Mate choice and the operational sex ratio: an experimental test with robotic crabs

Abstract

The operational sex ratio (OSR: sexually active males: receptive females) predicts the intensity of competition for mates. It is less clear, however, under what circumstances, the OSR predicts the strength of sexual selection -that is, the extent to which variation in mating success is attributable to traits that increase the bearer's attractiveness and/or fighting ability. To establish causality, experiments that manipulate the OSR are required. Furthermore, if it is possible to control for any OSR-dependent changes in the chosen sex (e.g. changes in male courtship), we can directly test whether the OSR affects the behaviour of the choosing sex (e.g. female choice decisions). We conducted female mate choice experiments in the field using robotic models of male fiddler crabs (Uca mjoebergi). We used a novel design with two females tested sequentially per trial. As in nature, the choice of the first female to mate therefore affected the mates available to the next female. In general, we detected significant sexual selection due to female choice for 'males' with larger claws. Importantly, the strength of sexual selection did not vary across five different OSR/density treatments. However, as the OSR decreased (hence the number of available males declined), females chose the 'males' with the largest claws available significantly more often than expected by chance. Possible reasons for this mismatch between the expected and observed effects of the OSR on the strength of sexual selection are discussed. whether the OSR affects the behaviour of the choosing sex (e.g. female choice 48 decisions). We conducted female mate choice experiments in the field using robotic 49 models of male fiddler crabs (Uca mjoebergi). We used a novel design with two 50 females tested sequentially per trial. As in nature, the choice of the first female to 51 mate therefore affected the mates available to the next female. In general we detected 52 significant sexual selection due to female choice for 'males' with larger claws. 53Importantly, the strength of sexual selection did not vary across five different 54 OSR/density treatments. However, as the OSR decreased (hence the number of 55 available males declined), females chose the 'males' with the largest claws available 56 significantly more often than expected by chance. Possible reasons for this mismatch 57 between the expected and observed effects of the OSR on the strength of sexual 58 selection are discussed. 59 60

Female mating competition alters female mating preferences in common gobies

“…A situation with an excess of females may at the same time be perceived as a shortage of nests and mating opportunities. Hayes et al. (2016) found similar results for female fiddler crabs exhibiting stronger preferences for larger claws under female biased conditions.…”

“…Female–female competition can be expected to affect not only reproductive effort, but also female mate choice behavior ( Heubel et al. 2008 ; Hayes et al. 2016 ).…”

Designing mate choice experiments

“…Given that this information is potentially informative, it should be reported in the methods section of studies as standard (e.g. Berglund, ; Bailey, ; Hayes et al ., ).…”

“…It is well supported that the operational sex ratio significantly influences the strength of sexual selection (Janicke & Morrow, ). However, because most studies of sexual selection measure mating outcomes, it is not clear if this effect is driven by changes in mate choice or in the strength of intrasexual competition between potential mates (Hayes et al ., ). Studies that compare mating preferences between choice tests with varying numbers of options, while also preventing intrasexual competition, are rare.…”

“…Studies that compare mating preferences between choice tests with varying numbers of options, while also preventing intrasexual competition, are rare. In the broadnosed pipefish ( Syngnathus typhle ), males show stronger preferences when more females are available during choice tests (Berglund, , ); however this pattern is not seen in two species of fiddler crabs (Hayes et al ., ; Kawano & Henmi, ).…”