Comparison of daily activity rhythms in individual workers of five Japanese ant species

Circadian rhythm is an endogenous 24 h variation in physiological processes and behaviors. It has evolved in the complexity of natural environments and enables organisms to adapt their behavior to the day and night cycle. To better understand the evolution of circadian rhythms, it is important to clarify the relationship between the morphological / ecological characteristics and phenotypes of daily rhythms. Daily rhythms in social insects are affected by social factors, such as tasks and interactions with individuals. However, data on daily rhythms of ants are available for a relatively small number of ant species. The purpose of this study was to understand how three morphological / ecological features affect the social rhythms of ants: 1) presence-absence / extent of morphological diversity, 2) nest types (underground, arboreal, or nomadic), and 3) feeding habits. We measured locomotor activity rhythms of isolated ants in five Japanese ant species under both light-dark and constant dark conditions. Under the constant dark conditions, workers did not show clear daily rhythms. No significant differences in the strength of daily rhythms were observed between the morphological castes of Camponotus vitiosus. The nomadic ants Pristomyrmex punctatus displayed clear daily rhythms with bimodal activity peaks under light-dark condition. Feeding habitats did not explain the activity patterns of foragers. In addition, we found that Formica japonica callow workers showed clear daily rhythms under light-dark conditions, suggesting the early development of circadian rhythms. This study provides data on individual daily activity rhythms in five new ant species, which further sheds light on the evolution of daily rhythms in ants.