John Hale, of Dunedin, asks :-

I have a hunch that our dog wags her tail from right to left, starting with a wag to her right. Is there any evidence of right-handedness or left-handedness in animals which don’t have hands? And those which do?

Ngaio Beausoleil, a veterinarian at Massey University, responded.

Indeed your hunch is correct – there is scientific evidence to support asymmetry in dog tail wagging as well as asymmetrical behaviour of many other sorts in many other animals. This phenomenon is termed 'lateralization' or 'laterality' and can occur at the level of the individual and population. Lateralized behaviour is associated with specialization of the left or right hemisphere of the brain for a particular task, e.g. a structure on one side of the brain may be specialized for threat detection or social communication.

Most humans show a strong tendency to favour one hand or limb when performing certain activities e.g. most people show a strong preference to use one hand for writing – this is individual laterality. Population level laterality refers to a bias in the distribution of individuals with a particular laterality, e.g. most people are right-handed for writing. There is some evidence that population level biases occur more often in social species when it is beneficial for behaviour to be coordinated.

There is evidence of lateralized behaviour and asymmetry in the structure and function of the nervous system in a wide range of non-human species, including other mammals, birds, fish, reptiles, amphibians and even invertebrates. Laterality is evident in naturally expressed behaviours, such as predator inspection and avoidance, prey capture, food manipulation, movement and social interactions. It is also observed in experimentally-induced behaviours such as negotiation of barriers and in mazes or choice tests.

In dogs and other domestic mammals, lateralization of important social behaviours such as tail wagging and facial expression has recently been described and seems to reflect different emotional states or underlying motivations. For example, the left eyebrow moved more than the right when dogs were reunited with their owners but not when introduced to an unfamiliar person. Interestingly, a different study reported a right-sided bias in the amplitude of tail wagging when dogs could see their owners. A similar right-bias was seen with an unfamiliar human but the wagging was less pronounced. Other dogs appear to perceive and respond differently to left- or right-biased wagging, suggesting that the asymmetry in this behaviour has a function in social communication.

Many behavioural asymmetries are stable individual traits and are likely to confer an advantage to the individual or group in terms of improving behavioural efficiency, social communication or cohesion or reducing the work of the nervous system.