Blakeslee writes a review (PDF here) of work by Vallortigara et al (PDF here) on emotional asymmetric tail wagging by dogs that is a further reflection of lateralized functions of the brain. Some edited clips from her article:
In most animals, including birds, fish and frogs, the left brain specializes in behaviors involving what the scientists call approach and energy enrichment. In humans, that means the left brain is associated with positive feelings, like love, a sense of attachment, a feeling of safety and calm. It is also associated with physiological markers, like a slow heart rate.
At a fundamental level, the right brain specializes in behaviors involving withdrawal and energy expenditure. In humans, these behaviors, like fleeing, are associated with feelings like fear and depression. Physiological signals include a rapid heart rate and the shutdown of the digestive system.
Because the left brain controls the right side of the body and the right brain controls the left side of the body, such asymmetries are usually manifest in opposite sides of the body. Thus many birds seek food with their right eye (left brain/nourishment) and watch for predators with their left eye (right brain/danger).
In humans, the muscles on the right side of the face tend to reflect happiness (left brain) whereas muscles on the left side of the face reflect unhappiness (right brain).
Dog tails are interesting...because they are in the midline of the dog’s body, neither left nor right. So do they show emotional asymmetry, or not?
Vallortigara et al show that when dogs were attracted to something, including a benign, approachable cat, their tails wagged right, and when they were fearful, their tails went left. It suggests that the muscles in the right side of the tail reflect positive emotions while the muscles in the left side express negative ones.
Brain asymmetry for approach and withdrawal seems to be an ancient trait..Thus it must confer some sort of survival advantage on organisms.
Animals that can do two important things at the same time, like eat and watch for predators, might be better off. And animals with two brain hemispheres could avoid duplication of function, making maximal use of neural tissue.
The asymmetry may also arise from how major nerves in the body connect up to the brain... Nerves that carry information from the skin, heart, liver, lungs and other internal organs are inherently asymmetrical, he said. Thus information from the body that prompts an animal to slow down, eat, relax and restore itself is biased toward the left brain. Information from the body that tells an animal to run, fight, breathe faster and look out for danger is biased toward the right brain.