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Conditional strategies occur when the relative fitness pay-off from expressing a given phenotype is contingent upon environmental circumstances. This conditional strategy model underlies cases of alternative reproductive tactics, in which individuals of one sex employ different means to obtain reproduction. How kin structure affects the expression of alternative reproductive tactics remains unexplored. We address this using the mite Rhizoglyphus echinopus, in which large males develop into aggressive 'fighters' and small males develop into non-aggressive 'scramblers.' Because only fighters kill their rivals, they should incur a greater indirect fitness cost when competing with their relatives, and thus fighter expression could be reduced in the presence of relatives. We raised mites in full-sibling or mixed-sibship groups and found that fighters were more common at higher body weights in full-sibling groups, not less common as we predicted (small individuals were almost exclusively scramblers in both treatments). This result could be explained if relatedness and cue variability are interpreted signals 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 in this species. We provide the first evidence of kin-mediated plasticity in the expression of alternative reproductive tactics. [ABSTRACT FROM AUTHOR]

Males of many species adjust their reproductive investment to the number of rivals present simultaneously. However, few studies have investigated whether males sum previous encounters with rivals, and the total level of competition has never been explicitly separated from social familiarity. Social familiarity can be an important component of kin recognition and has been suggested as a cue that males use to avoid harming females when competing with relatives. Previous work has succeeded in independently manipulating social familiarity and relatedness among rivals, but experimental manipulations of familiarity are confounded with manipulations of the total number of rivals that males encounter. Using the seed beetle Callosobruchus maculatus, we manipulated three factors: familiarity among rival males, the maximum number of rivals encountered simultaneously and the total number of rivals encountered over a 48 h period. Males produced smaller ejaculates when exposed to more rivals in total, regardless of the maximum number of rivals they encountered simultaneously. Males did not respond to familiarity. Our results demonstrate that males of this species can sum the number of rivals encountered over separate days, and therefore the confounding of familiarity with the total level of competition in previous studies should not be ignored. [ABSTRACT FROM AUTHOR]

Sexual conflict occurs when reproductive partners have different fitness optima, and can lead to the evolution of traits in one sex that inflict fitness costs on the opposite sex. Recently, it has been proposed that antagonism by males towards females should be reduced when they compete with relatives, because reducing the future productivity of a female would result in an indirect fitness cost for a harmful male. We tested this prediction in the seed beetle Callosobruchus maculatus, the males of which harm females with genital spines and pre-copulatory harassment. We compared lifespan, lifetime egg production and lifetime offspring production among females housed with groups of males that varied in their familiarity and relatedness. Females produced significantly more eggs and offspring when grouped with males who were both related and familiar to each other. Therewas no effect of male relatedness or familiarity on female lifespan. Our results suggest that males plastically adjust their harmfulness towards females in response to changes in inclusive fitness payoffs, and that in this species both genetic relatedness and social familiarity mediate this effect. [ABSTRACT FROM AUTHOR]