Are flies kind to kin? The role of intra- and inter-sexual relatedness in mediating reproductive conflict

Abstract

As individual success often comes at the expense of others, interactions between the members of a species are frequently antagonistic, especially in the context of reproduction. In theory, this conflict may be reduced in magnitude when kin interact, as cooperative behaviour between relatives can result in increased inclusive fitness. Recent tests of the potential role of cooperative behaviour between brothers in Drosophila melanogaster have proved to be both exciting and controversial. We set out to replicate these experiments, which have profound implications for the study of kin selection and sexual conflict, and to expand upon them by also examining the potential role of kinship between males and females in reproductive interactions. While we did observe reduced fighting and courtship effort between competing brothers, contrary to previous studies we did not detect any fitness benefit to females as a result of the modification of male antagonistic behaviours. Furthermore, we did not observe any differential treatment of females by their brothers, as would be expected if the intensity of sexual conflict was mediated by kin selection. In the light of these results, we propose an alternative explanation for observed differences in male-male conflict and provide preliminary empirical support for this hypothesis.

Keywords: aggression; inclusive fitness; kin selection; sexual conflict; sexual selection; social behaviour.

Figure 1.

Boxplots showing distribution of daily fighting rates observed in vials of D. melanogaster containing one female and a trio of males, for treatments differing in the type of inter- and intra-sexual relatedness. In ‘all-related’ vials the males are related to each other and to the female, in ‘males-related’ vials the males are related to each other but not to the female, and in ‘all-unrelated’ vials all flies are from different familial lineages. While statistically significant, male–male relatedness is only a marginal predictor of overall fighting rates between treatments (Akaike information criterion (AIC) = 194.841, adjusted R² = 0.042). The box encloses values between the first and third quartiles of the data (the inter-quartile range, IQR), while the horizontal bar within the box indicates the median. Whiskers extend from the box to largest/smallest values that are within 1.5 × the IQR of the box. Values outside that range are outliers and are indicated by circles.

Figure 2.

Boxplots showing distribution of female longevities (in days) observed in vials of D. melanogaster containing one female and male(s), in treatments differing in the number of males and/or the type of inter- and intra-sexual relatedness. Vials in the ‘related-pair’ consist of a single male and female from the same familial lineage; while in the ‘unrelated-pair’ treatment, the flies are from different lineages. In ‘all-related’ vials the males are related to each other and to the female, in ‘males-related’ vials the males are related to each other but not to the female and in ‘all-unrelated’ vials the all flies are from different familial lineages. Shading of boxes indicates whether the male(s) in a vial are related to the female (white) or are unrelated (grey). Boxplot components are as described in figure 1.