Hamilton's rule predicts sibling cannibalism in a wild population of insects

Despite its seminal status in evolutionary biology, empirical studies that fully assess the different components of Hamilton's rule in natural systems remain limited. This is in part due to the difficulty of quantifying the costs and benefits to the actor and recipient of a given behaviour, compared with the relative ease of quantifying relatedness. Sibling cannibalism represents a rare opportunity where the costs and benefits can be more easily quantified, in particular, because the cannibalized individual achieves zero fitness. We show that parameterizing Hamilton's rule for sibling cannibalism by wild first instar Lygaeus creticus seed bugs predicts the observed cannibalism behaviour: conspecific eggs are cannibalized without regard to relatedness, but the large survival benefit of eating a conspecific egg outweighs any loss of inclusive fitness from eating kin. In addition, we show that L. creticus nymphs prefer to cannibalize conspecific eggs over heterospecific Spilostethus pandurus eggs but that this preference depends on hatching environment. This suggests a potential role of learning in the expression of cannibalism preferences, adding ecology to the evolutionary context of Hamilton's rule.