Domestication

Introductions

During our first class on September 11, I'd like each of you to quickly introduce yourself, giving your name, telling us what pets you have, and letting us know something that you'd like to learn or topics you'd like to discuss in this course.  After we get to know each other a little we'll review the material below on domestication.

Context

Our hominin ancestors split from their chimpanzee cousins around 5 million years ago and for most of the past 5 million years, they have  lived in small bands as hunter gatherers.   Anatomically modern homo sapiens arose in Africa around 300,000 years ago, but they were not very successful and almost died out, in part because the harsh conditions of the ice ages which descended over the earth every 100,000 years or so.  In the midst of the last ice age, around 60,000 years ago, humans escaped from Africa and quickly settled across much of the rest of the globe, again, travelling in small hunter gatherer bands.    Sometime around 20,000 to 40,000 years ago, when the glacial ice sheets were at their maximum extent, an extraordinary event occured: humans domesticated their first animal: turning a now extinct form of Eurasian (perhaps Siberian) wolf into the dog.  This event only happened once; all dogs living today are derived from that first domestication event.  Humans have subsequently gone on to domesticate a total of only about 14 mammals, so the process isn't easy.   How did humans do it?  Maybe they didn't, maybe it was the wolves that domesticated themselves.  We'll look at that theory below.

Wikipedia defines Domestication, from the Latin domesticus, 'belonging to the house', as "a sustained multi-generational, mutualistic relationship in which one organism assumes a significant degree of influence over the reproduction and care of another organism in order to secure a more predictable supply of a resource of interest, and through which the partner organism gains advantage over individuals that remain outside this relationship, thereby benefitting and often increasing the fitness of both the domesticator and the target domesticate."   Life during the harshest portion of the ice age couldn't have been easy, and wolf sized dogs ate 3-4 pounds of meat per day, yet they were undoubtedly popular with people and dogs quickly spread around the globe with people.  

What do you think were the benefits that dogs brought to these early humans?

The oldest confirmed archeological finding of a dog is that of a dog skull found buried with two humans in a grave in Germany. The burial was made 14,000 years ago, just a few thousand years before the end of the ice age.  The dog was about 7 months old and showed evidence of being cared for during a severe illness.  The Bonn-Oberkassel dog was confirmed to be a dog, not a wolf, based on genetic tests and the shape of its jaw.


Dated to 12,000 years ago, just after the ice age, a woman in Israel was buried with her hands on a 3-5 month old puppy.

Perhaps the earliest depiction of a dog was found inscribed on a stone bowl found at Hallan Çemi Tepesi, a site in eastern Turkey, dated to 11,500 years ago.  This was just at the start of the Neolithic era, after the end of the last ice age, when the climate had warmed and people began living in villages.  There was evidence at Hallan Çemi Tepesi that people had begun to domesticate pigs, probably by stealing a litter from a wild sow and allowing the pigs to imprint on humans instead of their mother.  It cannot be proven that this image of a canine was a dog and not a wolf, but dog tails are typically curlier and longer than wolf tails.  The tail up display among dogs or wolves is a visible sign of confidence and authority.

Domestication of Cats

Very close to Hallan Çemi Tepesi, in southern Turkey after the ice age, there were great fields of wild wheat which the newly settled humans could collect as an abundant food source.  The Natufian people built houses and stored grain in plaster covered wooden grainaries.   The house mouse emerged around that time, taking advantage of the permanent settlements and rich sources of food.  And this was also about the time that felis sylvestris catus, or the house cat, separated from the African wild cat, Felis silvestris lybica.  House cats are only semi-domesticated; they have retained nearly all of the behaviors and hunting skills of their fully wild forebears and until very recently they were not dependent on humans for food or mating.  A primary difference between house cats and wild cats are that house cats are (relatively) tame, and tolerate humans to some degree, whereas wild cats do not tolerate humans much at all.  Given the similarities between the two subspecies, it might seem hard to tell when cats were domesticated, but for the discovery of a cat buried with a human in a Neolithic village on the island of Cyprus, 9,500 years ago (see picture to the right).  There were no native cats on the island of Cyprus prior to the arrival of Neolithic voyagers so they must have been brought by boat, and the burial of the cat together with a person implies some degree of affection and perhaps domestication.

Did humans deliberately tame cats and if so for what purpose?  As pets, or to control mice?  Before we answer that, let's look at how tameness arises in domestic animals.

Cat buried with human on the island of Cyprus, dated to 9,500 years ago

Dmitry Belyaev's Fox Experiment

One of the most important experiments on the evolution of domestication was conducted, starting in the 1950's by Dmitry Belyaev, a Soviet geneticist.  Stalin didn't like genetics or evolution because survival of the fittest smacked of the bourgeois oppression of the working class, and after WWII, conjured up the racism of the Nazis.  Serious geneticists were being fired, imprisoned or executed.  Belyaev wisely applied for an out of the way position on a Siberian fox farm which raised silver foxes for their fur,  to conduct a program to tame the foxes and make them easier to handle.    

Belyaev's experiment was simple: a gloved hand would be placed into a fox's cage.  The foxes which showed the highest fear reaction: biting, snarling, or cowering, were not allowed to breed.  Those few percentage of foxes which had a quiet exploratory reaction to the glove were allowed to breed.  The experiment was conducted again each year ahead of the winter breeding cycle.

As shown in the video to your left, the results were extraordinary: after just 8 annual breeding cycles, the foxes were showing signs of tameness.  After 18 cycles, the foxes were fully tame: they would follow people around, come when called, began barking, and acted playful and loving to people.  The experimental foxes had resting levels of cortisol, a stress hormone that were 25% as large as normal foxes, and their levels of serotonin were higher.

What's more, the foxes showed signs of Domestication Syndrome.  This was described by Charles Darwin as a suite of morphological characteristics that are common in domesticated animals but rare in wild animals.  Such characteristics include:

None of these characteristics were selected for, they all arose spontaneously after selecting solely for lack of aggression.

Brian Hare, a canine cognition expert at Duke University, visited the Siberian fox farm, where the experiment is still ongoing and did some experiments with the tamed foxes.  In his book, The Genius of Dogs, he remarked on the tameness of the foxes, and then noted, "More surprising were the physiological and physical changes that happened “ accidentally ” as a result of selection for the foxes ’ behavior . The experimental foxes had more flexible breeding cycles . They reached sexual maturity a month earlier and had a longer breeding season than the control foxes . Their skulls were more feminized than the control foxes ’, resulting in shorter and wider muzzles , similar to differences observed between dogs and wolves . The experimental foxes were more likely to have floppy ears , curly tails , and splotchy coats . And all these features appeared as by - products of breeding foxes who were less aggressive and more social toward people . These are the same differences we see between domesticated animals and their wild ancestors . Belyaev had done it . He had taken a population of wild animals and essentially domesticated them . And not just that , but he had figured out the mechanism by which it happened — not by intentionally breeding for each physical trait but by selecting only for behavior , that is , by allowing animals to breed who were friendly toward people . All the other changes associated with domestication occurred as a by - product."

What causes Domestication Syndrome?

How does selecting for tameness generate these morphological side-effects?  That's not entirely clear.  Tecumseh Fitch has a theory (you can see his video HERE) that the embryonic cells which give rise to the adrenal glands (fight or flight hormones) arise in the neural crest during embryonic development.  Adjacent to the adrenal cells are cells which develop into coat pigment, cartilage, snout and teeth.  In his theory, selecting for reduced adrenal response reduces the number of neural crest cells which then leads to shorter snouts, smaller teeth, floppy ears and curled tails and piebald coats.

There are other genetic differences between dogs and wolves as well as house cats and wild cats, which may have been selected for after initial domestication.  Domestic cat genomes showed evidence of recent selection in genes linked to memory, fear-conditioning, and stimulus-reward learning -- which are all related to the evolution of tameness.  

Genetic studies comparing wolves and dogs (whose genomes are 99.9% similar), show that dogs have a variety of changes in neurotransmitter pathways (norepinephrine and dopamine) involved in flight or fight response.   In addition, dogs have variants of two genes, which when deleted in humans cause Williams syndrome, in which patients tend to love everyone.

Dogs and cats and other domesticated animals exhibit neoteny or juvenilization in which adults exhibit juvenile appearances or behaviors.  The shortened snouts and wider faces with big eyes make adult dogs and cats look more like kittens or babies, compared to wild cats or wolves, and in doing so may trigger nurturing behaviors in humans.  One of the juvenile behaviors is a fixation or preoccupation on what their mothers are doing.  In domesticated dogs and cats, the fixation on the mother is carried over to a fixation on their owners, and dogs especially are especially alert to cues that their owners give.  Brian Hare decided to test the tame foxes at Belyaev's fox farm to see if selecting for tameness also gave them insight into human cues.   Hare ran experiments to see if tame foxes which had not had much socialization with humans were more alert to human cues, such as pointing, as dogs are.  He found that indeed, selecting for lack of aggression resulted in foxes which could follow human cues like pointing, not as well as dogs, but better than chimpanzees or non-tame control foxes which had been raised with people.  Hare concluded, "Belyaev’s experiment had changed the foxes ’ ability to read human gestures as a direct result of experimental domestication.  Domestication, selecting the friendliest foxes for breeding, had caused cognitive evolution ."

Similar experiments run on dogs which have been wild for hundreds or thousands of years, such as New Guinea Singing dogs or Australian dingoes show that they also have abilities to read human cues better than wolves or chimpanzees.

Belyaev's experiments suggest a plausible mechanism for the domestication of dogs and cats.  Rather than humans taking in wild puppies or kittens and selectively breeding for certain traits, the self-domestication hypothesis suggests that dogs arose from wolves which had sufficiently lost their fear of humans so that they hung around nomadic camp sites, scavenging bones and carcasses from the hunter-gatherer bands.  Hanging around campsites was the Belyaev equivalent of selecting for lack of fear and aggression.  After a handful of generations, these wolves with less fear would begin to look and act different from wild wolves: they would become more responsive toward humans, they would begin to bark (unlike wild wolves), and they would retain playfulness and tameness into adulthood.  If a hunter-gatherer band had sufficient resources to tolerate the scavenging, dogs could arise from wolves within 20-40 years.  

Given what you know about dogs, what are the characteristics of these proto-dogs that could have been valuable to early humans?

The domestication of cats could have arisen along a similar path.  With the first sedentary people storing wild grains after the ice age, house mice would have been plentiful around their settlements.  Only the least fearful of cats would tolerate living nearby humans in order to feast on the mice.  After several generations where the least fearful cats were most successful in reproducing, the wildcat would have evolved into the house cat with no direct human intervention.

Careful genetic analysis of house cats as compared to wildcats supports the neural crest domestication theory and find differences in genes associated with fear conditioning and several neurotransmitters.

Given what you know about cats, what are the characteristics of these proto-house cats that could have been valuable to early settled humans?

See the Scientific American article on your right for more information about cat domestication.

Taming of the Cat.pdf

For most of our history with dogs they were working breeds: helping us hunt, herd, pull sleds, and protecting us from dangerous mammals.  It was only in the Victorian era (1830-1900) that dog breeding became popular which gave rise to dog clubs and the over 500 types of dog breeds today.  While most of these breeds were bred for adherence to visual standards, at the expense of behavioral characteristics, some working breeds were selected for their incredible abilities at herding (Border Collies), scenting (Bloodhounds), retrieving (Labradors), pointing (various Pointers), swimming (Newfies), racing (Greyhounds), farm rodent catchers  (West Highland Terriers) and protecting flocks (Shepherds).


For those who are interested, here's a 13 minute video from 60 Minutes on the Evolution of Dogs, The Survival of the Friendliest (Thanks to Harriet!)

And here's an 8 minute PBS video on how we domesticated cats.

The Domestication of People

I'll conclude this chapter with an observation that may or may not be relevant to our relationship with pets.  Over the past 50,000 years, the human skull has undergone significant morphological changes.  Compared to ice age humans, our skulls are dramatically thinner, or more gracile,  our jaws are shorter to the point that 35% of the population lack wisdom teeth, and our brains are on average 15% smaller than our forebears.  All of these trends could be explained by self domestication: humans are domesticating themselves by selecting for less aggressive mates.  This could be because faced with ample supply, women prefer the less aggressive mate, or perhaps the most aggressive males went off to war and died, leaving the least aggressive males home to mate.  In addition to more crowded teeth we may have gotten a more sociable species, more in tune with each others wants.  The trend may be yet ongoing: the murder rate in America has dropped 75% over the past 300 years.  A downside to this trend is that juvenile behaviors are extending more often into adulthood.  But perhaps an upside to this trend is that people are bonding more with their pets.  We'll look more at this in the next chapter when we examine the pyschology of the house cat.