Sex roles and sexual selection: lessons from a dynamic model system

Abstract

Our understanding of sexual selection has greatly improved during the last decades. The focus is no longer solely on males, but also on how female competition and male mate choice shape ornamentation and other sexually selected traits in females. At the same time, the focus has shifted from documenting sexual selection to exploring variation and spatiotemporal dynamics of sexual selection, and their evolutionary consequences. Here, I review insights from a model system with exceptionally dynamic sexual selection, the two-spotted goby fish Gobiusculus flavescens. The species displays a complete reversal of sex roles over a 3-month breeding season. The reversal is driven by a dramatic change in the operational sex ratio, which is heavily male-biased at the start of the season and heavily female-biased late in the season. Early in the season, breeding-ready males outnumber mature females, causing males to be highly competitive, and leading to sexual selection on males. Late in the season, mating-ready females are in excess, engage more in courtship and aggression than males, and rarely reject mating opportunities. With typically many females simultaneously courting available males late in the season, males become selective and prefer more colorful females. This variable sexual selection regime likely explains why both male and female G. flavescens have ornamental colors. The G. flavescens model system reveals that sexual behavior and sexual selection can be astonishingly dynamic in response to short-term fluctuations in mating competition. Future work should explore whether sexual selection is equally dynamic on a spatial scale, and related spatiotemporal dynamics.

Evolution of female coloration: What have we learned from birds in general and blue tits in particular

“…For instance, in the two-spotted goby fish Gobiusculus flavescens", sexual selection is varying within the breeding season. This temporal variation is due to a complete reversal of sex roles across the breeding season, driven by a change in the operational sex ratio heavily male-biased at the start of the season then heavily female-biased towards the end of the season (Amundsen, 2018). Last, the assumption that all ornaments need to be costly or are subjected to allocation trade-offs is the subject of debate (Prum, 2010;Weaver, Koch, & Hill, 2017).…”

Elevated temperature, but not decreased pH, impairs reproduction in a temperate fish

“…However, disregarding reproduction can put at risk the long-term survival of the species 2 . In the two-spotted goby, Gobiusculus flavescens , used in this study, reproduction success includes physiological 3 and behavioural 4 processes. Females can allocate energy for partner selection, courtship, egg production, and offspring development 5 , while males energy can be allocated for partner selection, courtship, nest construction and defence 6 , 7 .…”

“…Females can allocate energy for partner selection, courtship, egg production, and offspring development 5 , while males energy can be allocated for partner selection, courtship, nest construction and defence 6 , 7 . The ability of a couple to successfully mate and produce clutches, as well as their potential investment in taking care of the clutches (parental care), is determinant aspects for their offspring’s fitness 4 .…”

Male mate choice, female competition, and female ornaments as components of sexual selection

“…Another paper—also on gobies—by Amundsen (2018) reports a similar effect. The operational sex ratio in another small fish, the two-spotted goby, is key to a functional sex role reversal over the breeding season.…”