Small dog breeds were thought to arose after domestication. But a new study proves otherwise.

Dogs come in a striking number of shapes and sizes. Some dog breeds, such as Golden Retrievers and German Shepherds, can grow up to two feet high, while Spitz and Chihuahuas hang around eight inches. It was long thought that such “lap dogs” arose after dogs were domesticated and humans started breeding small, adorable companions. But that’s just a bit different.

gene mutation

Small dog breeds all have a specific gene mutation, namely the growth factor IGF1. IGF1 is one of the major genetic mutations responsible for the small size of lap dogs. Scientists have theorized that dogs were initially quite large in size and started getting smaller and smaller about 20,000 years ago, around the time they were domesticated. But now researchers in a new study discovered that the genetic mutation in the growth hormone-regulating gene was also present in wolves more than 50,000 years old.


The search for the gene mutation in dogs has been going on for more than ten years. “It took a long time to find the mutation,” researcher Elaine Ostrander said in an interview with “We searched in the coding region, in regions between genes and in regions responsible for the regulation of the gene. But in the end we only found it when we decided in the gene, to see if there was anything interesting on the other strand.” The researchers eventually found the mutation in the DNA that controls the expression of the gene. According to the researcher, it has been very helpful that the genomes of more than 250 dog breeds have been mapped in the meantime. “It allowed us to compare large and small dogs,” says Ostrander.


Next, to determine when the IGF-1 mutation first showed up, the researchers searched through ancient wolf DNA. To do this, they examined the DNA of a 54,000-year-old steppe wolf, among other things (Canis lupus campestris). It leads to an interesting discovery. Because this great wolf also appears to have the relevant gene mutation. And that while this wolf lived well before domestication. “We were very surprised by the small allele (a particular variant of a gene, ed.). in ancient DNA,” says Ostrander. “It’s as if nature had it in her back pocket for tens of thousands of years, until it was needed,” Ostrander says.


The findings don’t just apply to dogs and wolves, though. For example, the IGF1 mutation also appears to have played a key role in the evolution of smaller canids such as jackals, coyotes and African wild dogs. Although scientists thought that the allele for a shorter stature was much newer, that turns out not to be the case. “It shows that certain things that we think are very modern are actually very old,” says Ostrander.

How canids have become smaller, partly due to the IGF1 mutation. Image: Plassais et al./Current Biology

According to the researcher, the discovery of the gene mutation is very important. “It represents an important piece of the puzzle of how size is regulated in breeds,” she says. “Dogs show more variation in size than any other mammal on Earth. Learning how nature does that is a major step forward in our understanding of mammalian growth and regulation.”

dog breeds

Although the IGF1 gene plays a very important role in determining a dog’s body size, it is not the only gene that influences it. Researchers therefore plan to study the other genes involved as well. “Since dogs evolved quite recently, there are actually very few genes that regulate body size,” Ostrander says. “Canids only own 25, while humans have several hundred.”

The ultimate goal is to better understand how the different genes together determine the exact size of each breed. “We want to better understand how exactly those 25 genes work together to produce dogs of varying sizes; from Chihuahuas to Great Danes,” concludes Ostrander.