Why do dogs wag their tails?

Have you ever wondered: “Why do dogs wag their tails and why do humans like it?" Domestic dogs may be "man's best friend": one-third of all households worldwide own one and our coexistence began around 35,000 years ago. However, many dog behaviours remain a scientific enigma. The rhythmic tail wagging of domestic dogs has always caught our attention. Through our intuition and a handful of scientific studies, we have attributed different meanings to this behaviour, leading to fragmented and conflicting answers. Here, we summarise existing research on the mechanisms, development, evolution and function of tail wagging in domestic dogs, highlighting where the results converge, diverge or do not exist. We propose investigating this behaviour from its evolutionary roots and we suggest two hypotheses to explain its increased occurrence and frequency in dogs compared to other canids. This behaviour could have arisen during the domestication process following two paths: a) as a by-product of selection for other traits, such as docility, or b) as a trait directly selected by humans who are attracted to repetitive and rhythmic movements. We invite testing these hypotheses through neuro-cognitive studies on both dogs and humans, thus shedding light not only on a key canine behavior but also on the evolutionary history of characteristic human traits, such as the propensity for rhythm.