Beyond bold versus shy: Zebrafish exploratory behavior falls into several behavioral clusters and is influenced by strain and sex

Individual differences in exploratory behavior have been found across a
range of taxa and are thought to contribute to evolutionary fitness.
Animals that explore more of a novel environment and visit areas of high
predation risk are considered bold, whereas animals with the opposite
behavioral pattern are shy. Here, we determined whether this bimodal
characterization of bold versus shy adequately captures the breadth of
behavioral variation in zebrafish or if there are more than these two
subtypes. To identify behavioral categories, we applied unsupervised
machine to three-dimensional swim traces from over 400 adult zebrafish
across four strains (AB, TL, TU, and WIK) and both sexes. We found that
behavior stratified into four distinct clusters: previously described bold
and shy behavior and two new behavioral types we call wall-huggers and
active explorers. Clusters were stable across time and influenced by
strain and sex where we found that TLs were shy, female TU fish were bold,
male TU fish were active explorers, and male ABs were wall-huggers. Our
work suggests that zebrafish exploratory behavior has greater complexity
than previously recognized and lays the groundwork for the use of
zebrafish in understanding the biological basis of individual differences
in behavior.