Kin-mediated plasticity in alternative reproductive tactics

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Conditional strategies occur when the fitness payoff an individual receives from expressing a given phenotype (from a range of two or more possible phenotypes) is contingent upon that individual’s environmental circumstances. This conditional strategy model underlies many cases of alternative reproductive tactics, in which individuals of one sex employ different means to obtain reproductive opportunities. How genetic relatedness and indirect fitness effects could affect the expression of alternative reproductive tactics remains unexplored. Here, we address this gap using the Acarid mite Rhizoglyphus echinopus, in which large males develop into aggressive ‘fighters’ and small males develop into non-aggressive ‘scramblers.’ Because fighters kill their rivals whereas scramblers do not, fighters should incur a greater indirect fitness cost when competing with their relatives, and thus the expression of the fighter phenotype could be reduced in the presences of relatives. To test this, we raised mites in full-sibling or mixed-sibship groups and found that fighters were more common in full-sibling groups, not less common as we predicted. This apparently counter-intuitive result could be explained if relatedness and cue variability are interpreted as a signal of population density, since fighters are more common at low densities in this species. Alternatively, our results may indicate that males compete more intensely with relatives than non-relatives in this species. Our study provides the first evidence of kin-mediated plasticity in the expression of alternative reproductive tactics.