Behavioral lateralization in zebrafish and four related species of minnows (Osteichthyes: Cyprinidae)



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Lateralization occurs when the left and right sides of some portions of the body differ from each other either in structure or function. It was once believed that humans were the only animals to exhibit lateralization of the brain. However, research throughout the last few decades has clearly shown that lateralization is not unique to humans. In fact, many vertebrates are known to exhibit lateralization of a type that is similar to humans.

In a monocular test, the subject is exposed to a stimulus and the eye that it uses to inspect the stimulus is recorded. The binary data obtained from these tests is often pooled among individuals of the same species in order to test hypotheses of species-level behavioral lateralization. Hypotheses of individual level behavioral lateralization are rarely tested. Therefore, I examined nine different statistical tests that are capable of testing binary data, which is commonly recorded in these types of behavioral tests. Using the p-value as an indication of the effectiveness of a test to show departures from randomness when such departures actually exist, and considering the growing necessity for providing confidence intervals associated with a given test, I found the score test to be the most satisfactory.

Behavioral lateralization has been observed in many species of fishes during stimulus specific tasks. However, one area that has been overlooked is the study of naïve side bias in motor behavior of fishes in the absence of any direct stimulus. Therefore, I also examined naïve side biases in motor behavior in five species of minnows (Osteichthyes: Cyprinidae). Fifteen individuals of each species were subjected to a ‘T’ shaped test arena, with 40 replicates/individual. This experiment took advantage of rheotaxis by running a slow current of water through each arm of the test apparatus. A total of 55 of 75 individuals tested exhibited a rightward turning preference. The significantly right-biased behavior observed in these fishes in the absence of any systematic stimulus suggests that it is imperative in future studies of laterality in fishes, and presumably in other organisms, that a stimulus-free control condition be included in the experimental design.