Most modern dogs have wolf DNA from relatively recent interbreeding. Here's which breeds are the most and least 'wolfish.'

Most modern dog breeds have small amounts of wolf ancestry from long after dogs were domesticated, according to a new study.

The wolf DNA isn't left over from when dogs and wolves diverged; instead, it most likely came from interbreeding in the past few thousand years. That wolfish influence may be linked to certain characteristics, such as size and personality traits, in different dog breeds, researchers reported Nov. 24 in the journal PNAS.

"Dogs are our buddies, but apparently wolves have been a big part of shaping them into the companions we know and love today," study co-author Logan Kistler, curator of archaeobotany and archaeogenomics at the Smithsonian National Museum of Natural History in Washington, D.C., said in a statement.

Wolves and dogs genetically split more than 20,000 years ago. Since then, there has been some gene flow between dogs and wolves, thanks to their genetic compatibility. To measure the extent of intermixing and its effects on both animals, researchers studied the previously published genomes of nearly 2,700 dogs and wolves from the Late Pleistocene (the last ice age) to the present. This group included 146 ancient dogs and wolves, 1,872 modern dogs and about 300 "village dogs" that lived around humans but weren't pets.

At least 264 modern dog breeds have wolf ancestry passed on from mating that occurred an average of 900 dog generations ago, which equates to about 2,600 years ago — long after dogs became domesticated at least 20,000 years ago, the team found. The most wolfish dogs had up to 40% wolf ancestry in their genomes, but most had between zero and 5% wolf ancestry.

"Prior to this study, the leading science seemed to suggest that in order for a dog to be a dog, there can't be very much wolf DNA present, if any," study co-author Audrey Lin, an evolutionary biologist at the American Museum of Natural History in New York City, said in the statement. "But we found if you look very closely in modern dog genomes, wolf is there. This suggests that dog genomes can "tolerate" wolf DNA up to an unknown level and still remain the dogs we know and love."

Czechoslovakian and Saarloos wolfdogs had the highest degree of wolf ancestry, which is perhaps unsurprising, since they were intentionally bred by crossing domestic dogs with wolves in the 20th century. Larger dogs and certain working breeds — such as Arctic sled dogs, hunting dogs and certain guardian dog breeds from west and Central Asia, such as Anatolian shepherds — tended to have higher levels of wolf ancestry.

But plenty of breeds didn't fit these patterns. Some large guardian dogs, such as bullmastiffs and Saint Bernards, didn't have any detectable wolf ancestry. And some smaller dogs had small amounts of wolfish DNA. For instance, 0.2% of the Chihuahua's genome can be traced back to wolves, the team found.

"This completely makes sense to anyone who owns a Chihuahua," Lin said in the statement. "And what we've found is that this is the norm — most dogs are a little bit wolfy."

Meanwhile, every tested "village dog" had wolf DNA in its genome, the scientists found. And the reason why might be related to their survival. "The stretches of wolf DNA we found in village dog genomes contained genes related to olfactory receptors," Lin and Kistler wrote in The Conversation. "We imagine that olfactory abilities influenced by wolf genes may have helped these free-living dogs survive in harsh, volatile environments."

Some personality traits that kennel clubs use to describe certain breeds also tracked with the amount of wolfish influence. Breeds with lower wolf ancestry were frequently described as "friendly," "easy to train" or "lively," while breeds with more wolf DNA were pegged as "suspicious of strangers," "independent" and "dignified." It's not yet clear if wolf genes are directly responsible for these traits.