Kinship as a double-edged sword: relatedness among burying beetle larvae enhances growth but increases mortality.

Theoretical and empirical considerations suggest that relatedness can have complex effects on social life. While high relatedness may promote sibling cooperation and altruism through indirect fitness benefits, it can also intensify competition if siblings share similar needs and competitive strategies. Moreover, low genetic diversity in highly related groups may heighten susceptibility to pathogens. Hence, due to these potential opposing effects, the consequences of relatedness for offspring fitness within a family context are not fully understood. Here, we investigated how relatedness among interacting offspring influences their fitness in the burying beetle Nicrophorus vespilloides, a species exhibiting facultative parental care, with larvae developing in a microbially rich and challenging environment. To assess offspring effects without parental influence, we raised larvae in the absence of care, thereby eliminating parental buffering and exposing them to a more stressful environment. We compared the growth and survival rates of broods consisting of full siblings and broods with unrelated larvae and found both benefits and costs of relatedness. Larvae gained weight more rapidly in the early stages when surrounded by siblings but suffered higher mortality later in development. These findings suggest that high relatedness facilitates cooperative effects but comes at a cost, potentially reducing social immunocompetence.