Why is the dog wagging?
Even after more than 100 studies on the topic, tail-wagging remains misunderstood and contradictory. This is now shown by a survey of the most common carnivore on earth.
While other vertebrates balance with their tails, support themselves (kangaroos) or swat at annoying flies, the appendage in the dog family has taken on a task of its own: It is primarily used for communication. A tail tucked between the legs, for example, is known to signal fear and insecurity, both in domestic dogs and in their wild relatives, the wolves. However, the conspicuous tail wagging is less pronounced in wolves. It appears to be a behaviour that only developed in dogs in the course of domestication. It is now one of the most common forms of communication on the planet: With an estimated one billion individuals, domestic dogs are among the most widespread mammals of all and the most common carnivores.
What the dog wants to express with its sometimes excessive tail wagging and what role humans have played in the development of this behaviour was to be shown in an overview study, for which experts from the Max Planck Institute for Psycholinguistics in Nijmegen and the University of Veterinary Medicine Vienna evaluated more than 100 studies on the topic. The results have been published in the current issue of "Biology Letters".
However, instead of clear findings, the team still identifies major knowledge gaps in research. For many popular assumptions - above all that a dog wagging its tail is happy and not stressed - there is no really reliable evidence. Studies in which the stress hormone cortisol was measured revealed no clear connection between wagging frequency and well-being. The study therefore states that wagging can express different messages in different situations, depending on whether the recipient is a conspecific or a human.
In wolves, a slow wag with a low held tail typically signals submission or appeasement. Excited wagging with a raised tail, on the other hand, does not occur in wild pack animals. However, the basic message - appeasement and friendly relationship management - does not seem to have changed in domesticated dogs either. When addressed to a master or mistress, it sometimes turns into begging.
Wagging with a side of impact
The simple equation according to which a dog that wags its tail feels good does not work, as the "favourite result" of co-author Taylor Hersh shows, as she revealed to the science magazine "Science" in an interview: "Tail wagging is an asymmetrical behaviour. When a dog encounters something it wants to approach, it often wags more to the right side of its body, while it wags to the left side of its body when it wants to retreat from something." This result, first observed in 2007, has since been confirmed in numerous studies. The animals can also recognise the direction of the wagging of conspecifics and react accordingly.
It seems clear that tail wagging became increasingly prominent in the course of domestication. However, it is unclear what role humans played in this process, Hersh and her team summarise. It may have developed as a side effect of selection for the tamest, most compatible dogs. If these traits are genetically linked to wagging behaviour, this would explain why the dogs became increasingly waggy over time.
However, it is also possible that dog owners have more or less consciously favoured those animals that have a particularly pronounced wagging behaviour. Rhythmic movements exert a certain attraction on people. "Several studies have shown that humans are particularly attracted to isochronous patterns, i.e. a rhythm in which all the intervals between events are the same, like a metronome," explains behavioural researcher and co-author Silvia Leonetti. According to this, dogs wag because humans have always liked it.
Which of the two hypotheses is correct - this is also one of the areas that Hersch, Leonetti and their colleagues believe would benefit from more research. As a first step, they suggest recording more than just frequency and duration in future studies of tail wagging, but also using modern technology to record parameters such as tail height, lateral deflection or the direction of the animals' gaze. This is the only way to finally get to the bottom of this "ubiquitous but scientifically elusive phenomenon" and its evolutionary history.
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