Abstract
Olfactory communication can be of critical importance for mate choice decisions. Lepidoptera are key model systems for understanding olfactory communication, particularly considering sex pheromone signaling in the context of sexual selection. Solvent extraction or rinsing of pheromone-producing structures is a widespread method for quantifying sex pheromones, but such measures reflect what is stored and may not represent what is actually emitted by an individual during courtship. Here, we address this point for the first time by quantifying the components of the male sex pheromone (MSP) of interacting Bicyclus anynana butterflies, a species for which much information is available onthe role played by MSPs in affecting mating success. Using headspace sampling during courtship and solvent extraction after completion of experiments using the same males, we were able to track individual traits. Our results show that solvent extracts do not reflect quantities of MSP components emitted by live butterflies. We further show that MSP amounts obtained using headspace sampling correlated with male mating success, but solvent extracts did not. Our results further strongly suggest that males actively control MSP emission when faced with increased male-male competition. Common practice solvent extracts may thus not serve as an adequate proxy for male sex pheromone signaling as they are perceived by choosy females. Our study serves as a proof of principle that quantification of male sex pheromone components depends on the method of collection, which could apply to many other insects using short-range chemical signals. This affects our understanding of how sexual selection shapes the evolution of sexually-selected chemical traits.
Highlights
Sexual selection was first defined by Charles Darwin and Alfred Russel Wallace as a type of natural selection where access to reproduction depends on a specific part of the environment, i.e., the other sex (Darwin, 1859; Wallace, 1892)
How sexual selection shapes the evolution of sex pheromones remains largely unclear, and different schools of thought assume that sexual selection produces either stabilizing selection on the presence and amount of chemical compounds, or directional evolution for increased amounts of specific compounds that are preferred by the other sex (Groot et al, 2016)
We aimed to determine if one of the most commonly used methods in studies on olfactory communication, tissue extraction/rinsing in a solvent, reflects olfactory signals as they are emitted in the air
Summary
Sexual selection was first defined by Charles Darwin and Alfred Russel Wallace as a type of natural selection where access to reproduction depends on a specific part of the environment, i.e., the other sex (Darwin, 1859; Wallace, 1892). Many male moths and butterflies produce sex pheromones, typically during courtship, and emitted at close range (Phelan and Baker, 1987; Birch et al, 1990; Sappington and Taylor, 1990; Andersson et al, 2007; Nieberding et al, 2008). It is these close-range signals that are expected to play a major role in mate-choice decisions, yet close-range chemical communication by male moths and butterflies has received relatively little attention (Nieberding et al, 2008; Heuskin et al, 2014)
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