A surprising study reveals that there is a trace of "wolf" lurking within the tiny body of a Chihuahua and the gigantic build of a St. Bernard.

An international research team from the American Museum of Natural History and the National Museum of Natural History analyzed the genomes of 2,693 dogs and wolves and found that 64.1 percent of purebred dogs carry fragments of wolf DNA. Furthermore, a study of village dogs (free-roaming dogs living in or near human communities) from around the world found genetic traces of wolves in all 280 analyzed pups.

Dogs are thought to have evolved from populations of gray wolves, which became extinct during the Late Pleistocene epoch about 20,000 years ago. Although wolves and dogs still share habitats and can produce fertile offspring, interbreeding between the two has been thought to be extremely rare. Apart from deliberate breeding, there has been little evidence of genetic mixing since domestication.

“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,” explains Audrey Lynn, a postdoctoral fellow at the American Museum of Natural History who specializes in bioinformatics. “But we found if you look very closely in modern dog genomes, wolf is there.”

Genomes Reveals Memories From 3,000 Years Ago

Lynn and her colleagues collected large-scale genome data from the National Center for Biotechnology Information and the European Nucleotide Archive (ENA) and used sensitive genomic methods such as local ancestry analysis (LAI) and phylogenetic analysis to study wolves, purebred canines, village dogs, and other canids from the late Pleistocene to the present. The research was conducted using highly sensitive genomic methods such as local ancestry estimation (LAI) and phylogenetic analysis.

By combining these advanced methods, the researchers were able to divide the entire genome into smaller sections, estimate the ancestry of each section, and capture minute amounts of gene flow that could not be detected using conventional statistical methods.

They found that the gene flow from wolves to dogs occurred on average about 1,000 generations ago (equivalent to about 3,000 years ago). In contrast, the gene flow from dogs to wolves was much more recent, concentrated around the beginning of the 19th century. This is thought to be related to the increase in stray dog populations that accompanied urbanization and the expansion of human activity.

At the whole genome level, canines and wolves are clearly separated. However, when the research team constructed phylogenetic trees for each of the 1,582 genes, they found that not a single gene supported the monophyly of dogs. Further examination of the mitochondrial DNA and Y-chromosome phylogenetic trees revealed a complex interplay between canine and wolf lineages. This discrepancy is evidence of multiple rounds of gene transfer in the past.

Imprints on Physique and Personality

The researchers found that wolf genes are linked to a variety of dog traits, most notably body size. Larger dogs tend to have more wolf ancestry, and certain working breeds, such as Arctic sled dogs, wild-dog breeds, and hunting dogs, are more likely to have this trait. Terriers, bird dogs , and scent hounds, on the other hand, were least influenced by wolf genes.

Notably, among large service dogs, the influence of wolf ancestry varied greatly among breeds. The Sarabi Dog, Central Asian Shepherd Dog, and Anatolian Shepherd Dog, which are livestock guard dogs from Turkey and Central Asia, inherited 0.5-1.2 percent of their genes from wolf ancestors, while Neapolitan Mastiffs, Bull Mastiffs, and St. Bernards showed almost no trace of wolf ancestry

On the other hand, even the Chihuahua, the world's smallest dog breed, was found to have a small proportion of wolf ancestry, about 0.2 percent. These differences speak to the historical specificity of the breed's evolution.

The personalities of dog breeds also showed striking patterns depending on the proportion of wolf genes they had. When compared with breed-specific personalities described by the Kennel Club, the organization responsible for certifying dog breeds, breeds with fewer wolf genes were more likely to be described as “friendly,” “willing to obey,” “easy to train,” and “affectionate.”

In contrast, breeds with strong wolf ancestry tended to be described as “suspicious of strangers,” “independent,” “wary,” and “territorial.” However, researchers cautioned that these descriptions are based on subjective human observations and that it's unclear whether wolf DNA is directly related

Adaptive Genes That Support Survival

Wolf-derived genes are not simply a remnant of evolution, but may actually contribute to the survival of dogs. When gene ontology analysis was performed on the regions of the village dog genome that were enriched for wolf ancestral elements, the only significant functional category was the olfactory transduction pathway. This result suggests that gene inflow from wolves may have enhanced the village dog's sense of smell, thereby improving its ability to search for human food waste.

Village dogs that are not directly cared for by humans generally have a very low survival rate. For stray dogs in urban environments, the survival rate at five months of age is less than 37 percent, with some reports suggesting it is as low as 16 percent. A keen sense of smell may play a key role in their survival in these harsh environments.

Physiological changes during domestication are thought to have weakened dogs' sense of smell compared to wolves, and it seems likely that the influx of wolf genes provided village dogs with an advantage by reinforcing this weakened sense of smell.

Another adaptation has also been identified: Tibetan mastiffs carry a mutation in the EPAS1 gene, derived from the Tibetan wolf, that allows them to survive in the low-oxygen environments of the Tibetan Plateau and the Himalayas.

All 10 individuals analyzed by the research team were homozygous for this mutation (a condition in which an organism has the same pair of alleles), and it was also widely distributed among surrounding village dogs and other highland dog breeds, suggesting that gene introgression from wolves could be an efficient solution for dogs to cope with new environmental challenges.

Wolves as Evolutionary Tools

The breeds with the highest percentage of wolf ancestry were the Czechoslovakian Wolfdog and the Saarloos Wolfdog, which were developed through deliberate breeding, at 23-40 percent. Among typical dog breeds, the Grand Anglo-Français Tricolore, which originated in France, had 4.7-5.7 percent, and the Shiloh Shepherd, which originated in the United States, had 2.7 percent, showing strong traces of wolf ancestry.

Additionally, the Tamaskan Husky, which was developed with the aim of achieving a wolf-like appearance, retains 3.7 percent of its wolf ancestors' genes. Although this is a relatively new breed, the length of its gene fragments suggests that it was developed not by directly crossbreeding wolves or wolfdogs, but rather by concentrating wolf-derived gene fragments that were individually contained in the constituent breeds, such as the Alaskan Malamute, through selection targeted at wolf-like phenotypes.

“Through the years, dogs have had to solve all kinds of evolutionary problems that come with living with humans,” says Logan Kistler, a curator at the National Museum of Natural History. “And it seems like they use wolf genes as part of a toolkit to continue their evolutionary success story.”

This story originally appeared on WIRED Japan and has been translated from Japanese.