Theory suggests that reproductive effort generally increases with age, but life-history models indicate that other outcomes are possible. Empirical data are needed to quantify variation in actual age-dependence. Data are readily attainable for females (e.g. clutch per egg size), but not for males (e.g. courtship effort). To quantify male effort one must: (i) experimentally control for potential age-dependent changes in female presence; and, crucially, (ii) distinguish between the likelihood of courtship being initiated, the display rate, and the total time invested in courting before stopping ('courtship persistence'). We provide a simple experimental protocol, suitable for many taxa, to illustrate how to obtain this information. We studied courtship waving by male fiddler crabs, Uca annulipes. Given indeterminate growth, body size is correlated with age. Larger males were more likely to wave at females and waved more persistently. They did not, however, have a higher courtship rate (waves per second). A known female preference for males with higher display rates explains why, once waving is initiated, all males display at the same rate.
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
Cooperation between neighbors in territory defense is expected when the cost of helping a neighbor is less than that of establishing new boundaries with a successful usurper of a neighboring territory. Cooperation has been documented in 3 species of fiddler crab and is understood to depend strongly on the relative sizes of participants-large residents will help smaller neighbors repel intermediate-sized intruders. Simply meeting these criteria does not, however, guarantee that helping occurs, and additional factors are likely to affect the benefits of providing help. We tested whether the likelihood that a large resident would help his smaller neighbor was affected by neighbor familiarity or the relative size of the smaller neighbor, by replacing neighbors with smaller, larger, or size-matched individuals and then simulating intrusions onto their territories. The likelihood of helping did not differ between familiar and unfamiliar neighbors of the same size, but it decreased when the replacement resident differed in size from the original resident. These results suggest that although residents do not recognize their neighbors individually, size acts as a cue to neighbor identity.
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