Fearless fish use both sides of their brain

Wild-born black-lined rainbowfish (Melanotaenia nigrans) are bolder than their captive counterparts, and those that analyse information with both sides of their brains are also more fearless.

This is the conclusion of a study by Macquarie University researchers Drs Culum Brown and Anne-Laurence Bibost, who first compared wild rainbowfish and captive rainbowfish.

They were not surprised that wild fish were significantly bolder than captive-reared fish, as previous work they had done showed that populations hunted by predators were braver than those from low-predation areas.

The researchers then assessed the fish for lateralisation. The preference to analyse and react to information with either the left or right hemisphere of the brain is called cerebral lateralisation and is widespread among vertebrates.

Lateralisation is commonly seen in hand or turn preferences. For example, the majority of humans are right-handed.

The researchers used a modified version of the mirror test to determine whether a fish showed a lateral preference to view itself with its left or right eye. Levels of boldness were tested by timing how long it took a fish to emerge from a safe hiding place.

Non-lateralised fish (those that used both eyes to view their mirror image and thus that did not analyse information in a specific brain hemisphere) were significantly bolder than left- and right-lateralised fish. This suggests that fear is heightened when primarily processed by a single hemisphere, making lateralised fish less bold.

Previous studies have shown that complex tasks are more difficult to perform when information processing is shared between two hemispheres. It therefore boils down to a question of speed.

A non-lateralised fish in a potentially life-threatening situation must first draw information from both hemispheres, and compare and integrate it before it can make a decision. Strongly lateralised fish, on the other hand, act more quickly because they only draw on information from a single hemisphere.

If non-lateralised fish process fear-related stimuli comparatively slowly or less efficiently than lateralised fish, it may be that the moderating effect of fear is somewhat lessened in comparison to strongly lateralised fish.

The researchers propose this may result in a reduced level of fear generally, or perhaps the decision to explore is already made before the moderating effect of fear comes into play. Either scenario would explain their observation that non-lateralised fish are bolder than lateralised fish.

“The similarities between personality and laterality are certainly intriguing and hint at a single underlying mechanism,” said Brown. “We suggest that these aspects of personality traits are actually caused by variation in laterality.”

The study was recently published in Behavioral Ecology and Sociobiology