Dog and elderly woman

The close relationship between humans and dogs is the longest of all the domestic animals, spanning more than 15,000 years. Modern-day dog ownership has also been linked with improved health and wellbeing, and even longevity. New research by scientists in Sweden and the U.K. now suggests that whether we choose to own a dog or not may be largely determined by our genes. Studies in more than 50,000 pairs of twins by scientists at Uppsala University in Sweden and the University of Liverpool in the U.K. estimated that the heritability of dog ownership due to additive genetic effects was 57% in women, and 51% in men.

“We were surprised to see that a person’s genetic make-up appears to be a significant influence in whether they own a dog,” commented Tove Fall, PhD, professor in molecular epidemiology at the department of medical sciences and the Science for Life Laboratory at Uppsala University. “As such, these findings have major implications in several different fields related to understanding dog-human interaction throughout history and in modern times.” Fall is lead author of the team’s published paper, released today in Scientific Reports, and titled, “Evidence of large genetic influences on dog ownership in the Swedish Twin Registry has implications for understanding domestication and health associations.”

Dogs were the first animals to be domesticated, and although we don’t know where, when, and why humans first developed a relationship with the wolf, there is “incontrovertible evidence” that domesticated dogs were a part of pre-farming hunter-gather societies in Europe 15,000 years ago, in the Far East going back 12,500 years, and in the Americas 10,000 years ago, the authors wrote.

As well as being trained to hunt, herd, and protect, as an “extension to the human toolkit,” dogs are key companions, and regular contact with dogs is widely believed to have improved both our physical and mental wellbeing. Dogs visit patients in hospital and care establishments and are used to help rehabilitate prisoners, while dog-ownership is sometimes recommended as an intervention to improve health. “We have previously shown that dog ownership is associated with longevity and lower risk of childhood asthma,” the team noted. However, as Fall noted, “although dogs and other pets are common household members across the globe, little is known how they impact our daily life and health. Perhaps some people have a higher innate propensity to care for a pet than others.”

What we don’t yet know is whether the health effects of dog ownership are down to the fact of owning the dog and the benefits associated with that relationship and physical care, or do people who want to own dogs already have pre-existing differences in their personality, health, and genetics? To try and untangle this further and investigate the heritability of dog ownership, Fall and colleagues turned to the Swedish Twin Registry of both identical (monozygotic) and non-identical (dizygotic) twins born in Sweden, and the country’s dog registries, which detail ownership. The Swedish Twin Registry is the largest twin cohort in the world.

Studying identical and non-identical twins is a well-recognized approach to help unpick genetic versus environmental influences on biology, health, and behavior, the authors explained. “The heritability of a trait can be estimated from studies comparing concordance of the trait in monozygotic (MZ) and dizygotic twins (DZ) using structural equation modeling. These estimations rely on the underlying assumptions that MZ and DZ twin pairs share environment to a similar degree, and that MZ twins share their entire genome, and that DZ twins on average share 50% of their segregating alleles.” The investigators’ final dataset included 85,542 twins from 50,507 twin pairs, where 35,035 pairs included both individuals, and 115,472 pairs included only one individual. Of the 85,542 participants, 8,503 (9.9%) were identified as dog owners.

Results from the structural equation modeling indicated that genetic factors played a major contribution (57% for females and 51% for males) to dog ownership in Sweden, with much of the remaining variance explained by non-shared environmental factors. Shared environment had only a minor influence (0% to 6%), and only contributed in early adulthood.

The authors say the findings have two main implications. Firstly, that genetic variation may have contributed to our ability to domesticate dogs and other animals, and secondly that the potential pleiotropic effects of genetic variation affecting dog ownership should be considered when looking at the health effects of owning a dog. “These findings are important as they suggest that supposed health benefits of owning a dog reported in some studies may be partly explained by different genetics of the people studied,” pointed out study co-author Carri Westgarth, PhD, lecturer in human-animal interaction at the University of Liverpool.

Interestingly, the authors pointed out, the findings are also in line with a previous study in male twin pairs, which found a 37% heritable component to how often individuals played with pets. They acknowledge that their study does have some limitations, and what the results don’t provide is any information on is which genes might be involved in the heritability of dog ownership. “Although our current study is the first to provide evidence that human genetic factors may perhaps be involved in our choice to keep dogs, our finding does not inform us as to which genes are involved,” the team commented.

“These kind of twin studies cannot tell us exactly which genes are involved, but at least demonstrate for the first time that genetics and environment play about equal roles in determining dog ownership,” stated Patrik Magnusson, PhD, senior author of the study and associate professor in epidemiology at the department of medical epidemiology and biostatistics at Karolinska Insitutet, Sweden, and head of the Swedish Twin Registry. “The next obvious step is to try to identify which genetic variants affect this choice and how they relate to personality traits and other factors such as allergy.” The authors suggest that future genome-wide association studies may help provide greater insights. “Like other personality-related traits, we expect polygenic inheritance,” they wrote.

The reported findings do provide a foundation on which to base new studies, the investigators pointed out. “In view of the deep history of animal domestication (the first and oldest being the dog) and our long and changing relationship with them, this evidence may be an important first step in unraveling some of the most fundamental and largely unanswered questions regarding animal domestication—i.e., how and why?”

“The study has major implications for understanding the deep and enigmatic history of dog domestication,” concluded zooarchaeologist and co-author of the study Keith Dobney, PhD, chair of human palaeoecology in the department of archaeology, classics, and Egyptology at the University of Liverpool. “Decades of archaeological research have helped us construct a better picture of where and when dogs entered into the human world, but modern and ancient genetic data are now allowing us to directly explore why and how?”

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