Joint effects of brood size and resource availability on sibling competition

The evolution of sibling competition is promoted when the brood's demand for resources (brood size) exceeds the parents' supply of resources (resource availability). However, little is known about the joint effects of brood size and resource availability and whether these effects are independent of each other. We conducted a study on the burying beetle Nicrophorus vespilloides, in which we manipulated both brood size and resource availability. We manipulated brood size by providing parents with 5, 10 or 20 larvae and resource availability by providing parents with a 5, 10 or 20 g mouse carcass. We found that resource availability had positive effects on parental care, larval body mass and larval survival, while brood size had a negative effect on larval body mass and larval survival. There were positive effects of the interaction between brood size and resource availability on larval begging and larval body mass, suggesting that the slopes describing the effect of brood size on larval begging and larval body mass became less negative as carcass size increased. When we repeated the analysis using larval density (i.e. brood size/resource availability) as a proxy for the shortage of resources, there were negative effects on parental care, larval body mass and larval survival. Our results have important implications by showing that there were main effects of both brood size and resource availability, and that their effects were not always independent of each other. Thus, treating brood size and resource availability as independent factors is preferential to using offspring density.