Selection for conspicuous visual signals in a fiddler crab

Sexual selection has an undeniable influence in the evolution of the spectacular diversity of courtship signals in the animal kingdom. A long history of study has pointed to mechanisms through which sexual selection can act: it can favor signals that are reliable indicators of species identity or effectively transfer mate quality information. In some species, these mechanisms have the potential to shape signal evolution. This is the case in fiddler crabs. Males court females by waving their sexually dimorphic claw. Females recognize conspecific males by the species-specific display, while intraspecific variants of the display appear to be indicators of male quality. We investigated which of these mechanisms prevail by using robotics to test female responses to waves of different heights in the fiddler crab, Austruca mjoebergi. We reveal that, although the studied species shares a sympatric habitat, females did not significantly more often approach the species’ average signal. We found evidence that more conspicuous, higher signals were more likely to attract females, although the effect was not particularly strong. We discuss our results in the light of other possible scenarios from which sexual selection can act in the evolution of signals.This study was funded by Australian Research Council Discovery Grant (DP160100316) to P.R.Y.B

Territorial battles between fiddler crab species

Many species worldwide are impacted by habitat loss. This may result in increased competition both within species and between species. Many studies have demonstrated that when two previously non-overlapping species are forced to compete over a resource, one species is likely to become dominant over the other. This study explores the impact a larger species of fiddler crab (Tabuca elegans-previously known as Uca elegans) has when invading an area previously used solely by a smaller species (Austruca mjoebergi-previously known as Uca mjoebergi). Here we show that, while there are some detrimental effects of living next to a heterospecific, they are relatively minor. New heterospecific neighbours fight more regularly with resident crabs, but each fight is no longer or more escalated than those between the resident and a new conspecific male. The residents are not specifically targeted by intruding heterospecifics, thus, given the large advantage of having a heterospecific neighbour in terms of lowered competition for females, the overall impact of species mixing is probably not as negative as might have been predicted.Funding was provided by the Australian National University and the Australian Research Council (ARC DP120101427)

Aggressive interactions and intermale spacing in choruses of the leaf-folding frog, Afrixalus delicatus

Intermale spacing was examined in caged Afrixalus delicatuschoruses. Males maintained an individual distance using an advertisement call, an encounter call and physical combat (or the threat of combat). An increase from low to intermediate chorus size (2-4 males) led to a decrease in nearest calling neighbour distances. At high chorus sizes (5-8 males) males maintained a minimum individual distance of 30-35 cm. New calling males were not accommodated in the chorus once eight males were calling. As the chorus size increased, a greater number of aggressive interactions were required to maintain the minimum distance. The spatial separation of calling males reduces the number of competitors when space is limited. The maintenance of a preferred minimum individual distance may increase a male’s reproductive success by reducing acoustic interference with neighbouring males as well as providing a clear pathway for female approach

Keeping up appearances: male fiddler crabs wave faster in a crowd

Courtship displays are often energetically and temporally costly as well as highly conspicuous to predators. Selection should therefore favour signalling tactics that minimize courtship costs while maintaining or increasing signal attractiveness. In fiddler crabs, males court females by waving their one greatly enlarged claw in a highly conspicuous and costly display. Here, we investigate whether courting males adjust their wave rate, and therefore the cost of courtship, to the current level of competition.We show that display rate increases as competition increases and that when competition is removed, males reduce their display rate by 30 per cent. These results suggest that male fiddler crabs actively reduce the cost of courtship by adjusting their wave rate in response to the immediate level of competition

The effects of claw regeneration on territory ownership and mating success in the fiddler crab Uca mjoebergi

Underlying male quality is often reflected in the condition of sexually selected traits. In fiddler crabs, male success in both intra- and interspecific interactions is highly dependent on the size of the major claw. However, males are often forced to autotomize their major claw. Claw regeneration significantly altered the structure of a males' major claw in Uca mjoebergi. We found, however, that claw regeneration did not affect signal quality. Both males and females were unable to visually distinguish a regenerated claw from an original claw. Although regenerated males were inferior fighters, males were able to compensate for this fighting disadvantage by avoiding fights with other males. Regenerated males were, however, less likely to acquire and defend high-quality territories and consequently suffered a decrease in mating success

Multimodal communication in courting fiddler crabs reveals male performance capacities

Courting males often perform different behavioural displays that demonstrate aspects of their quality. Male fiddler crabs, Uca sp., are well known for their repetitive claw-waving display during courtship. However, in some species, males produce an additional signal by rapidly stridulating their claw, creating a ‘drumming’ vibrational signal through the substrate as a female approaches, and even continue to drum once inside their burrow. Here, we show that the switch from waving to drumming might provide additional information to the female about the quality of a male, and the properties of his burrow (multiple message hypothesis). Across males there was, however, a strong positive relationship between aspects of their waving and drumming displays, suggesting that drumming adheres to some predictions of the redundant signal hypothesis for multimodal signalling. In field experiments, we show that recent courtship is associated with a significant reduction in male sprint speed, which is commensurate with an oxygen debt. Even so, males that wave and drum more vigorously than their counterparts have a higher sprint speed. Drumming appears to be an energetically costly multimodal display of quality that females should attend to when making their mate choice decisions

Robotic crabs reveal that female fiddler crabs are sensitive to changes in male display rate

Males often produce dynamic, repetitive courtship displays that can be demanding to perform and might advertise male quality to females. A key feature of demanding displays is that they can change in intensity: escalating as a male increases his signalling effort, but de-escalating as a signaller becomes fatigued. Here, we investigated whether female fiddler crabs, Uca mjoebergi, are sensitive to changes in male courtship wave rate. We performed playback experiments using robotic male crabs that had the same mean wave rate, but either escalated, de-escalated or remained constant. Females demonstrated a strong preference for escalating robots, but showed mixed responses to robots that de-escalated (‘fast’ to ‘slow’) compared to those that waved at a constant ‘medium’ rate. These findings demonstrate that females can discern changes in male display rate, and prefer males that escalate, but that females are also sensitive to past display rates indicative of prior vigour

Testing female preferences under more natural conditions: a case study on a fiddler crab

Mate choice is often affected by multiple factors, and there are often trade-offs associated with choosing a mate. Additionally, experiments that test for mate preferences usually rely on simple two-choice tests. These tests are, however, often less complex than the scenarios that individuals face in natural populations. Here, we test female choice in the fiddler crab Uca mjoebergi. We looked at female preference for wave rates and proximity to males in simple two-choice tests. We then mimicked a more natural choice scenario, where females faced a cluster of six courting males that differed in their distance from the female as well as in their wave rate. In addition, we tested whether female preferences under these more complex conditions were affected by the risk of predation. We found a preference for faster wave rates and closer males in two-choice tests. The preference for closer males was, however, only evident when the difference in distance was large (15 cm), not when it was small (3 cm). When females chose between six males, they preferred the males that waved faster, even if they were further away. We did not, however, find any difference in female choice when a simulated predator was present or absent. By examining a more realistic set of options that females face, we can paint a better picture of how females’ trade-off costs and benefits during mate choice. Significance statement: Mate choice experiments often rely on two-choice tests. Mate choice, however, is often more complex under totally natural conditions. Using a two-choice experiment, we show that female fiddler crabs U. mjoebergi show a preference for faster wave rates and closer males. Under a more natural choice scenario, when choosing between six males, females preferred to travel longer to reach faster waving males. We found that female responses did not differ when a predator was present or absent. Designing choice experiments to more accurately mimic natural conditions will allow assessing trade-offs that occur in mate choiceThis work was supported by the Australian Research Council Discovery Grant to P.R.Y.B. (DP120101427). R.V.-T. was supported by the fellowships from Consejo Nacional de Ciencia y Tecnología-México and the Research School of Biology

Risks and rewards: balancing costs and benefits of predator avoidance in a fiddler crab

The decision to take risks in the presence of a predator involves complex trade-offs between immediate survival and future reproduction. Individuals may gain fitness advantages if they are able to optimally alter their risk-taking strategies depending on the differential costs and benefits of risky behaviours across contexts. Male fiddler crabs (Austruca mjoebergi) exhibited a higher propensity to take risks in the presence of a female compared with conspecifics that were not presented with a female during both mating and nonmating periods. Contrary to predictions, however, risk-taking behaviour did not differ between mating and nonmating periods.Australian Research Council Discovery Grant to P.R.Y.B

Effects of temperature on reproductive timing and hatching success in a tropical fiddler crab

The timing of reproduction is critical to reproductive success in many animal species. Parents that can perceive and respond to environmental cues and time the hatching/birth of their offspring to optimal environmental conditions show higher reproductive success. Intertidal ectotherms are under particularly strong selection because larval development rates are temperature-dependent, and larvae must hatch during the highest spring tides to avoid high levels of inshore predation. Here we investigate whether female fiddler crabs, Austruca mjoebergi, can mitigate the effects of high temperatures by adjusting the timing of reproductive events and/or by behavioural compensation. We experimentally manipulated incubation temperatures between 30 and 36 °C, based on natural and predicted temperature conditions, and found that hatching success decreased linearly with increasing temperatures. However, temperature had no effect on the timing of fertilization or hatching, suggesting that larval development rate was not temperature-dependent. Across the tested temperatures, females did not adjust egg size, the amount of yolk in each egg, larvae size or clutch size. In conclusion, high temperatures prevented clutches from reaching the hatching stage, but within the range of temperatures that facilitated hatching, there was no evidence of behavioural compensation and no discernible effect of temperature on reproductive timing.The study was funded by an Australian Research Council Discovery Grant to P.R.Y.B. (DP120101427)