LAUSR

The killing of conspecifics returns indirect benefits when killers eliminate competitors for resources, and direct benefits when cannibals obtain nutrients directly after killing. Cannibals risk pathogen infection and both killers and cannibals risk reducing their inclusive fitness when killing kin. When competing with kin over food during a prolonged period of starvation, we surmise that this cost–benefit ratio will stay almost constant for killers, as their benefits are indirect so that they direct killing away from kin (unless in extremis), but that this ratio will reduce over time for cannibals, as their benefits are direct, such that they will start consuming kin when food stressed. We tested this hypothesis using the male-dimorphic bulb mite Rhizoglyphus robini in which males are armed ‘fighters’ that can kill and cannibalize or unarmed ‘scramblers’. In four-fighter groups that were either starved or fed for 2 weeks, and that comprised either kin (brothers) or nonkin (at most cousins), we assessed the number of dead and cannibalized fighters. The cumulative number of dead and cannibalized fighters was higher under starved than under fed conditions, and increased over time, rejecting our hypothesis. Instead, we found that fewer kin than nonkin were cannibalized, but fighters discriminated to a much lesser extent between kin and nonkin when killing. Hypothetically, cannibalism of kin incurs a greater risk of infection by pathogens to which cannibals are genetically susceptible. Our results highlight the importance of distinguishing killing from cannibalism, as they can have contrasting effects on the demography of populations.