Legs of male fiddler crabs evolved to compensate for claw exaggeration and enhance claw functionality during waving displays

Extreme range in adult body size reveals hidden trade-offs among sexually selected traits

Sexually selected weapons used to monopolize mating opportunities are predicted to trade-off with traits used in competition for fertilization. Yet, the limited size range typically found among adults of a species often precludes clear comparisons between population-level and individual-level relative trait investment. The jousting weevil, Brentus anchorago (Coleoptera: Brentidae), varies more than 26-fold in body mass, which is among the most extreme adult body size ranges of any solitary terrestrial species. We reveal a trade-off at a population level: hypermetric scaling in male weapons (slope = 1.59) and a closely mirrored reversal in allocation to postcopulatory traits (slope = 0.54). Yet, at the individual level, we find the opposite pattern; males that invest relatively more in weapons for their size class also invest more in postcopulatory traits. Across 36 dung beetle and 41 brentine weevil species, we find the allometric slope explains more trait variation at larger body size ranges; in brentines, population-level scaling patterns become more detectable in species with a larger range in adult body size. Our findings reveal that population-level allometries and individual-level trade-offs can both be important in shaping relative trait allocation; we highlight that the adult body size range is rarely examined but may be integral to gaining a deeper understanding of trade-offs in reproductive allocation.

Can Sex-Specific Metabolic Rates Provide Insight Into Patterns of Metabolic Scaling?

Females and males can exhibit striking differences in body size, relative trait size, physiology and behavior. As a consequence the sexes can have very different rates of whole-body energy use, or converge on similar rates through different physiological mechanisms. Yet many studies that measure the relationship between metabolic rate and body size only pay attention to a single sex (more often males), or do not distinguish between sexes. We present four reasons why explicit attention to energy-use between the sexes can yield insight into the physiological mechanisms that shape broader patterns of metabolic scaling in nature. First, the sexes often differ considerably in their relative investment in reproduction which shapes much of life-history and rates of energy use. Second, males and females share a majority of their genome but may experience different selective pressures. Sex-specific energy profiles can reveal how the energetic needs of individuals are met despite the challenge of within-species genetic constraints. Third, sexual selection often pushes growth and behavior to physiological extremes. Exaggerated sexually selected traits are often most prominent in one sex, can comprise up to 50% of body mass and thus provide opportunities to uncover energetic constraints of trait growth and maintenance. Finally, sex-differences in behavior such as mating-displays, long-distance dispersal and courtship can lead to drastically different energy allocation among the sexes; the physiology to support this behavior can shape patterns of metabolic scaling. The mechanisms underlying metabolic scaling in females, males and hermaphroditic animals can provide opportunities to develop testable predictions that enhance our understanding of energetic scaling patterns in nature.

Biomechanics influence sexual dimorphism in the giant mesquite bug, Thasus neocalifornicus

In many species, males possess specialized weaponry that confers benefits during male-male combat. Because male weapons are often disproportionately larger versions of preexisting body parts, females often possess reduced versions of male weaponry. Most research focuses exclusively on sexual dimorphism in the size of male and female weapons, even though other aspects such as weapon performance can also explain the evolution of weapon sexual dimorphism. In the giant mesquite bug, Thasus neocalifornicus, males wield exaggerated hindlegs that aid in locomotion and are used as weapons to generate forceful squeezes during combat. However, female T. neocalifornicus hindlegs are relatively inconspicuous and only used for locomotion. To understand the intricacies of weapon sexual dimorphism in T. neocalifornicus hindlegs, we measured the allometry of their hindlegs morphology, biomechanics, and performance. Males and females had relatively similar sized legs when concerning only linear measurements: hindleg length did not differ between the sexes (both for intercept and slope), but males do have relatively wider hindlegs (greater intercepts). Regarding performance, however, males were relatively and proportionally stronger than females. Furthermore, the output lever of male hindlegs scales hypoallometrically and the tibial spine maintains its position as the hindlegs grows, both of which maintain the hindlegs' biomechanical efficiency as they increase in size. Overall, our finding demonstrates that selection on the performance and biomechanics of sexually selected weapons can influence the expression of sexual dimorphism, by exaggerating some aspects of the weapons morphology-but constraining others.

Impacts of pollution, sex, and tide on the time allocations to behaviours of Uca arcuata in mangroves

Fiddler crabs (Uca) are ecosystem engineers in coastal ecosystems. Many anthropogenic and natural factors can affect the time allocated to various behaviours in Uca. However, the behaviour of U. arcuata, a widely distributed fiddler crab in Asia, has not been studied in mainland China. Here, we used binoculars to record the time budget of ten behaviours of U. arcuata to investigate the potential effects of sex, tides, and pollution on these behaviours. We found that the crabs spent 42.3%, 27.0%, and 10.6% of their time on feeding, feeding while walking, and stationary respectively. The crabs spent <1.5% of their time on copulation and grooming. The total foraging time (feeding + feeding while walking) did not differ among the three polluted sites. However, crabs spent more time on feeding but less time on feeding while walking. The feeding rate and probability of burrowing and grooming decreased while the possibility of locomotion and stay in burrow increased with increasing nutrient concentration. Females spent 13.9% more time on feeding and fed 54.9% faster than males. Males had a higher tendency to grooming and combat while they were less likely to walk than females. Regarding to the influence of tide, fiddler crabs fed 11.2% faster at ebb tides than at flood tides, and they were more likely to walk and stay in burrows at flood tides than at ebb tides. Our results indicated that nutrient pollution had stronger impacts on the behaviours of crabs than sex and tide. In polluted mangroves, increasing nutrient concentration reduced the quantity of sediment processed by fiddler crabs due to their smaller feeding area, slower feeding rate, and reduced frequency of burrowing activities. These results imply that mitigating nutrient pollution in mangroves may benefit the restoration and management of coastal ecosystems through the enhanced engineering functions of fiddler crabs.

Morphology, performance and fluid dynamics of the crayfish escape response

Sexual selection can result in an exaggerated morphology that constrains locomotor performance. We studied the relationship between morphology and the tail-flip escape response in male and female rusty crayfish (Faxonius rusticus), a species in which males have enlarged claws (chelae). We found that females had wider abdomens and longer uropods (terminal appendage of the tail fan) than males, while males possessed deeper abdomens and larger chelae, relative to total length. Chelae size was negatively associated with escape velocity, whereas longer abdomens and uropods were positively associated with escape velocity. We found no sex-specific differences in maximum force generated during the tail flip, but uropod length was strongly, positively correlated with tail-flip force in males. Particle image velocimetry (PIV) revealed that the formation of a vortex, rather than the expulsion of fluid between two closing body surfaces, generates propulsion in rusty crayfish. PIV also revealed that the pleopods (ventral abdominal appendages) contribute to the momentum generated by the tail. To our knowledge, this is the first confirmation of vortex formation in a decapod crustacean.

Ecology and signal structure drive the evolution of synchronous displays

Animal synchrony is found in phylogenetically distant animal groups, indicating behavioral adaptations to different selective pressures and in different signaling modalities. A notable example of synchronous display is found in fiddler crabs in that males wave their single enlarged claw during courtship. They present species-specific signals, which are composed of distinctive movement signatures. Given that synchronous waving has been reported for several fiddler crab species, the display pattern could influence the ability of a given species to sufficiently adjust wave timing to allow for synchrony. In this study, we quantified the wave displays of fiddler crabs to predict their synchronous behavior. We combined this information with the group's phylogenetic relationships to trace the evolution of display synchrony in an animal taxon. We found no phylogenetic signal in interspecific variation in predicted wave synchrony, which mirrors the general nonphylogenetic pattern of synchrony across animal taxa. Interestingly, our analyses show that the phenomenon of synchronization stems from the peculiarities of display pattern, mating systems, and the complexity of microhabitats. This is the first study to combine mathematical simulations and phylogenetic comparative methods to reveal how ecological factors and the mechanics of animal signals affect the evolution of the synchronous phenomena.

Underwater compensation for exaggerated weaponry: The role of morphology and environment on crab locomotor performance

Exaggerated morphologies may increase fitness, but they might be costly to bear; heavy weight, for instance, might hinder locomotion. Evidence supporting these costs are sparse because animals that move on land or swim have traits reducing those costs, called compensatory traits. Animals that walk underwater, however, are under different environmental pressures than land animals. Buoyancy, for instance, reduces the effective weight of any object, which could decrease the locomotion costs of carrying exagerrated traits. Hence, underwater species might maintain performance without compensation. To test this, we compared males of the freshwater anomuran Aegla longirostri that bear an exaggerated claw to females (the natural control). We first tested whether the exaggerated claw decreased male locomotor performance. Next, we tested if sexual dimorphism in performance is associated with differences in leg asymmetry, length, and muscle size. Lastly, we tested if large males have proportionally heavier legs than smaller males. Unexpectedly, females are faster than males while also having relatively longer legs than males. Therefore, females might walk faster because of the longer legs, which might be unrelated to the male exaggerated claw. Furthermore, larger males did not have proportionally heavier legs than smaller males, further suggesting no compensation. Hence, even though aeglid's claw weigh ~25% of their total body weight, we did not find evidence for burden or compensation on males. The environment might thus decrease the costs of exaggerated traits.