Male Fiddler Crabs Research Articles

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.

Size‐dependent mating preference of the male fiddler crab Austruca perplexa

AbstractIn many animals, females prefer large males to small males, which allow large males to be choosier than small males when selecting a mate. We investigated the courtship intensity of small‐ and large‐sized male fiddler crabs (Austruca perplexa) by examining their claw‐waving rates (waves/min) towards small‐ and large‐sized females. We found that large males showed a greater preference for large females by producing more waves/min towards them, whereas small males did not show any apparent preference for either large or small females. Moreover, the waving rate of large males was positively correlated with female size, but there was no correlation between waving rate and female size in small males. These results indicate that large males in a population become choosier and show strong mate choice, which is most likely due to their greater preference among females.

Staying cool: the importance of shade availability for tropical ectotherms

Understanding how organisms mitigate the impacts of climate warming is one of the biggest challenges facing modern-day biologists. For tropical ectotherms, staying cool is critical for avoiding thermal stress, so individuals that are able to maintain territories in cool microhabitats are likely to gain fitness advantages. This study evaluated the importance of shade availability in the tropical fiddler crab, Austruca mjoebergi, by investigating temperature variation and behavioral responses (distribution, activity level, and time budget) in sun-exposed and shaded microhabitats. We found that the daily temperatures in sun-exposed microhabitats were significantly higher than in shady microhabitats, and that more male crabs held territories in the shade. Shaded males were active on the sediment surface for longer periods each day, allowing more time for feeding and more opportunities to detect mate-searching females. The greater number of males resident in the shade, and shorter distances between male neighbors in the shade, suggested that there was less available space for individuals to move into the shaded microhabitats. The behavioral differences between sun- and shade-living residents are likely to have consequences for male fitness. We highlight the importance of shade in providing thermal refugia for tropical ectotherms. The impact of climate warming on tropical organisms could be less dire if individuals are able to access shade. Shade can offer thermal refugia for tropical organisms that may have important consequences for both behavior and fitness. Relatively low and stable temperatures in shaded areas allow male fiddler crabs to remain on the surface for longer compared to sun-exposed males, allowing for longer feeding periods and more mating opportunities.

Autotomy and running performance of fiddler crabs (Decapoda: Brachyura: Ocypodidae)

Abstract The sexually dimorphic, enlarged major claw is a notorious trait among male fiddler crabs, but comes with potential locomotor costs. Possessing the ability to autotomize the enlarged claw is thus potentially advantageous to not only to escape a predator’s grip, but also to increase running performance. Previous studies concluded that autotomy either has no effect or even a negative effect on running performance. If the claw does not aid in locomotion, then shedding the enlarged claw that accounts for 40% of a fiddler crab’s mass should positively affect running performance. I therefore investigated autotomy and running performance in the Atlantic sand fiddler crab Leptuca pugilator (Bosc, 1801) with a focus on improving upon the methods of previous studies. Crabs were given substantial recovery time between collection, running trials, and autotomy induction. Maximum sprint speed was assessed by running crabs on a 1 m sand and mud track where individuals were significantly faster after autotomy of the enlarged claw (N = 64, t63=-7.25, P < 0.001). Intact running velocity was furthermore a significant predictor of autotomized running velocity (R2 = 0.194, P < 0.001). This study is the first to show a significant increase in fiddler crab sprint velocity after autotomy on a flat surface, and I propose where methodological pitfalls may have occurred in previous studies.

Territorial defense in a network: audiences only matter to male fiddler crabs primed for confrontation

Territorial defense in a network: audiences only matter to male fiddler crabs primed for confrontation

Color changes of fiddler crab between seasons and under stressful conditions: Patterns of changes in lightness differ between carapace and claw

In animals, coloration on the body surface could have various functions, such as thermoregulation, camouflage, and intraspecific communication. Because individuals often experience different ecological and social contexts/environments, changes of coloration due to these contexts/environments would be adaptive. The functions of coloration, in addition, may differ between different body regions. In this study, I compared the changes in coloration (lightness) between the carapace and the large claw of male fiddler crabs, Austruca lactea, to estimate the function of coloration in each body part in this species. I conducted monthly field surveys to assess seasonal changes of coloration of the crabs' carapace and large claw. I also carried out a field experiment to determine short-term changes in coloration under stressful conditions. Monthly surveys revealed that claw color was always lighter than the carapace color. Both of these were lightest during the breeding period. Interestingly, carapace lightness positively correlated with carapace width in the peak of the breeding season (July), while it was negatively correlated with carapace width in the non-breeding season (May and September). Claw lightness also differed significantly between months, although claw lightness was less variable than carapace lightness. In the field experiment, carapace coloration rapidly (within ten minutes) became darker under stressful conditions, but claw coloration changed very little, even after crabs were exposed to stressful conditions for 30 min. These results suggest that the function of coloration differs between carapace and claw. Carapace coloration may have functions related to thermoregulation and/or camouflage, and may therefore rapidly respond to fluctuating contexts/environments. Carapace coloration may also have a nuptial function for courtship that is related to body size. Claw coloration could be used to convey relatively more stable information, and may play a role in species recognition for mate choice, and resource holding potential for territorial fighting. Multiple factors, including physical and social factors, may affect each other, and consequently influence coloration of each body part in this species.

A case of bilateral hypertrophy in the chelae of a male specimen of Minuca rapax (Smith, 1870) (Decapoda, Brachyura, Ocypodidae)

Abstract The present study describes the first record of bilaterally hypertrophied chelipeds in a male fiddler crab, Minuca rapax (Smith, 1870), collected in the mangrove area near the city of Jaguaribe, located in the northern portion of Itamaraca Island in Pernambuco, Brazil. This particular specimen was captured by hand as it emerged from its burrow. Of the total 126 individuals (48 females and 78 males) of the genus Minuca (Bott, 1954) captured, only one presented chelipeds similar in size and shape. Some of the causes of malformation in decapods have been attributed to a double generation of growth areas or non-synchronous regenerative coordination in wound healing. Due to the lack of studies on the definitive cause of these abnormalities and deformations, further investigations are required to determine the roles of individuals with abnormalities within populations as well as the principal causes of these abnormalities.

Autotomy of the major claw stimulates molting and suppresses feeding in fiddler crabs

Autotomy is a predator escape mechanism in many taxa, including crustaceans. Male fiddler crabs autotomize the major claw, despite its importance in mating, territoriality, and thermoregulation. This claw can be regenerated through molting. We investigated major claw autotomy in the fiddler crab Leptuca pugilator, testing hypotheses that (1) autotomy of the major claw accelerates the molting process, reflecting the importance of this appendage, (2) autotomy of the major claw results in increased feeding to accumulate energy reserves needed for regeneration, (3) this change in feeding is hormonally driven. Autotomy of the major claw accelerated molting. Yet contrary to expectations, autotomy of the claw initially inhibited feeding, with evidence for release of feeding inhibitory factor. These effects are specific to the major claw, and were not seen following autotomy of a walking leg. Acceleration of molting following autotomy of the major claw appears to be an adaptive response reflecting the costs of being without this multi-function structure.

Investigating the role of a mud structure in a fiddler crab: do semidomes have a reproductive function?

Sensory traps occur when a signal evolves to mimic a cue to which the receiver responds favorably in another context. For example, if females are attracted to the noise a prey item makes, then a conspecific male can mimic that noise, thereby attracting females in the context of mating. Thus, sensory traps can be beneficial to receivers since attraction helps locating males more efficiently. But what happens when receivers are less attracted by the sensory trap outside of the original context (due to the cost of the response for instance)? Does this weaken and eventually “break” the sensory trap? Here, we show that a rare, small mud structure (a semidome) built by some male fiddler crabs (Austruca mjoebergi) at their burrow entrances are most likely relics of the sensory traps used by other fiddler crabs (the larger, far more abundant hoods of Leptuca species). Females of A. mjoebergi have retained their tendency to move towards vertical structures for protection when threatened by a predator—the original context thought to be important in the evolution of sensory traps in other fiddler crab species. However, females show no significant preference for males with semidomes. Additionally, males do not use the semidomes as landmarks to relocate their burrows when courting. The results suggest that in A. mjoebergi, semidomes do not function as sensory traps, despite the existence of a pre-existing bias in females. We suggest that semidomes have lost their attractiveness in the mating context due to ecological features, such as predation and environmental heterogeneity. There is growing evidence of sensory traps in many animal species, with an ever-widening range of contexts from nuptial gift giving in spiders to egg recognition in non-brood-parasitized birds. While we are starting to understand the significance of sensory traps, little thought has been given to what happens when they break down. If there is a change in the original context that favored the receiver’s response, then there is likely to be a subsequent change in the receiver’s bias and finally, in the sensory trap. If the mimic of the original stimulus does not induce a beneficial response in the receiver anymore, the sensory trap would become inefficient and stop being selected. The effect will be most profound when the sensory trap was costly to set up, since there will be selection against its retention. Here, we show that a sensory trap can become a relic when ecological conditions change and make it not functional anymore. To our knowledge, it is the first study providing such empirical evidence.

Multimodal signaling in fiddler crab: waving to attract mates is condition-dependent but other sexual signals are not

Many animals utilize multimodal signals for mate attraction and courtship. Each one of these signals could possibly provide information to receivers about aspects of the sender’s quality. Demonstrating condition-dependence of each of the multiple signals may shed light on which are related to the sender’s quality. This is because condition-dependence can reliably convey the sender’s quality due to the costs associated with signal production; only males in good condition are able to pay the costs of producing the signal. Male fiddler crabs use multimodal signals to attract females from a distance (they build semidomes and produce attraction waves) and for courting the females once they have approached (they produce courtship waves and substrate vibrations). Here, we investigate condition-dependence of the multimodal signals produced by the fiddler crab Austruca lactea. We manipulated the food levels of focal crabs (added food, control, removed food) for approximately 2 weeks in the field. We then compared the intensity of the male’s signals and the male’s blood lactate level. We found that the number of attraction waves significantly differed among treatments; the number of the waves increased with food availability. Blood lactate concentration also increased in the food-addition treatment. However, other signal traits were not significantly affected by the level of food available. These results suggest that adding food facilitated an increase in the male’s ability to produce attraction waves, and this would impose higher energy costs. In this species, only attraction waving reflects male nutritional condition and so could potentially be used by females to distinguish between low- and high-quality males. Animals often employ multimodal signals for mating. Here, we examine whether each of the multimodal signals is dependent on male condition in Austruca lactea. We compared the intensity of their multimodal signals, i.e., semidome, attraction waving, courtship waving, and vibration, among three nutritional treatments (food-addition, control, food-removal). Energy costs were additionally estimated by measuring blood lactate level. The number of attraction waves and the blood lactate level increased in food-adding treatment, while the frequency of semidome construction, rate of courtship waving, and pulse interval in vibrations were not influenced by nutritional condition. This suggests that additional food improved male condition and increased the number of attraction waving. Only attraction waving potentially conveys information about the male nutritional quality. Attraction waves could potentially be used by females to distinguish between low- and high-quality males.

Taking a risk: how far will male fiddler crabs go?

Courtship is costly for males when it increases their energy expenditure and predation risk. There are several ways in which males might be able to mitigate these costs, or compensate for them by elevating the benefits of courtship. First, they could selectively court more profitable females. Second, they could adjust the amount of risk they take against their residual reproductive value. Third, they could sometimes use cheaper signals to deceive females. In the fiddler crab Leptuca terpsichores (Crane, 1941), males risk losing their burrow to another crab and falling prey to a bird when they leave their burrow to intercept a mate-searching female and lead her back to the burrow for mating. Some males build sand hoods at their burrow entrances, which are landmarks that attract females and allow males to relocate their burrows quickly with little error. Here, we show that (1) males took greater risks when courting larger females by travelling farther away from their burrows; (2) the distance a male moved from his burrow did not depend on his size (hence, age); and (3) males with sand hoods did not travel farther away from their burrows than males without hoods, and they were not more likely to reach females. Taking greater risks when courting larger (more fecund) females appears to be a key means through which male fiddler crabs can achieve a more favourable balance between the costs and benefits of courtship. Courtship and mate choice can be costly for males. Males may improve the balance between courtship costs and benefits by modifying their risk-taking during courtship according to the perceived value of the female, or their expectations of future reproduction. Male fiddler crabs move away from their burrows to court females, which is risky because they may lose the burrow or be attacked by predators. We used the distance a male travels from his burrow as an index for the level of risk he is willing to take. We explored the effects of female size, male age, and the presence of a sand hood at the burrow entrance on distance travelled during courtship. Males took greater risks to court larger females, but did not adjust risk-taking according to their age (expected future reproduction) or whether their burrow had a sand hood.

Mudflat Use and Predation on Male Southwestern Atlantic Fiddler Crabs (Uca uruguayensis) by Gull-billed Terns (Gelochelidon nilotica)

Abstract. Gull-billed Terns (Gelochelidon nilotica) comprise a group of cosmopolitan subspecies that make use of a wide range of aquatic ecosystems and have a broad dietary spectrum. This study documents the use of a southwestern Atlantic mudflat in Argentina and depredation of the southwestern Atlantic fiddler crab (Uca uruguayensis) by Gull-billed Terns. Gull-billed Terns preyed exclusively on male fiddler crabs. This may have been due to the observed presence of Gull-billed Terns mainly between 2 and 3 hr after low tide, when more than 80% of fiddler crabs active on the surface were males. Gull-billed Terns spent 66.9% of the time flying over and feeding on fiddler crabs. Gull-billed Terns landed without feeding 9.3% of the time. Gull-billed Terns succeeded in capturing prey in 29.8% of cases. Food capture rate of Gull-billed Terns was 68.9 items/hr, which is higher than rates reported for the species feeding on fiddler crabs in Mauritania and Guinea Bissau.

Dynamics of energy reserves and the cost of reproduction in female and male fiddler crabs

The physiological costs of reproduction can be measured as the energy allocated to reproductive activities. In fiddler crabs, females allocate energy to vitellogenesis and brooding, whereas males perform expensive courtship behaviors. We evaluated in a large-scale study the reproduction cost of females and males of Leptuca uruguayensis in a temperate estuary, where their reproductive efforts are synchronized in a short reproductive season. The reproductive investments (vitellogenesis, spermatophore production, and male reproductive behaviors) were measured and related to the dynamics of storage and expenditure of energy reserves (glycogen, total lipids, and total protein) in the hepatopancreas, ovary, and muscle of the enlarged cheliped, throughout one annual cycle. Maximum energy storage occurred in winter, a period of low activity, whereas minimum energy storage occurred during the reproductive period, characterized by expensive activities. The glycogen reserves of the hepatopancreas decreased about 66% in females and 61% in males, suggesting high and similar physiological costs of reproduction between sexes, despite their different reproductive strategies to maximize their fitness.

Simultaneous multitasking behaviour affects feeding efforts of male fiddler crabs (Brachyura, Ocypodidae)

Abstract In animals, both monotasking and multitasking behaviours are observed. Multitasking behaviour helps to increase output quantity, but one activity may distract attention from another, and activities may interact negatively as they compete for the same amount of time available. To test this, we video recorded 23 Austruca bengali (Crane, 1975) males who were ‘feeding only’ (monotasking) and 24 males who were ‘feeding and courting (waving major claws) females’ simultaneously (multitasking). We measured the feeding and waving rates for ‘feeding and waving’ males, and the feeding rate for ‘feeding only’ males. We found that the feeding rate of ‘feeding only’ males was higher, but they reduced their feeding rate when they shared the time for feeding and courting. Moreover, feeding rate was negatively correlated with waving rate for ‘feeding and waving’ males. Our results indicate that courting activity distracted attention from feeding activity, and that activities interacted negatively with each other.

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.

The metabolic cost of carrying a sexually selected trait in the male fiddler crab Uca pugilator.

Models proposed to explain sexually selected structures assume that these traits are costly. However, studies investigating the impact of such structures on locomotory costs have produced inconsistent results. Male fiddler crabs possess a large sexually selected claw and are ideal for assessing the impact of a sexually selected trait on the cost of locomotion. Here, we measured the energy expenditure of clawed, declawed and artificially loaded crabs during sustained exercise by measuring oxygen consumption and blood lactate levels. We also measured blood lactate levels of clawed and declawed crabs following a sprint and forced walk to assess energy expenditure during non-sustainable, strenuous locomotion. Clawed and declawed crabs consumed the same amount of oxygen and had the same blood lactate concentration during sustained locomotion, suggesting that the large claw does not increase energetic costs during sustainable locomotion. Following non-sustainable, strenuous locomotion, however, there was a trend for clawed crabs to have higher concentrations of lactate in their blood than declawed crabs, suggesting that bearing a large claw may increase energetic costs during strenuous locomotion and lengthen recovery time. Artificially loaded crabs produced more lactic acid than clawed and declawed crabs during sustainable locomotion, suggesting that compensatory traits help mitigate the energetic cost of carrying the large claw. Overall, our results show that the impact of exaggerated structures on energy expenditure may depend on exercise intensity and highlight the need to examine various locomotory intensities when attempting to assess the costs of bearing a sexually selected trait.

The effect of audience on intrasexual interaction in the male fiddler crab, Uca maracoani (Latreille 1802–1803, Decapoda, Ocypodidae)

Animal communication occurring in wide networks can involve signals sent to several receivers. The animals composing the audience may affect how individuals display during an interaction and may change the message that is sent. In this study, we investigated whether the presence of a conspecific affected the intensity of agonistic interaction between male fiddler crabs, Uca maracoani. Pairs of males of the same size were observed when in the presence of a male, a female or no crab as audience. We found that if there is a female audience, males became more aggressive and interacted the most. Also, the female audience leads to an increase in incidence of male foaming, possibly indicating predisposition for mating. If the audience was a male or if there was no audience, there was no significant difference in interaction. These results indicate that the presence of an audience affects the way male fiddler crabs interact and the type of displays they show. Therefore, the context seems to guide the fiddler crab behavior in terms of how they perform in order to send information about themselves to conspecifics.

Burrow ornamentation in the fiddler crab (Uca leptodactyla): female mate choice and male–male competition

Non-biological ornamentation is found in the nests and burrows of different kinds of animals. We evaluated here whether sand hoods constructed by male fiddler crabs (Uca leptodactyla) are one of the signals used by males to attract females during courtship. We observed females when they were walking among the males, and we quantified the proportion of females that visited male burrows with and without ornamentation and the choice to stay in a male’s burrow. Females visited more burrows with hoods than burrows without hoods, and they chose significantly more builder males. Male investment in ornamentation nevertheless decreased when the proportion of females increased in the area. Male investment was not correlated with the proportion of non-builder males nearby, but was positively correlated with overall density. The density sex ratio, however, was more male-biased in high-density than in low-density areas suggesting that even if building attracts females, the function could be related to male competition for mates.

The vibrational signals that male fiddler crabs (Uca lactea) use to attract females into their burrows.

In some fiddler crab species, males emit vibrations from their burrows to mate-searching females after they have attracted a female to the burrow entrance using a waving display. Although the vibrations are considered acoustic signals to induce mating, it has not been demonstrated whether the vibrations attract the females into the burrow and, consequently, influence females' mating decisions. We investigated the structures and patterns of the vibrations using a dummy female and demonstrated experimentally a female preference for male vibrations in Uca lactea in the field. The acoustic signals consisted of repetitions of pulses. The dominant frequency of the pulses decreased with male carapace width. The pulse length decreased slightly with an increasing number of vibrational repetitions, and the pulse interval increased with increasing repetitions. These factors imply that the vibrations convey information on male characteristics, such as body size and stamina. In the experiment on female mate choice, the females significantly preferred males with higher pulse repetition rates when they were positioned at the entrance of the burrow, indicating that the females use the male vibrational signals to decide whether to enter the burrow. However, females showed no preference for the vibrations once they were inside a burrow, i.e., whether they decided to copulate, suggesting that the vibrations do not independently affect a female's final decision of mate choice. The vibrations inside the burrow might influence a female's decision by interaction with other male traits such as the burrow structure.

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

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.