Daily activity patterns and circadian rhythm in a nicrophorine burying beetle (Nicrophorus marginatus)

Circadian rhythms are endogenous mechanisms responsible for the onset and termination of many important physiological processes related to energy allocation, and are thought to evolve to maximize fitness in a particular biotic and abiotic context.  These rhythms and the related daily activity regime of an organism are often the phenotypical expression of interspecific niche competition, and this expression is particularly interesting in species that compete for hyperephemeral resource such as carrion. Here we examine the daily activity and circadian patterns in a Nicrophorine burying beetle species, Nicrophorus marginatus, which is often found in sympatry with multiple congeners and is one of few in its clade that exhibits diurnality. We tested the hypothesis that this species would segregate activity to periods associated with lower risk of dessication and predation by avian predators, and that activity phenotype would be governed by circadian regulatory mechanisms rather than external cues. We recorded daily activity patterns in a 12:12 LD condition using a Trikinetics digital locomotion monitor, and found that N. marginatus exhibit a strongly crepuscular activity pattern, with females displaying greater overall activity than males during light but not dark cycles. We then recorded activity in dark-only conditions, and show that the crepuscular activity patterns persisted in the absence of external light cues, suggesting an endogenous mechanism of activity regulation. Crepuscular activity in the species may have evolved as a compromise between intrinsic physiological constraints associated with desiccation and extrinsic predation and competition risks.