The Good, the Bad, and the Mites: Unraveling the relationship between burying beetles and phoretic mites

Abstract

Interspecific interactions can shift along a spectrum from mutualism to antagonism depending on ecological context, but the mechanisms and consequences of these shifts remain poorly understood. The relationship between burying beetles (Nicrophorus orbicollis) and their phoretic mites (Poecilochirus spp.) exemplifies this complexity. Despite a co-evolutionary history spanning millions of years and decades of research, whether these mites are threats, harmless passengers, or beneficial partners remains unresolved, with evidence to date suggesting a highly context-dependent relationship. I investigated how environmental factors influence the beetle-mite relationship using competitive trials in which two same-sex but size-mismatched beetles competed for a mouse carcass – an essential reproductive resource. I first ran baseline trials without mites to confirm that, under standard conditions, the larger individual typically wins, and to determine the size difference where the smaller beetle has a non-zero probability of winning. I then ran trials in which one beetle carried a typical pre-breeding mite load, alongside controls where neither beetle carried mites, at two temperatures (15°C or 25°C). Mites affected the outcome of these competitive interactions by preferentially transferring to the larger beetle and reducing their competitive success, while temperature had no detectable effect. In a complementary study designed to assess whether the beetles’ innate response to mites is tolerant or aversive, I conducted a behavioural assay, and found that beetles exposed to mites engaged more in self-grooming and escape behaviours relative to controls, suggesting an evolved aversive response. Together, these results suggest that Poecilochirus mites impose a net fitness cost on N. orbicollis, despite occasional context-dependent benefits. My research adds to the literature by demonstrating that even long-standing associations can shift along the mutualism-antagonism spectrum depending on context and illustrating the value of a simple behavioural assay to evaluate the net fitness effects of interspecific interactions over evolutionary timespans.