Why do burying beetles bury? Competition and the fitness benefits of parental care.

Parental care increases the fitness of current offspring at a cost to their parents, compromising parental resources for future reproduction. Given this inherent conflict and cost, complex parental care is relatively uncommon. Reliance on a rare and critical resource for reproduction that necessitates intense competition and defence could be a key reason for the evolution of costly and diverse parental care behaviors. Burying beetles (genus Nicrophorus) rely on carrion for reproduction, and have a suite of parental care behaviors, including the burying of small carcasses within underground breeding chambers. Carcass burial is energetically costly, but may confer fitness benefits to beetles by creating a safe, controlled environment for rearing offspring and securing the carrion. Yet, no study has directly tested whether carcass burial is adaptive, and if so, the selective pressures favoring this trait. Here, we manipulated the ability of beetle parents to bury carrion in nature to see the effect on reproductive success. Preventing beetles from burying led to complete reproductive failure, primarily due to competition with other invertebrates and vertebrates. Results align with previous ideas that competition played an important role in the evolution of carrion burial in Nicrophorus burying beetles, but do not exclude a role of other selective pressures (eg, microbial competition, microclimate). Competition is likely an important selective pressure driving the evolution of parental care, especially for species where intense competition for rare and critical resources persists throughout the growth and development of offspring, impacting the likelihood of reproductive failure.