Shrinking bodies and shrinking brains

How did we get from majestic howling wolves and prowling tigers to the stumpy pugs and aloof lap-warmers we now welcome into our homes and hearts?

Two new papers have deepened our understanding of the process and effects of domestication on our favourite four-legged friends.

The popular narrative of domestication holds that the transformation of dangerous large animals into household pets was a gradual process – the work of humans selectively breeding for small, cute companions. While this may yet be true for some domesticated animals, a new study published in Current Biology contends that the process may have been a lot less complex for dogs than we thought.

Researchers at the National Institutes of Health (NIH) have identified a genetic mutation in a growth-hormone-regulating gene in dogs that corresponds to small body size, and which was present in wolves more than 50,000 years ago, long before humans eyed these wild beasts for best friends.

The search for such a mutation has been underway at the NIH for more than a decade, but it took lateral thinking to finally uncover it. The team looked for sequences around the growth-hormone-regulating gene that were positioned backwards in domestic dogs, working from there to confirm if these sequences were also present in other canids and in ancient DNA.

This novel approach was successful, identifying a reverse form of the insulin-like growth factor 1 (IGF1) gene with variants that correlated to body size in each of the 200 breeds looked at.

Collaborating with researchers at Oxford University and Ludwig Maximillian University, the team then turned its focus to trawling through the DNA of a 54,000-year-old Siberian wolf – lo and behold, the mutation was there too, precluding the possibility that it is a direct by-product of human domestication efforts, which began about 20,000 years ago.

Graph showing distribution of small body size mutation across different species of canids credit plassais et al
The gene mutation for small body size can be tracked through many canids. Credit: Plassais et al. / Cell Press

The discovery completely changes the evolutionary narrative of man’s best friend.

“It’s as though nature had kept it tucked in her back pocket for tens of thousands of years until it was needed,” says Dr Elaine Ostrander of the NIH.

The finding holds not just for dogs and wolves, but also for coyotes, jackals, African hunting dogs and other canids.

“This is tying together so much about canine domestication and body size, and the things that we think are very modern are actually very ancient,” says Ostrander.

Cats, however, may not have had such a simple journey to domestication, according to a new study published in Royal Society Open Science.

The study, compiled by a team of international researchers, describes a shrinking of cats’ brains as they transitioned from wild creatures to domesticated moggies.

The team says a link between domestication and a reduction in brain volume has been reported before, but only in old and inaccessible literature that, in some cases, compared domestic animals with wild species that are no longer considered to be their true ancestors.

Looking to begin the process of weeding out fact from fiction, the team compared the cranial volume of domestic cats with European wildcats, African wildcats and hybrids of wild and domestic cat breeds.

The result was a clear continuum, with brain size steadily decreasing from wild cats, through hybrids, to our domestic felines. The team says this is clear evidence of brain size decreasing in step with domestication, but it’s unsure at this stage why the two are linked, and whether the trend might be true of other domesticated animals.

So the shrinking stature of our fluffy friends may have been accompanied by shrinking smarts – lucky we love them all the same.

Please login to favourite this article.