Biological and Hormonal Approaches to the Evolution of Human-Canine Relationships

Domestication has profoundly changed dogs’ morphology, physiology, and behavior compared to that of wolves. However, it remains unclear how dogs evolved the social capacities allowing them to effectively interact and bond with humans. Since endocrine systems may be prime targets of selection and driving forces of behavioral changes, some authors suggested that domestication changed dog’s glucocorticoid and oxytocin profiles, facilitating their human-directed behavior. However, studies directly comparing dogs’ and wolves’ hormonal and behavioral profiles while accounting for developmental effects are largely lacking. Here we summarize the dog domestication process, the functions of the prime hormonal systems likely to have been affected by domestication, and three studies investigating human-directed behavior in similarly raised wolves and dogs, as well as dogs with different life experiences (pet dogs, free-ranging dogs). The studies demonstrate that similarly raised wolves and dogs showed some behavioral differences in their human-directed behaviors, yet, at the same time, more similarities in their hormonal response to human contact than the different dog populations. Indeed, the different dog populations behaved similarly but showed different hormonal reactions when interacting with humans. Together, these results point to the added value of approaching questions from both a behavioral and hormonal perspective and highlight the need to carefully control for the animals’ previous experiences in comparative studies on domestication-related effects. We conclude that the current data question a general, clear-cut domestication effect as predicted by the most prominent domestication hypotheses (emotional reactivity and hyper-sociability hypotheses) and rather suggest that life experiences during development may play a key role in shaping dogs’ responses to humans.