Getting into the brain of sharks

Check out the guest post our BEEF lab student Evan Byrnes wrote for Fish Thinkers.

Evan is studying personality and spatial learning in Port Jackson sharks and has recently published some of his work in Behavioural Brain Research.

Getting into the brain of sharks

You have probably heard the idea that some people are more left brained than right brained, or vice versa. This idea is called brain lateralization, and can influence how individuals think, learn, and behave.

Brain lateralization refers to the fact that the brain is made up of two different hemispheres, and that these hemispheres are not alike. In the human brain, each hemisphere specializes in processing different kinds of information. The left hemisphere is supposed to be better with languages and organizing sequential information, whereas the right brain is more is more creative and deals with visualizing objects in spaces, making music, and processing emotions. People often vary in how strongly lateralized their brain are, and this is often reflected in their behaviour. For example, people vary in their handedness or dominant eye use when viewing particular scenes. Until recently, brain lateralization was believed to be a characteristic unique to humans because of its close association with complex language. However, over the last decade it has become evident that a wide range of animals also have lateralized brains.

In a recent study, we learned that sharks also show differences in their brain lateralization. Similar to patterns observed in humans, Port Jackson sharks ranged from being strongly right lateralized to left lateralized and everything in between.

Evolutionarily speaking, the fact that individuals differ in brain lateralization has been quite perplexing puzzle for scientists to explain. Research so far has shown a few advantages of strong brain lateralization, such as the enhancement of cognitive abilities and social cohesion. Strongly lateralized parrots, for example, are better at problem solving1, and more strongly lateralized fishes show better cohesive movements during shoaling2. On the flip side, research has not identified any disadvantages of being lateralized, and presumably there should not be any real evolutionary advantage to being left brained or right brained. Therefore, based on the current evidence it is quite difficult to understand why evolutionary hasn’t selected for all animals to be strongly lateralized.

Laterality has previously been linked to a number of emotions in humans and animals, including fear and aggression. Baboons, for example, are more likely to attack individuals that approach from the left3, and dogs tend to wag their tail to the left when they feel threatened or sad4. Recently the idea was proposed that the link between laterality and these emotional states has put some sort of evolutionary constraint on laterality, causing large variation to be maintained.

Over the past year, our lab has been investigating this idea in one of the oldest evolutionary lineages of fishes, the elasmobranchs (sharks, rays, and chimaeras). We examined lateralization through turn preferences when swimming in a maze and during circular swimming within their home tank. Using these tests, we could observe which eye an individual preferred to use when exploring both a novel and familiar environment, and the strength of these preferences were used to calculate lateralization strength. We then went on to explore how observed lateralization differences may be linked to sharks’ reactions to a stressful situation. Sharks were handled and exposed to air for a short period of time, after which we recorded increases in activity rate and assigned stress reactivity scores. Our results showed that the laterality strength strongly associated with the manner that individuals responded to handling stress, such that stronger lateralized individuals were more reactive to stress.

Our results suggest that an increased stress response in lateralized sharks may help them escape from predators more quickly than weakly lateralized sharks. However, this enhanced stress response may also come at a cost. The same sharks that are adapted to escape predators and survive in the wild may be more susceptible to capture stress during fishing. While study provides some of the first evidence for brain lateralization in sharks, it is a small piece of the evolutionary puzzle in understanding how animal brains have evolved. It is important that we continue to study how individual differences in brain morphology and function influences individual behaviours and reactions to anthropogenic pressures.

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