Domestic dogs (Canis lupus familiaris) are the oldest and one of the most popular species of companion animals 1,2. Research has uncovered that their closest relatives are wolves 3,4, and that modern dog breeds were domesticated into human society over 10,000 years ago 5. However, their origins and the process of domestication have yet to be fully understood.
Domestication involves the selection of traits that fundamentally alter wild species to become more useful to humans 6. Dogs have been closely associated with humans for many centuries and there are now more than 400 breeds of domestic dogs 7. Dogs are thought to have been selected for human proximity 8, a wide range of physical traits 9, behaviour, and more broadly, the ability to build social relationships 10.
Research comparing the behaviour of dogs and wolves has indicated that dogs show less avoidance and aggression toward familiar humans 11. In addition, dogs show better understanding of human communicative signals than wolves 12,13 and, in some studies, dogs have even demonstrated an ability to form social bonds with humans 10,14. Although the leading force behind dog domestication remains unclear, it is certain that these behavioural adaptations, including docility and the ability to form social bonds with humans, are important factors which enabled dogs to be incorporated into human society.
Animal behaviour, particularly social behaviour, is modulated and influenced by the actions of various hormones in the brain. For example, glucocorticoid is widely known as a hormone which is positively related to anxiety and social avoidance 15,16. The role of glucocorticoid in the domestication process has been proposed in many studies. One example is the domestication experiment on silver foxes (Vulpes vulpes), in which domesticated foxes were selectively bred for non-aggressiveness or fear toward humans. They showed reduced cortisol secretion and exhibited greater exploratory behaviour in, and tolerance of, new environments 17. Another hormone that is of interest is oxytocin (OT). OT is known to play an important role in the bonding and attachment between mother and young in mammalian species, as well as in other types of relationships, such as in pair bonding 18,19. Recently, the study of OT-mediated bonding has been expanded to interspecies relationships, including those between dogs and humans 10. OT was also shown to have a function in enabling dogs to respond to human social cues, such as pointing 20,21. Moreover, the marked variations in OT receptor (OTR) genes between purebred dogs of different breeds, free-ranging dog populations, wolf subspecies, and golden jackals have suggested that the OTR gene could have been a target gene during domestication 22.
Dog breeds show differences in temperament and variations in emotionality, vocalization, activity, problem solving, reactions to human handling, trainability 23,24, and the ability to understand human communicative signals 25. As wolves, which share the same ancestors, do not show good human-communication skills, genetic changes should have occurred during the domestication of dogs, and the behavioural differences between dogs and wolves should appear as different phenotypes, depending on the genetic backgrounds of different dog breeds 26.
Owing to remarkable advances in technology, it has become possible to study the genetic relationships between domestic dog breeds to construct a genetic classification for them 27,28. There appear to have been two population bottlenecks during the dog domestication process, one likely associated with their initial domestication from wolves to dogs and the other with the recent formation of modern dog breeds 3,29. A neighbor-joining tree revealed several breeds with ancient origins that could be separated from the remaining breeds with modern European origins 27,28,30. The ancient breeds showed lower attachment behaviour toward their owners, suggesting that this attachment behaviour was acquired during the relatively recent domestication process 31. Interestingly, as even these ancient breeds with little human contact were able to use communicative signals, dogs may have acquired these abilities before the recent formation of dog breeds 25,32. These results suggest that comparisons between and within dog breeds regarding gene–behaviour associations is useful for understanding the underlying genes responsible for dog domestication.
The analysis of genes related to the behaviour of various dog breeds holds great potential for understanding the domestication process. An investigation using genome sequencing data from a globally diverse collection of village dogs and wolves suggested that the initial selection during the early stages of dog domestication was for a behavioural trait influenced by genes associated with the perturbation of crucial neural crest signalling pathways 26. A genome-wide polymorphism analysis of a global sample of dogs and wolves suggested that there had been selection pressure on the ‘adrenaline and noradrenaline biosynthesis pathway,’ well known for its involvement in the fight-or-flight response 33. Other genome-wide analyses have also identified various genomic regions as candidates for affecting dog cognitive phenotypes, such as inhibitory control, sociability, and interspecies communication 34. An analysis of the dog genome region which is known in humans to be linked to Williams–Beuren syndrome (WBS), a multisystem congenital disorder characterized by hyper-social behaviour, provided evidence that structural variants in genes previously implicated in the behavioural phenotype of patients with WBS and contained within the WBS locus, contribute to extreme sociability in dogs 8. However, the genes associated with the skill of reading human gestures and attachment behaviour toward humans differ between dogs and wolves, and are not well-understood.
Genomic research has identified a list of genes that underwent positive selection during domestication 27,35. The functions of these candidate selected genes are diverse, including digestion, gene ontology, neurological processes, and sexual reproduction. Some of these selections may have influenced the endocrine system, resulting in domestic dogs acquiring their unique behaviours during the domestication process 16. We focused on cortisol and OT as important hormones related to dog domestication. We hypothesized that cortisol regulation of social tolerance and non-fearful response to humans may have been the most important turning point in the domestication of dogs 16. A decrease of cortisol can facilitate social cognitive skills in dogs, probably owing to a reduction of their fear response to humans, as suggested previously by other researchers 36. However, cortisol suppression does not fully explain the substantial ability of dogs to understand human communicative signals and form social bonds with humans. Therefore, we focused on the OT system. We hypothesized that OT is another hormone which played a key role in the dog domestication process 16. Another gene which is of interest is WBSCR17. WBSCR17 is in the critical region of patients with Williams–Beuren Syndrome and is predominantly expressed in the brain and heart. WBSCR17 is thought to be involved in brain development, through the O-glycosylation of neuron proteins 37.
In this study, to explore the genes related to behavioural and cognitive changes in the dog domestication process, we investigated the association between the human-related cognitive ability of dogs and polymorphisms in the following four genes: melanocortin 2 receptor (MC2R), WBSCR17, OT, and OTR. We selected two distinctive social cognitive skills in dogs that are clearly different from wolves: the ability to follow pointing and gestures from humans, tested using a two-way choice test 13, and gaze behaviour, tested using a problem-solving test 12.