Inclusive fitness benefits of scent-marking predators

Because relatedness is high and dispersal is limited, one would expect to see higher levels of altruistic behaviour among clonal organisms than among animals of lesser relatedness. Enigmatically, however, parthenogenetic aphids do not emit alarm signals when a predator first enters a colony but only after being captured. Here, we report that an aphid smearing alarm pheromone directly onto a predator decreases the predation risk for clone-mates as the predator continues to search for additional prey. Adult multicoloured Asian ladybird beetles, Harmonia axyridis, daubed with alarm pheromone caused a greater proportion of aphids to drop off a plant and escape predation than did predators lacking pheromone droplets. Thus, along with direct fitness benefits of individual protection, aphid alarm signalling behaviour may have evolved through inclusive fitness benefits of protecting clone-mates by scent-marking predators.