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Key points
- The split between small and large in dogs may be the most ancient divide.
- Genetic evidence points to early distinction between large and small body size in canids.
- The oldest dog remains in the Americas were found in British Columbia and dated back more than 13,000 years ago.
While working on Dog’s Best Friend, I noted that many dog people had more than one dog, quite often a large one and a small one, such that the large, or yard, dog lived outside, often just under the porch where it could best serve as guardian; while the little dog was a house pet and pest remover. That led me to conclude that “the divide between big dogs and little dogs is among the most ancient and fundamental in dogdom” (p. 53).
Imagine my surprise when these dozen-plus years later an international group of geneticists published a paper in Current Biology detailing how both natural and artificial (human) selection for two specific versions of messenger RNA appear to dictate the general size of a dog, that is, whether it is large or small. The two variants (alleles) are found in all members of the family dog, including gray wolves, coyotes, jackals, Ethiopian wolves, African wild dogs, dholes, and foxes. Writing for the group, geneticist Jocelyn Plassais, on leave from INSERM-University of Rennes in France to work in Elaine Ostrander’s National Institutes of Health lab, said the team analyzed more than 1000 genomes of wild and domestic canids. They concluded that the split between big and little dogs not only was the oldest in domestic dogdom but also appeared well before dogs split from wild wolves. They estimate the two RNA variants showed up in gray wolves more than 53,000 years ago.
Much of the discussion in the paper, “Natural and human-driven selection of a single non-coding body size variant in ancient and modern canids,” is devoted to the timing of the emergence of the mutations. The team concludes that given the evidence, the two emerged at about the same time. Across the board, dogs with two copies of one of the variants weigh under 33 pounds while those with two of the other variant weigh over 55 pounds. Human breeders have fixed smallness by selectively breeding like to like; this turns out to correspond to having two copies of the variant that conveys smallness.
By the late Pleistocene, the variant for smallness had nearly disappeared, perhaps due to the harsh conditions of the last Ice Age. But it continued to exist in canids and actually became more prevalent in populations again around 9500 years ago, suggesting that humans had begun to selectively breed small dogs. At that time, there may also have been natural forces at work, e.g., the rise in the earth’s average temperature, which would tend to favor smaller animals. Over the last 200 years, human breeders have greatly increased the abundance of the smallness variant to create globally popular small breeds.
Earlier this year, archaeologists announced the discovery of the oldest dog remains in the Americas. These were found in a limestone cave in Haida Gwaii in British Columbia and dated back more than 13,000 years ago. Such findings make clear that dogs likely arrived on the continent with humans and that already they segregated into both large and small, the inheritors of the genetic split found by the Plassais team.
A Final Unrelated Note: In 2015, I reported (you can find the piece here on my blog or reprinted here) on research suggesting that dogs had become humans’ best friend by hijacking the oxytocin pathway that controlled the maternal-infant bond. News of this research traveled widely around the world as it bolstered much of the sentimentality regarding dogs. Last month, researchers Anne Burrows and Kailey Olmstead of Duquesne University presented at the Experimental Biology conference in Philadelphia on their work with slow- and fast-twitch muscles in dogs’ faces. They hypothesized “that dog facial muscle physiology would be more similar to humans than to wolves,” in that dogs would have more fast-twitch muscles that would enable them to be more expressive. In analyzing samples, they found dogs had a substantially greater number of those muscles than wolves. It will be intriguing to watch whether or how this finding will be squared with the oxytocin hypothesis.
