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 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 variability and repeatability of behavioral activity 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 collective group behavior in daylight obscures the expression of individuality, whereas the weak group effects in nighttime reveal a substantially wider continuum of individually consistent activity types. Our findings imply that 1) studying activity variation only on diurnal basis may underestimate the total activity variation among social individuals and may thus bias the repeatability estimates, and 2) accounting for diel variation in social effects may be essential for detecting ecologically realistic behavioral variation within and between animal groups. To conclude, this study highlights the complex interactions between circadian activity rhythms, individual behavioral differences, and group dynamics, and thereby provides new insights for understanding overall behavioral diversity in social animals.