Why do dogs wag their tails?
Have you ever wondered why dogs wag their tails and why humans like this behaviour? Domestic dogs are said to be humans' best friend: one-third of all households in the world own one and our coexistence with them started about 35,000 years ago. Many of their behaviours, however, remain a scientific enigma. A team of researchers from Turin, Vienna and Nijmegen, coordinated by Professor Andrea Ravignani of Sapienza University of Rome, conducted a study published in the journal Biology Letters that summarises the results of work to date on the mechanisms, ontogeny, evolution and function of tail wagging in domestic dogs.
Domestication - that of the dog probably began during the Upper Palaeolithic - is a lengthy process that leads to a series of physiological, morphological and behavioural changes resulting from ecological interaction: one species actively manages the survival and reproduction of another, which provides resources and services to the former.
In dogs and some other mammals, these changes may include depigmentation of fur, reduction in the size of the facial skeleton and teeth, overall body size and proportions, the appearance of physical attributes such as floppy ears and a curled tail, reduced aggression, increased docility and changes in hormone levels resulting in behavioural changes, such as reduced stress response. Furthermore, comparative studies between wolves and dogs have shown that the domestication process has shaped the cognition and sociability of dogs in both dog-dog and dog-human interactions.
The various hypotheses that have attempted to explain how these changes have occurred can be traced back to two processes: desirable characteristics in domesticated species are primarily the result of adaptation to a human-dominated environment, i.e. a by-product of selection for other characteristics, or they are the result of genetic selection operated directly by humans. In particular, tail wagging may have emerged during the domestication process following two paths: either as a by-product of the selection of other traits, such as docility or as a trait directly selected by humans who are attracted to repetitive and rhythmic movements.
In the first case, there would be a genetic link between docility and tail anatomy: initial selections for docility may have led to alterations in neural crest cells during development, affecting various phenotypic traits, including tail anatomy. This is in line with the results of a long-term experiment that attempted to replicate the mammalian domestication process and track changes in behaviour, genetics and development. The experiment was conducted on silver foxes (Vulpes vulpes) bred for 40 generations and directly selected for tameness and docility. The resulting population of foxes showed behavioural, physiological and morphological traits similar to those observed in dogs: although the wagging behaviour was not directly selected, the domesticated foxes showed similar wagging behaviour to dogs and had more curled tails. This would support the hypothesis that the domestication process led to behavioural and anatomical changes that altered the wagging behaviour of dogs so that the latter wag their tails more often and in more contexts than non-domesticated canids.
The second hypothesis is that of "domesticated rhythmic wagging": wagging behaviour may have been one of the goals of the domestication process, with humans (un)consciously selecting dogs that wagged their tails more often and, potentially, more rhythmically. Indeed, multidisciplinary evidence shows that humans have remarkable abilities to perceive and produce rhythmic sequences, particularly isochronous patterns in which events are evenly spaced in time. It is not yet clear how this behavioural trait appeared in humans, but cognitive neuroscience shows that the human brain prefers rhythmic stimuli, which trigger pleasurable responses and involve brain networks that are part of the reward system. This propensity for isochronous rhythms may have driven human selection for the conspicuous rhythmic wag in dogs and may explain why dogs display it so often in human-dog interactions.
According to both hypotheses, the selection of wagging behaviour may not have been uniform among the various breeds; for example, hunting dogs wag their tails more than herding dogs, and have undergone different selective pressures during domestication. "The combination of techniques of behavioural analysis, computer vision and physiology with neuroscience will be able to help distinguish between tail movements under control, hence under possible selection, and those resulting from mere mechanical effects such as, for example, the tip of the tail moving as a consequence of several cranial portions of the tail being subjected to a selection action", says Andrea Ravignani, professor of General Psychology "A more systematic and in-depth investigation of tail wagging will not only allow a better mapping of this iconic behavioural manifestation of the dog but will also provide indirect insights into the evolution of human traits, such as the perception and production of rhythmic stimuli".
References:
Why do dogs wag their tails? - Silvia Leonetti, Giulia Cimarelli, Taylor A. Hersh, Andrea Ravignani - Biology Letters – DOI: https://doi.org/10.1098/
Further Information
Andrea Ravignani
Department of Human Neuroscience
andrea.ravignani@uniroma1.it