By Maneka Gandhi
We humans have always considered ourselves different from every other species on Earth. But, as time goes on, scientists repeatedly show that the traits we consider unique and based on a superior intelligence are found in most animals as well. Tool using, for instance, is done by insects, fish, crows and apes.
One of these is the trait of laterality. The term laterality refers to the primary use of the left or right hemispheres of the brain. The two halves of the animal brain are not exactly alike, and each hemisphere differs in function. The left hemisphere controls the right side of the body and the right hemisphere controls the left side.
Most people are right handed / right sided because it is thought that the left hemisphere makes the right side stronger. Also, in 90-92% of all humans, the left hemisphere controls language.
Are animals any different?
Despite the varied structures of animal brains, many mammals, birds, fish and invertebrates have the same trait of laterality. Many animals use their left eye and left ear (indicating right brain activation) more often than the right ones when investigating objects that are potentially frightening.
Left-handed people are more vulnerable to stress, as are left pawed dogs and many other animals.
Right-handed animals have better immunity.
A 1987 study by leading animal behaviour scientists, MacNeilage, Studdert-Kennedy and Lindblom, showed that macaque monkeys have a left-hand preference. For primates in general, a left-hand preference was observed for visually guided reaching movements and a right-hand preference for manipulating objects, acts that required dexterity.
In prosimians (non-ape primates like bush-babies, lemurs, etc.), the right hand is used for major tasks, like grasping a branch after a leap, and the left is used for quick small movements, like catching insects.
Young horses prefer breathing with their right nostrils. Horses with a higher emotional content prefer looking with their left eye! Recent research in Applied Animal Behaviour Science, though, showed that about 53% of horses prefer to lead with the right leg, 40% with the left and 7% showed no preference. Racehorses use the same leg in their stride unless they are forced to change it while turning, injured, or fatigued.
Dogs prefer one paw to another – except when they are frightened by noise (Bransonn and Rogers, 2006). Trainers of guide dogs, in fact, test them for laterality, as the dog may be better at walking to the left, or the right, of their blind owner.
Cattle prefer viewing new things with their left eye (as do magpies, chicks, toads and fish) and familiar things with their right eye.
Bats show a left-hand bias for climbing or grasping.
Kangaroos use their left hand for things that require fine manipulation, but the right hand for behaviour that requires physical strength.
Most parrots favour one foot when grasping objects, usually the left.
Rainbow fish were made to examine their reflection in a mirror. Fish which looked with their right eye swam on the left side of the group. Conversely, fish that looked with their left eye swam on the right.
The Zebra finch male views the female with his right eye when courting her (Workman & Andrew, 1986).
Emei Music frogs listen to positive signals (like a mating call) with their right ear, and with their left to negative signals such as predatory attacks.
Humpback whales exhibit a 75% predilection for slapping their right flukes on the water surface.
Chickens show a right foot preference in scratching the ground in their search for food. This is coupled with a dominance of the right eye in tasks requiring them to perform visual discrimination learning, such as a search for food. Studies in the 1980s, by scientists Andrew, Mench, Rainey, Zappia and Rogers, have indicated that the right eye of a chicken learns to tell the difference between food grains and small pebbles faster than the left eye. On the other hand, the left eye is used for control of attack and copulation responses. Pigeons have similar right eye dominance, but since they do not use their feet to scratch the ground for food, they have not developed a complementary right footedness.
Even invertebrates, like snails, show laterality. Their shells spiral in either a clockwise or anticlockwise direction. This asymmetry, called ‘chirality’, means that snails can only mate with matching snails.
The presence of laterality in the brains of birds is particularly interesting, as humans had always believed that it could only be possible if there was a corpus callosum in the brain. Avian brains do not have a corpus callosum at all, but still display laterality.
In fact, there is similarity between the avian and human brain when it comes to laterality. The songs of the song-bird species is also controlled by the left hemisphere. There is also similarity in the syntax of bird songs and human languages. Birds go one better, however, as the brains of birds have both hemispheres working to control singing/language, while humans only possess this ability in one hemisphere.
In the same way, as we humans can open a door with either hand equally, yet struggle to write legibly with our non-dominant hand, complexity of the task appears to be an important factor in the handedness of animals. Animals that do activities requiring significant levels of dexterity, show evidence of a preferred “handedness”.
Italian researchers, Quaranta, Siniscalchi and Vallortigara, found that dogs wag their tails to the right when they see positive stimuli which they want to approach, and they wag to the left when confronted with something they would like to avoid. This suggests that, just as for people, the right and left halves of the brain do different jobs in controlling emotions.
There is some evidence to suggest that dogs and cats can be right-or left-pawed, although the ratio seems to be more evenly split than in humans. In one study 46% cats used only the right paw in reaching for food, 44% the left, and 10% were ambilateral. One study indicates that laterality in this species is strongly related to temperament. Individuals with stronger paw preferences are rated as more confident, affectionate, active, and friendly. A study at Queen’s University Belfast went further, showing that preferences of left/right are based on gender, with 95% of female cats favouring the right paw and 95% of male cats favouring the left. “There is some suggestion that limb preference might be a useful indicator of vulnerability to stress. Ambilateral animals with no preference for one side or the other, and those that are more inclined to left-limb dominance, for example, seem more flighty and susceptible to poor welfare than those who lean more heavily towards right limb use,” says Dr Wells of QU, Belfast. She adds: “We have discovered that left-limbed dogs, for example, are more pessimistic in their outlook than right-limbed dogs. From a pet owner’s perspective, it might be useful to know if an animal is left or right limb dominant, as it may help them gauge how vulnerable that individual is to stressful situations.”
Determining which side of the brain dominates the other could change the way domestic animals are bred, raised, and used, including predicting which puppies will make the best service dogs, and which racehorses will race better on left or right curving tracks.
So, another myth of man’s superiority bites the dust.
(To join the animal welfare movement contact firstname.lastname@example.org, www.peopleforanimalsindia.org)