Dense, small and male-biased cages exacerbate male–male competition and reduce female choosiness in Bicyclus anynana

Abstract

Sexual selection is increasingly recognized to depend upon, and to fluctuate with, major ecological factors in natural environments. The operational sex ratio (OSR) and population density can affect the opportunity for, and strength of, sexual selection but their effects are rarely taken into account in laboratory behavioural studies. In Bicyclus anynana, a model butterfly for mate choice studies, the experimental set-up widely used in laboratories often involves very high densities compared to the field, malebiased sex ratios and small cage volumes. We hypothesized that these conditions impede the proper expression of female mating preference by promoting male competition and sexual harassment of females. Using various cage volumes, we separately manipulated OSR and density to cover the range of values used in B. anynana laboratory mating experiments and to approach field values. Male competition, quantified by the number and duration of courtships aborted by males, became stronger with increasing densities, specifically under more male-biased sex ratios, and decreasing cage volumes. Thus, male eagerness to mate was essentially due to the use of small cage volumes and very high experimental densities. Concomitantly, female choosiness, quantified by the proportion of rejections of male mating attempts, decreased with increasing densities under a male-biased sex ratio. Females also accepted more matings with decreasing cage volume, and mated more rapidly with increasing density. We conclude that the laboratory social environments frequently used to test mate choice in B. anynana, with unnaturally high densities and male-biased sex ratios, exacerbate maleemale competition and strongly hinder female choice, biasing the estimates of the strength and direction of sexual selection that were shaped under very different natural environments in the wild. We propose a set-up for B. anynana mate choice studies that allows the assessment of female choosiness in statistically robust mating experiments. © 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Understanding the processes of pair formation is a necessary, yet neglected, step to fully grasp how secondary sexual traits and mating preferences evolve under sexual selection. It requires taking into account the fact that sexual selection, like natural selection, takes place in ecologically complex environments and examining the two major selective agents of sexual selection, mate choice and within-sex competition, in an integrative way (Miller & Svensson, 2014). It is increasingly acknowledged that mating patterns depend on ecological and social constraints, which determine the