When the penny drops: sharks outsmart cichlids in serial reversal learning

Two species of fish were tested in a serial reversal learning task using two dimensional objects varying in colour and/or form. After reaching the learning criterion (LC) in the original discrim-ination task, individuals reversed for up to ten times, i.e., stimulus association was switched and animals rewarded for choosing the stimulus that had not been rewarded in the previous learning phase. This was conducted using the two stimuli green diamond (S+) vs. red square (S-) for the training of eight cichlids, and black star (S+) vs. black circle (S-) for the training of seven bamboo sharks. All fifteen animals completed the initial training phase and reversed successfully at least three times, indicating an ability to forsake a previously learned stimulus association in favour of a new one. Sharks, but not cichlids, also showed a decrease in the number of sessions needed to reach LC with an increase in the number of reversals conducted, showing that some sort of learning strategy, i.e., rule learning, had been applied. Animals that improve their performance within such learning paradigms are considered to possess more behavioural flexibility and advanced cognitive abilities than those who do not. More flexibility could specifically aid species living in complex habitats, social groups and subjected to changing environmental conditions.