Could a single hormone have helped us domesticate dogs (and vice versa)?
Originally published in slightly different form on December 7, 2010 at PsychologyToday.com.
“The primary factor that humans selected for was tameness, or low levels of aggression. The main mechanism through which this was accomplished was neotenization, or retention of juvenile low aggression into adult life. We also selected animals who paid attention to us." —Nigel Barber, Ph D.
The “Love Hormone” and Canine Behavior
In a previous article here, “The Love Hormone: The Key to Canine Evolution,” I proposed the idea that oxytocin probably played a major role in the development of the wolf’s social instincts, instincts which for years have been misunderstood and misinterpreted as being about the formation of a social, dominance hierarchy, based on rank and status.
My model of wolf evolution is based on the idea that oxytocin can affect social behaviors in mammals on a significant scale.
“It might sound improbable," I wrote, “that a simple neuropeptide, even though it has some pretty fancy tricks up its sleeve (it's a hormone and pheromone) could be responsible for such complex changes in behavior. But research by Insel & Young (2002) shows that in monogamous prairie voles, when the effects of both oxytocin and vasopressin (which are closely related), were disrupted—by derailing their connections to the reward circuits in the nucleus accumbens—the monogamous voles quickly became promiscuous. This was a huge reversal [which] took place immediately.
“In contrast, Lim et al (2004), did a study with promiscuous meadow voles, and found that the addition of oxytocin and vasopressin (again, in direct connection with the reward circuits in the brain), produced long term pair bonding where no such behavior had previously existed. And again, the results took place immediately, in real time.”
Lim et al conclude: “A change in the expression of a single gene in the larger context of pre-existing genetic and neural circuits can profoundly alter social behavior, providing a potential molecular mechanism for the rapid evolution of complex social behavior.”
Meanwhile, there are numerous studies showing that many (if not all of) the emotional connections made between dogs and their owners are based at least partially on increases in the production of oxytocin. Just looking into your dog’s eyes can increase oxytocin. (Miho Nagasawa, et al, 2009: “Dog’s gaze at its owner increases owner’s urinary oxytocin during social interaction,” Hormones and Behavior, 55, 434-441.)
Given all this evidence, I respectfully (and partially) disagree with Nigel Barber’s statement (above) about how dogs became domesticated. I don’t think early humans were deliberately selecting for tameness, per se, or for neotenous characteristics, though I agree that we probably liked animals that paid attention to us (particularly if by looking at us, those animals increased positive feelings in us). It’s more likely that the major thing we were selecting for (albeit quite unconsciously) was oxytocin.
Three Distinct Phases
In an article in the August issue of Current Anthropology, University of Pennsylvania anthropologist Pat Shipman has proposed that there were three distinct phases to what eventually became the domestication process and offers a testable hypothesis that “human adaptive changes were causally linked to the animal connection.”
Shipman says the first phase took place over 2½ million years ago, and was probably based on shared hunting patterns between pre-human hominids and proto wolves (both species are social hunters who target animals that are larger and more dangerous than themselves). This would’ve been a time when hominids and wolflike animals may have had a respectful, symbiotic relationship much like the one described in the folklore of many native cultures, and which may have echoed the way killer whales reportedly shared hunting duties with the Australian aborigines well into the early 20th Century.
This was not a period of humans domesticating, caring for, breeding, or selecting wolves (or proto-dogs) for any purpose, but the two species probably did interact very closely for a very long period of time (probably up until about 500,000 years ago).
Shipman says the second period probably began about 200,000 years ago, which is when homo sapiens emerged and we see the first examples of art depicting animals. Neolithic cave art (circa 10,000 B.C.), discovered at Tasilli-n-Ajjer, in Algeria, shows what appear to be dogs or doglike animals surrounding a wild ox or wildebeest, while a human hunter, carrying a spear, stands at a distance, ready and waiting to throw his weapon. This suggests that our Neolithic male ancestors probably used dog-like animals for hunting purposes.
The actual domestication process—where dogs became part of our families and our daily lives—began about 40,000 years ago, and continued through the last ice age. (Dogs may have kept us warm!)
Women, Tameness and Just Feeling Good
In her book The Lost History of the Canine Race, Mary Elizabeth Thurston suggests that females of the tribe may have been the ones caring for the first dogs. “Women early on assumed the task of gathering plant matter and collecting small game because the slower pace of these tasks blended more easily the need to carry and care for very young children. For this reason, prehistoric women might have been primarily responsible for forging the first intimate relationships with canids. (pg. 5)”
A major role for women in prehistoric times was nurturing the young. Oxytocin is sometimes called the “nurturing hormone.” Women naturally produce more oxytocin than men. So it seems to me that the primary things our ancestors selected for in their domestication of dogs was not tameness or neotenous characteristics, but for hunting skills and oxytocin. (Not to be sexist, but even today women tend to be more nurturing while men are more likely to roughhouse with their dogs.)
The Belyaev fox experiment (mentioned by Dr. Barber in his blog article, and also in two of my recent posts) shows that tameness runs concurrently with the reduction of adrenaline.
Adrenaline is a fear hormone; oxytocin is a trust hormone. I know of no studies showing an inverse relationship between them, but it has to exist; you can’t be full of adrenaline and trusting at the same time. And since—according to the Russian fox researchers—the genetic reduction of adrenaline is on the same pathway as the genes that code for neotenous morphological traits, it seems likely that there’s a correlation between oxytocin and neoteny.
So rather than “thinking” about what they were selecting for, I believe our ancestors went by what felt good to them. And dogs make us feel good, partly (or perhaps even primarily) because of oxytocin.
Domestication of Humans or Domestication of Dogs? Or Both?
In “A Woof at the Door,“ Pat Shipman discusses canid teeth, worn as ornaments by our ancestors. “Did someone who wore a perforated canid tooth 33,000 years ago proclaim him- or herself to be one of the group that domesticated dogs? Possibly. Domesticating dogs was a remarkable human achievement that doubtless provided a definite selective advantage to those who accomplished it successfully. They might well have had reason to brag about their accomplishment by wearing canid teeth.”
Personally, I don’t think our ancestors were bragging. I think they just felt very deeply connected to their dogs, the way we do to ours. And when their canine companions died, our ancestors probably mourned them just as deeply as we do. These days we write memoirs about our dogs (I’m currently writing one myself), or we put up a webpage in their memory, or we place an urn on the mantel. Perhaps, by wearing a set of canid teeth, our ancestors were memorializing their dogs too, but in their own way.
Whatever form the domestication process took, it seems to me that it was and still is a two-way street. Dogs have domesticated us as much as we’ve domesticated them.
And oxytocin was a major player in the process.
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1) Modern wolf research, done by L. David Mech, strongly suggests that no such hierarchical structure exists in some wild wolf packs. In fact, he and his colleagues don’t like to use the word alpha anymore because “It falsely implies a hierarchical system in which a wolf assumes a place in a linear pecking order.” (Mech, L. David. 1999. “Alpha status, dominance, and division of labor in wolf packs.” Canadian Journal of Zoology.)
Mech has complained that many people (including me) have misinterpreted his stand on this issue. In a more recent study (2010) Mech has stated, quite clearly, that dominance is one of the most prevalent and important behaviors of wolves in a pack, negating his statement in 1999 that dominance contests are so rare that during the 13 summers he observed the Ellesmere Island wolves, he never saw a single one take place.