this remarkable transformation from hostile to docile. For example, corticosteroids, which are stress hormones, appear to be suppressed as animals become more domesticated. In addition, serotonin, a neurotransmitter associated with lower predatory and defensive aggression, increases in the brains of animals as the domestication process unfolds.)
This is accompanied by a series of non-adaptive traits. In other words, traits that have no obvious beneficial evolutionary advantage to the survival of that species. Such traits include pigment discoloration of hair or other body parts, reduced brain size, smaller teeth, alterations to the shape and size of the face. Other changes can include changes to hormones, reproductive cycle and the nervous system.
(Recent investigations by Professor Brian Hare of Duke University in the US state of North Carolina indicate that it can also increase a species ability to coordinate and communicate with others.)
Charles Darwin who conceived of the theory of evolution via natural selection (Alfred Russell Wallace independently arrived at the same concept during the same period) and author of “THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION” observed this phenomenon in domestic animals. Compared to their wild ancestors, domestic animals are tamer or docile, and they also tend to show a variety of other features including more juvenile faces with smaller jaws, floppier ears, patches of white fur, curly tails, unique coat colors and patterns, reduced brain size and body mass, etc. Darwin was interested in domestication as a general model for evolutionary change, but did not give these oddities of the domestication process a name nor understood why it occurred.
Increased urbanization due to rapid human population growth appears to be the engine driving the emergence of certain aspects of the self-domestication process in some species Almost three-quarters of humans live in cities, and more than half of the landmass of the planet is urbanized.
(Some experts argue that a similar scenario unfolded during the Pleistocene or “Ice Age” that helped paved the way for the domestication of wild wolves into domestic dogs by early humans thousands of years ago.)
Brian Hare argues that the pioneering experiments of the late Russian geneticist Dimitry Belyaev and his protégé Ludmilla Trut, predicts that our increasing population density will be sufficient to fuel the next great self-domestication event via natural selection.
In the late 1950s, Belyaev began his decade’s long Farm fox experiment that replicated the domestication process in a controlled environment. Not only did successive generations of foxes become tamer and friendlier towards humans as less aggressive pups were selected and bred, but the experiment also produced the peculiar non-adaptive traits seen in domesticated and self-domesticated animals.
Scientists such as the late biologist Raymond Coppinger and his wife, biologist and science author Lorna Coppinger argue that there was a period of self-domestication in the wild prior to human intervention in the wolf to dog domestication process. They say that dogs did not evolve directly from wolves, nor were they trained by early humans, but domesticated themselves to exploit Mesolithic village dumps.
The most likely scenario, they believe, is that hunter/gatherers and later increasing sedentary populations created mounting garbage outside of their camps as they still do today. Such rubbish, which would have included food waste like bones and nutritious human feces, attracted scavengers such as wolves. Wolves that were calmer, with lower reactive aggressiveness, would have been brave enough to approach human camps. The genes of these tamer animals would have spread to subsequent generations as they thrived on increasing piles of Ice Age human rubbish.
It stands to reason that if self-domestication could happen in the wild wolf to domestic dog scenario without human interference (or with limited human interference), then couldn't this happen to other wild animals especially those who like those early wolves are encroaching upon human habitats today?
In recent years, researchers have described animals that lived in close association with humans over many generations and developed new characteristics linked to domestication, even though they were not selectively bred for those traits. Two examples are the free-living wild house mice (Mus musculus domesticus) and the White-backed munia (Lonchurastriata).
The White-backed munia birds, for example, have been kept in captivity for two centuries but not selectively bred, just allowed to reproduce, and they spontaneously developed into a new breed with distinct traits. It is a short step from those kinds of associations to “urbanization” as animals adapt to places with lots of people in them.
Some examples of urban wildlife exhibiting features of self-domestication includes urban and rural populations of red foxes (Vulpes vulpes) from London and surrounding boroughs are divergent in skull traits, according to J. K. Parsons of the Institute of Biodiversity, Animal Health, and Comparative Medicine, University of Glasgow and colleagues.
These changes are primarily found to be involved with the length of the animal's snout, with urban individuals tending to have shorter and wider muzzles relative to rural individuals, smaller brain cases and reduced sexual dimorphism (all part of the SDS). Changes were widespread and related to muscle attachment sites and thus are likely driven by differing biomechanical demands of feeding or cognition between habitats.
Biological anthropologist Brian Hare and his student James Brooks used camera-traps in their study of coyotes (Canis latrans) in the US state of North Carolina to help determine a link between temperament and human population density. The pair analyzed the data from camera traps all over North Carolina and concluded that urban coyotes were more likely to approach camera-traps than coyotes living in wild areas. This experiment demonstrated a lack of reactive aggression against an unfamiliar entity. In addition, when the researchers compared the self-control of some thirty-six different species, coyotes were not only better than dogs and wolves, they were the only other animal who were as good as great apes.
The researchers also documented several other SDS-related changes in these animals including various other behavioral and morphological changes including color coat changes in several coyotes (again all SDS-related changes).
Yet another example might be taking place in the southern United States. In the Florida Keys, there is a population of deer that is native to the islands called Key deer (Odocoileus virginianus clavium). These deer, which live in more urban areas, have become less fearful, bigger, more social, and more fertile than those out of contact with humans, writes P.M Harveson and colleagues. In other urban areas in the US, people have seen common white-tailed deer (Odocoileus virginianus) with unusual coloring like splotchy or albino coats. R.McCoy and colleagues says there are anecdotes of “deformities” in piebald and albino deer, including short legs, shorter, lower mandibles, and longer tails the kind of changes associated with the SDS.
It is not only mammals that are apparently going through this adaptation to urbanization process. According to investigations done by Jesko Partecke and colleagues, urban blackbirds in Europe are less aggressive than their rural relatives. They have higher breeding density and a longer breeding season. They also live longer and have lower corticosterone, a stress hormone, than their rural relatives.
According to Dr. Madeleine Geiger of the Palaeonotological Institute and Museum at the Universitat of Zurich, Switzerland:
“Although this pattern could be described as “self- domestication”, this does by no means mean that urban animals are on their way to being domesticated. They probably will never be. This just means that the underlying mechanism behind both phenomena [urbanization and domestication] are similar.”
But Richard Wrangham of Harvard University’s Department of Human Evolutionary Biology goes a step further. He believes that the underlying mechanism that causes these various changes to urbanized and domesticated animals are one and the same.
“It seems to me that the case for a meaningful connection between features of the domestication syndrome (DS traits) occurring in the Farm Fox experiments and in urbanized species is very strong,” says Wrangham.
“Given the persistence with which DS traits are found in animals that have been domesticated for millennia, and the rapid emergence of DS traits in experimental research on domestication, plus the growing evidence that DS traits are produced by at least one common mechanism, a mild neurocristopathy [a diverse class of pathologies that may arise from defects in the development of tissues containing cells commonly derived from the embryonic neural crest cell lineage], I fully expect that the urbanized populations of wild animals that show reduced fear and some DS traits will prove to conform to the same system. In other words, I expect that in general, the DS traits that urbanized species exhibit will be found to be traits of a self-domestication syndrome supported by a mild neurocristopathy. ”
“If that idea turns out to be wrong, the questions about why we see DS traits in urbanized animals will of course still be fascinating!”