Resolving the mechanisms that switch competition to cooperation is key to understand biological organization1. This is particularly relevant for intrasexual competition, which often leads to males harming females2. Recent theory proposes that kin selection may modulate female harm by relaxing competition among relatives3-5. We experimentally manipulated the relatedness of groups of male Drosophila melanogaster competing over females to demonstrate that, as expected, within group relatedness inhibits male competition and female harm. Females exposed to three brothers unrelated to the female had higher lifetime reproductive success and slower reproductive ageing compared to females exposed to triplets of males unrelated to each other. Triplets of brothers also fought less with each other, courted females less intensively and lived longer than triplets of unrelated males. However, associations among brothers may be vulnerable to invasion by minorities of unrelated males: when two brothers were matched with an unrelated male, the latter sired on average twice as many offspring as either brother. These results demonstrate that relatedness can profoundly affect fitness through its modulation of intrasexual competition, as flies plastically adjust sexual behaviour in a way consistent with kin selection theory.We first tested the effect of relatedness of males within a group on female fitness, by quantifying different aspects of fitness and life-history (experiment 1) in females exposed to male triplets. Males were unrelated to the female and either full-sib brothers of each other (AAA) or unrelated to each other (ABC), and were replaced weekly until female death. Consistent with expectations3-5, we found that females exposed to AAA-males had significantly higher lifetime reproductive success than females exposed to ABC-males (Fig. 1a). This was due to the fact that while total female lifespan did not differ on average between treatments (F 1, 119 = 1.66, P = 0.2), females exposed to AAA-males had significantly longer reproductive lifespan (from eclosion to last egg-laying day6, Fig. 1b), and female reproductive lifespan was positively correlated with female lifetime reproductive success (F 1, 117 = 484.59, P < 0.001). Two non-mutually exclusive mechanisms might cause this. First, high-fecundity females might die faster when exposed to ABC-males, leading to an average higher productivity of AAA-replicates ('selective death'). Second, individual Author contributions. Expt.1 was designed by P.C., S.W. and T.P., conducted by P.C. and F.A. and analysed by P.C. Expt. 2 was designed by P.C., C.K.W.T., S.W. and T.P., and conducted/analysed by P.C. Expt. 3 was designed and conducted by S.W. and P.C. and analysed by P.C. Expt. 4 was designed by C.K.W.T., T.P. and S.W., and conducted/analysed by C.K.W.T. The article was conceived and written by T.P. with input from P.C., C.K.W.T. and S.W. Europe PMC Funders Author Manuscripts females might suffer a steeper rate of age-dependent decline in reproductive output when exposed...
Hybridisation is increasingly recognised as an important cause of diversification and adaptation. Here, we show how divergence in male secondary sexual characters between two lineages of the common wall lizard (Podarcis muralis) gives rise to strong asymmetries in male competitive ability and mating success, resulting in asymmetric hybridisation upon secondary contact. Combined with no negative effects of hybridisation on survival or reproductive characters in F1-hybrids, these results suggest that introgression should be asymmetric, resulting in the displacement of sexual characters of the sub-dominant lineage. This prediction was confirmed in two types of secondary contact, across a natural contact zone and in two introduced populations. Our study illustrates how divergence in sexually selected traits via male competition can determine the direction and extent of introgression, contributing to geographic patterns of genetic and phenotypic diversity.
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