Although it is often expected that adverse environmental conditions depress the expression of condition-dependent sexually selected traits, the full consequences of environmental change for the action of sexual selection, in terms of the opportunity for total sexual selection and patterns of phenotypic selection, are unknown. Here we show that dietary stress in guppies, Poecilia reticulata, reduces the expression of several sexually selected traits and increases the opportunity for total sexual selection (standardized variance in reproductive success) in males. Furthermore, our results show that dietary stress modulates the relative importance of precopulatory (mating success) and postcopulatory (relative fertilization success) sexual selection, and that the form of multivariate sexual selection (linear vs. nonlinear) depends on dietary regime. Overall, our results are consistent with a pattern of heightened directional selection on condition-dependent sexually selected traits under environmental stress, and underscore the importance of sexual selection in shaping adaptation in a changing world. LETTEREnvironmental-dependent sexual selection 449

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There is considerable evidence that female reproductive fluid (FRF) interacts intimately with sperm, affecting several sperm traits, including sperm motility and longevity, and ultimately fertilization success. One of the first documented interactions between FRF and sperm is the ability of FRF to attract and guide sperm towards the eggs. However, most of the evidence of FRF’s chemoattraction proprieties comes from a limited number of taxa, specifically mammals and invertebrate broadcasting spawners. In other species, small FRF volumes and/or short sperm longevity often impose methodological difficulties resulting in this gap in chemoattraction studies in non-model species. One of the outcomes of sperm chemotaxis is sperm accumulation towards high chemoattractant concentrations, which can be easily quantified by measuring sperm concentration. Here, we tested sperm accumulation towards FRF in the zebrafish, Danio rerio, using an ad hoc developed, 3D printed, device (‘sperm selection chamber’). This easy-to-use tool allows to select and collect the sperm that swim towards a chemical gradient, and accumulate in a chemoattractant-filled well thus providing putative evidence for chemoattraction. We found that sperm accumulate in FRF in zebrafish. We also found that none of the sperm quality traits we measured (sperm swimming velocity and trajectory, sperm motility, and longevity) were correlated with this response. Together with the 3D printable project, we provide a detailed protocol for using the selection chamber. The chamber is optimized for the zebrafish, but it can be easily adapted for other species. Our device lays the foundation for a standardized way to measure sperm accumulation and in general chemoattraction, stimulating future research aimed at understanding the role and the mechanisms of sperm chemoattraction by FRF.

Anti-predator benefits associated with living in groups are multiple and taxonomically widespread. In fish shoals, individuals can exploit the confusion effect, whereby predators struggle to target a single individual among several individuals. Theory predicts that the confusion effect could be aided by homogeneity in appearance; thus, individuals should group by phenotypic characteristics, contributing to generating high within-shoal phenotypic homogeneity. While assortments by body size have been extensively documented, almost nothing is known about whether within-shoal homogeneity in body pigmentation affects shoaling preference. To investigate this issue, we used the Mediterranean killifish, , a shoaling species characterized by conspicuous vertical bars on body sides. Individual females were given a choice between two novel shoals characterized by either a high or low degree of homogeneity in the number of bars. As predicted, individual females preferentially associated with the shoal showing the higher phenotypic homogeneity. Our data demonstrated that fish might associate with the shoal that maximizes phenotypic homogeneity in body pigmentation, irrespective of their own phenotype.