Multiple mating predicts intensity but not mechanism of kin recognition

Understanding how animals recognize their kin has been a major challenge in biology. Most animals use one of 2 mechanisms: "familiarity" whereby kin are remembered from interactions early in life, such as in a nest, or "phenotype matching" whereby putative kin are compared with a template of what kin should look, smell, or sound like. Cross-species studies suggest that there is a link between which of these 2 mechanisms are used and the degree of female promiscuity (multiple mating). Phenotype matching is more likely to be used by promiscuous species because these species have lower average brood relatedness than monogamous species and familiarity is thus an unreliable cue of relatedness. However, it is unclear if this relationship holds within species, across populations that differ in their degree of promiscuity. Here, we take advantage of variation in brood relatedness across populations of guppies (Poecilia reticulata) to examine the relationship between kin recognition mechanisms and multiple mating within a single species. Contrary to the established hypothesis, we show that variation in recognition mechanism across populations is not governed by multiple mating. Instead, our data show that kin recognition, quantified as association preferences for shoalmates, is strongest when brood relatedness is high, consistent with Hamilton’s rule, but multiple mating does not otherwise influence the specific recognition mechanism used.