| Summary: | Biologists since Darwin have recognized that domestication, where species are selected to live in human-controlled environments, exerts strong selection on organisms and dramatically impacts their evolutionary trajectories. Across domesticated mammal species, characteristic morphological, physiological and behavioural changes occur simultaneously, as correlated traits, a phenomenon known as the domestication syndrome. Key behavioural alterations are connected with the domestication syndrome, in which domesticated animals express decreased aggression and fearfulness alongside increased sociability and playfulness compared to their wild counterparts. To investigate various aspects of the behavioural implications of domestication, we used the dog (Canis familiaris) and its extant ancestor, the grey wolf (Canis lupus), as our study species. Since we currently lack quantitative confirmation that correlated changes in behaviours follow domestication, we evaluated correlations among sociability, aggression, fearfulness and playfulness in more than 90,000 dogs in Paper I. Contrary to expectations, we found weak support for behavioural correlations in modern dog breeds, but observed exaggerated effect sizes of correlations in ancient breeds. We suggest that while selection on suites of behaviour have been relevant during early dog domestication, a recent shift in selection pressures in modern dog breeds affects the expression of domestication-related behaviours independently. In Paper II we therefore contrasted the expression of sociability, aggression, fearfulness and playfulness during domestication in wolf hybrids and dogs, and found that while wolf hybrids were less playful and overall more fearful than dogs, they were not less social or more aggressive than dogs. Our results suggest that behavioral alterations during domestication do not necessarily occur in concert as predicted by the domestication syndrome and point to an important, but previously overlooked, role of selection on playfulness during the ...
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