The rhythm of the night: Assessing temporal variation of activity patterns in arhythmic environments from field data

Ecologists and zoologists have worked for centuries to understand the spreading, extent, and regulation of biological rhythms among eukaryotes living in arhythmic ecosystems. Investigating whether the long-standing hypothesis that biological clocks went lost in species inhabiting arhythmic environments can thus have broad implications in assessing evolutionary and ecological pathways allowing colonization of new and physiologically challenging environments. Subterranean habitats generally lack temporal variation in environmental conditions. However, chronobiological research in such communities remains hindered by methodological, ethical, and logistical constraints, particularly for field-based studies. Here, we propose a flexible, noninvasive framework to assess temporal variation in the activity of animals using detection probability as a proxy for biological rhythms. Our approach is based on Bayesian occupancy and abundance models, which can incorporate quantitative (count), qualitative (presence/absence), or semiquantitative (abundance classes) data derived from repeated surveys. This framework is adaptable to a variety of taxa, monitoring strategies, and environmental conditions. It is particularly well-suited for sensitive ecosystems, as it minimizes disturbance and allows for standardized, repeatable analysis of activity patterns. We illustrate the utility of the framework through three case studies (Proteus anguinus Laurenti 1768, Niphargus sp. Schiødte 1849, and Spelaeomysis bottazzii Caroli 1924) encompassing different data types and showing its potential to detect rhythmicity even in stable subterranean environments. By facilitating robust in situ assessments of temporal activity, our method provides a novel and accessible tool for exploring biological rhythms in arhythmic environments, and can thus support the conservation of highly specialized, often vulnerable, fauna.