A social switch: daylength drives meadow vole group dynamics in automatically tracked habitats

Sociality has evolved on numerous occasions, with important fitness benefits and consequences for individuals in group-living species. Despite the prevalence of group living, it remains challenging to study the proximate factors that promote variation in this behaviour. Meadow voles, Microtus pennsylvanicus , vary in social organization in response to seasonal cues in the wild, providing a remarkable opportunity to study mechanisms that support changing grouping behaviour within a single species. In summer, female meadow voles are territorial and maintain exclusive home ranges, while in winter, they live in mixed-sex groups. Laboratory manipulations of photoperiod reproduce seasonal variation in social behaviour in this species, demonstrated by altered short-term dyadic interactions during behavioural tests. To assess changes in group social dynamics, we implemented an automated tracking system using radiofrequency identification technology to monitor free-moving voles housed in long daylengths and short daylengths in multichambered habitats over extended intervals. We developed open-source software to model each animal's position within the habitat and to describe co-occupancy patterns. Photoperiod drove robust and pronounced changes in group social behaviour, recapitulating natural seasonal transitions in the wild. Voles housed in short, winter-like days spent more time in groups, formed larger, more evenly mixed groups and shared a single home-chamber more often than long-day voles. Short-day voles were more active than long-day voles but rested in groups, thus cohabitation was strongly correlated with ultradian rhythms in activity. These metrics of group dynamics will allow more nuanced assessment of the effects of manipulations on social behaviour in voles and other species, as both habitat and software are adaptable for use with other rodents. • Voles in winter-like, short daylengths (vs long daylengths) spent more time in groups. • Voles in short daylengths also gathered in groups of larger size. • Photoperiod was sufficient to reproduce seasonal changes in grouping behaviour. • Tracking group dynamics improves study of social behaviour in group-living species.