In animals, skewed sex ratios can affect individual fitness via sexual (e.g., intersexual conflict or intrasexual mate competition) or nonsexual (e.g., sex-specific resource competition) interactions. Because most analyses of sex ratio focus on sexual interactions, the relative importance of sexual versus nonsexual mechanisms remains unclear. We tested both mechanisms in the flour beetle Tribolium castaneum, where male-biased sex ratios increase female fitness relative to unbiased or female-biased groups. Although flour beetles show both sexual and nonsexual (resource) competition, we found that sexual interactions did not explain female fitness. Instead, female fecundity was dramatically reduced even after a brief exposure to flour conditioned by other females. Earlier studies suggested that secreted toxins might mediate density-dependent population growth in flour beetles. We identified ethyl benzoquinone and methyl benzoquinone (quinones) as components of adult stink glands that regulate female fecundity. In female-biased groups (i.e., at high female density), females upregulated quinones and suppressed each other's reproduction. In male-biased groups, low female density and associated low quinone levels maximized fecundity. Thus, females appear to use quinones as weapons for female-specific, density-dependent interference competition. Our results underscore the importance of nonsexual interference competition that may often underlie the fitness consequences of skewed sex ratios.

1. Animals often have to choose between multiple food sources in their habitat, and these potentially complex decisions have a large impact on their fitness. Among other factors, previous experience with an alternative resource can significantly increase subsequent preference for the resource ('induction of preference').2. Induction of resource preference is particularly relevant in spatially or temporally heterogeneous habitats. Although most mobile species -especially generalists -probably frequently encounter habitat heterogeneity, the impact of preference induction on individual behaviour and fitness in heterogeneous habitats is poorly understood. This study analysed larval preference induction in wheat-adapted generalist red flour beetles, Tribolium castaneum (Coleoptera: Tenebrionidae), under three types of habitat heterogeneity.3. First, the induction of preference for novel cereal flours under temporal heterogeneity was analysed, exposing larvae to new resources during development. Larvae preferred a new resource if they experienced it recently, but the magnitude of induction varied across resources.4. Specific induction for a homogenous mix of wheat and a novel resource was also observed, with larvae preferring the mix over either pure resource.5. To analyse induction under spatial heterogeneity, beetle eggs were placed in one of two alternative resource patches and the preference of emerged larvae was tested. Hatching into a novel resource did not always induce preference.6. Finally, it was found that induction of preference for new resources could be maladaptive for larval development. In sum, this work demonstrates that experience-based plasticity of larval resource choice may strongly impact larval preference and fitness in heterogeneous habitats.