Parental care and competition with microbes in carrion beetles: a study of ecological adaptation

• We examined competitive adaptations to microbial challenge in two silphid species.
• The species lacking parental care had superior performance on aged carcasses.
• Antimicrobial treatment of a carcass did not affect reproductive output.
• A high level of antimicrobial function did not depend on parental care.
• A small carcass is a useful model for comparative methods of ecological function.

The investigation of the function of a complex adaptation can benefit from multiple approaches. We used comparative study and physical simulation to re-examine the hypothesis that parental adaptations enable carrion beetles (Silphidae) to better compete with carrion microbes. The comparative study used two closely related species, Nicrophorus orbicollis and Ptomascopus morio, which differ markedly in their level of care. Reproductive performance was measured using carcasses aged on field soil for 0, 3, 6 or 9 days. As expected, N. orbicollis converted more carrion biomass to offspring biomass than did P. morio (27.8% versus 16.6% on fresh carcasses) and both species produced smaller broods on aged carcasses. There was a negative interaction, however, between the level of care and carcass age on reproductive performance; the parental care advantage exhibited by N. orbicollis was less on older carcasses than on fresh carcasses, and was negligible on the oldest (6- and 9-day) carcasses. Ptomascopus morio also demonstrated superior ability to regulate the size of offspring when using carcasses of various ages. In a second experiment, we simulated antimicrobial activity of Nicrophorus anal exudates. A 6-day pretreatment with an antibacterial, an antifungal, or a combination treatment did not result in larger broods relative to those on untreated 6-day carcasses. The results call into question the hypothesis that preparation of the carcass by Nicrophorus evolved primarily as an adaptation for preventing microbial deterioration of carrion. We suggest that selection to compete with microbes may be relaxed in carrion beetles that provide elaborate parental care. A small vertebrate carcass is a potential model system for applying the comparative method to ecological function.