Table 1.

Outstanding questions on dog tail wagging behaviour and related suggestions for future studies.

Tinbergen domain	outstanding questions	methods
mechanism	physiology: are different tail wagging parameters associated with different arousal levels and/or physiological mechanisms?	—systematically quantify dog tail wagging parameters (see ‘Recommendations and future directions' section for details) while collecting real-time physiological (e.g. heart rate, cortisol, oxytocin) and behavioural (e.g. pacing, yawning) measures of arousal [22] in different contexts (e.g. affiliative, aggressive)
	biomechanics: is the entire tail under neurological control, or just a portion of it (e.g. base, tip)? Which brain circuits control tail wagging? Is any parameter of tail wagging under voluntary control or potentially learned?	—monitor dog tail activity while simultaneously conducting non-invasive electrophysiology (e.g. EEG) and neuroimaging (e.g. fMRI) [100–103].
	genetics: how much inter-individual phenotypic variation is there in tail wagging? How much of that variation can be explained by genetics?	—use whole-genome sequencing data and comparative genomics to investigate whether tail wagging is a genetically linked trait; a logical starting point would be to build on genomic research that has already been conducted on dog tail length [104] and shape [105].
ontogeny	development: does tail wagging behaviour in the same individual change from puppyhood to adulthood? Is any age-related variability in tail wagging linked to cognitive development and/or the production of other communicative signals?	—systematically quantify dog tail wagging parameters in the same individuals throughout development while also tracking changes in cognition and non-tail-based communication [106,107].
function	context: how do dog tail movements differ in social versus non-social contexts?	—systematically quantify dog tail wagging parameters when dogs are exposed to social and non-social stimuli and situations (e.g. [57]).
	audience: are there systematic differences in tail wagging behaviour when dogs are interacting with humans versus with conspecifics? How do receivers perceive different tail wagging parameters?	—systematically quantify dog tail wagging parameters in both intra- and inter-specific contexts (for an example of how differences in dog–dog and dog–human interactions can be investigated, see [108]) and investigate the effect on the receiver (human or conspecific, e.g. [22]).
evolution	phylogeny: which features of dog tail wagging are shared with other members of the class (mammals), suborder (caniforms) and family (canids)? Which features are more recent phylogenetic innovations?	—systematically quantify tail wagging parameters in a diversity of species and compare the resulting phylogeny of behavioural similarity (in terms of tail wagging) with a phylogeny of genetic relatedness (e.g. [109]).
	selection: is increased tail wagging in dogs a by-product of selection for other traits or was it directly selected? Is tail wagging correlated with other domestication traits (e.g. docility)?	—compare dog tail wagging with tail wagging in other domesticated mammals (e.g. pigs or cats) to shed light on whether tail wagging results from domestication in general (i.e. a general selection for tameness) or is specific to dog domestication. —quantify correlations between temperament traits such as docility, friendliness or deferentiality (as measured through temperament test batteries, e.g. [110]) and tail wagging parameters.
	perception: do humans and dogs have a preference for more rhythmically regular tail movements?	—test the perception of tail wagging parameters in humans and dogs (and ideally in non-human primates and other canids as well) through neuroimaging and physiological studies (e.g. expose both humans and dogs to tail wagging dogs and measure attention parameters such as eye fixations) [111–114]. —human studies should be conducted on diverse participants, given that age, cultural upbringing, and personal experience with dogs could all be confounding factors. —testing human populations living in close proximity to free-ranging dogs would be particularly interesting from an evolutionary perspective, since an unconscious positive selection for dogs wagging their tails, mediated by a potential preference for their rhythmicity, could still be taking place.