Night reveals individuality in a shoaling fish

Many animals rely heavily on visual cues from their environment, and therefore show circadian rhythmicity in their behavioral activities. In group-living animals, individuals’ activity rhythms must be synchronized not only with diel light-dark cycle but also with other group members. Increasing evidence has recently shown that circadian behaviors of animals are consistent within individuals and different between individuals, but the sources and consistency of diel activity variation in social context are less known. Using radio frequency identification (RFID) technology, we recorded individual moving activity of the Eurasian perch (Perca fluviatilis) in randomly formed groups through 10 full diel cycles in seminatural environment. We found diel changes in trait variability and repeatability estimates both within and between the groups: individual differences in activity were more repeatable in nighttime whereas group differences were more repeatable in daytime. The results suggest that ...