Uca Beebei Research Articles

A low-cost sexual ornament reliably signals male condition in the fiddler crab Uca beebei

Empirical studies of the reliability of sexual signals are needed to test the prediction that females should prefer males in good condition. We examined whether an attractive sexual signal used by male fiddler crabs, Uca beebei, reliably indicates male condition by measuring male blood glucose and lactate levels in the field. The signal is a mud structure, a ‘pillar’, that attracts females for mating. Blood lactate levels (a measure of energy expenditure) of pillar builders (P males) immediately after they built their pillars did not differ from those of nonbuilders (NP males), suggesting that the cost of building a pillar is small. However, P male lactate levels were significantly higher than those of NP males late in the activity cycle after P males had invested more in vigorous courtship using claw ‘waving’. Whereas P males maintained their glucose levels (a measure of energy availability as the major fuel for ATP production) at around 30 mg/dl throughout their daily activity period, NP males showed significantly lower levels in the middle of the period. These results confirm that pillars reliably signal a male’s condition, as measured by his ability to maintain a blood glucose level necessary for costly courtship, even though the construction of a pillar has minimal energetic cost.

THE DESIGN OF A BEAUTIFUL WEAPON: COMPENSATION FOR OPPOSING SEXUAL SELECTION ON A TRAIT WITH TWO FUNCTIONS

Male fiddler crabs, genus Uca, have one greatly enlarged claw with which they court females and threaten and fight other males. Longer claws are more effective signals but are thought to be less effective weapons because the relative closing force at the tip of the claw decreases with claw length. We studied claw morphology and fighting in Uca terpsichores and Uca beebei and found a mechanism that may resolve opposing selection for signaling and fighting ability. When males fought they delivered gripping forces not at the tips but at the tubercles on the inner margins of their claws' fingers. As claws grow, these tubercles remain relatively close to the apex of the gape. Consequently, the mechanical advantage that governs the forces that can be delivered at these tubercles decreases only slightly with increasing claw length allowing the claw to be an effective signal and a powerful weapon. Animal weapons are exceptionally diverse in form and detail of armature and the causes of this diversity are poorly understood. We suggest that the designs of weapons may often reflect compensatory patterns of growth and placement of armature that enhances the weapon's overall utility for multiple uses in competition for mates.

A comparative analysis of sperm storage and mating strategies in five species of brachyuran crabs

Variation in female sperm storage is explained, in part, by the amount of sperm transferred at mating. Laboratory mating experiments were conducted on Eurypanopeus depressus and Rhithropanopeus harrisii from the Chesapeake Bay and Pachygrapsus transversus from Florida, while mated pairs of Uca beebei and U. terpsichores were collected from mudflats in Panama. All experiments and collections were conducted during the summer of 2006 and 2007. More sperm was transferred to larger than smaller females, and by species with long copulation durations (R. harrisii and E. depressus). These two species live in cryptic habitats, have high sperm/egg ratios, and likely store sperm across multiple broods. In contrast, P. transversus and U. beebei mate conspicuously, have short copulations, transfer fewer sperm, and have low sperm/egg ratios. Comparisons of sperm transfer across different mating strategies and habitats provide a better understanding of female sperm storage in the Brachyura.

Density affects female and male mate searching in the fiddler crab, Uca beebei

In most species, only one sex searches for mates while the other waits. Models of sex-specific mate-searching behavior predict single-sex searching, but the factors that determine which sex searches are not understood. In this study, we examine the effects of den- sity and predation risk on mate-searching behavior in the fiddler crab Uca beebei. U. beebei is one of the few fiddler-crab species in which both sexes search for mates. In a field experiment conducted in Panama, we manipulated crab density and perceived predation risk in replicate plots. Females searched more and males searched less at high densities. At high levels of per- ceived predation risk, both sexes similarly reduced their search rates. Observations of plots that naturally varied in crab density show that females were more likely to search for mates in areas of higher density, where there were more males. Females may preferentially search for mates in high-density areas because the abundance of nearby burrows, into which they can run to escape preda- tors, decreases their costs of searching and because the abundance of males and male burrows facilitates com- parisons and thus may increase their benefits from searching. Males at high densities decrease their mate- searching rate perhaps in response to the increase in fe- male searching and to the corresponding increase in the intensity of their competitors' mate-attraction signals.

Interspecific attractiveness of structures built by courting male fiddler crabs: experimental evidence of a sensory trap

Male fiddler crabs Uca musica sometimes build sand hoods and male Uca beebei sometimes build mud pillars next to their burrows to which they attract females for mating. Mate-searching females preferentially approach these structures and subsequently mate with structure builders. Here we show that the preference for structures is not species-specific and argue that it may not have evolved for mate choice. When not near burrows, many species of fiddler crabs approach and temporarily hide near objects, suggesting that hoods and pillars may attract females because they elicit this general predator-avoidance behavior. To test this sensory trap hypothesis we individually released female U. musica, U. beebei and Uca stenodactylus, a non-builder, in the center of a circular array of empty burrows to which we added hoods and pillars and then moved a model predator toward the females. All species ran to structures to escape the predator and the two builders preferred hoods. Next, we put hood replicas on male U. beebei burrows and pillar replicas on male U. musica burrows. When courted, females of both species preferentially approached hoods as they did when chased with a predator. However, males of both species with hoods did not have higher mating rates than males with pillars perhaps because hoods block more of a male's visual field so he sees and courts fewer females. Sexual selection may often favor male signals that attract females because they facilitate general orientation or navigation mechanisms that reduce predation risk in many contexts, including during mate search.

Male-biased predation of a fiddler crab

Great-tailed grackles, Quiscalus mexicanus, prey on fiddler crabs, Uca beebei, either by running straight at them or by running past them then angling sharply back to strike. Grackles that used angled runs caught only males and were twice as successful as birds that used straight runs and caught equal numbers of males and females. Why do grackles that use angled runs catch only males? Males were not differentially available, nor did grackles prefer them to females in choice tests. Fewer crabs entered their burrows when we moved a model predator past them than when we moved it directly towards them and crabs allowed the model to get closer when it passed then returned to strike. Although crabs may take more risks when birds use angled runs, the sexes did not differ in their escape responses. Hence, differential risk taking cannot explain male-biased predation by birds using angled runs. We suggest that males, with their large claws and lighter colours, are more conspicuous than cryptic females. When a bird runs past then turns to dash back at a crab it may be able to keep track of a male much better than a female. Indeed, grackles missed all females they struck at using angled runs. Hence, males may be conspicuous and preferred prey to birds using angled runs. Males' enlarged claws, bright colours and other sexually selected traits may increase male predation rate in this context.

Planktivorous fishes as selective agents for reproductive synchrony

Most shallow-water crabs release larvae during nocturnal maximum amplitude high tides, when larvae are rapidly transported by strong ebb tides to deeper waters at night. Larvae released at this time may avoid being eaten by planktivorous fishes, which abound in shallow water and feed during the day. Eurypanopeus planus (Smith, 1869), Uca beebei (Crane, 1941) and Pachygrapsus transversus (Gibbes, 1850) live on the Pacific coast of Panama and often release larvae during the day. Eurypanopeus planus also releases larvae without regard to tidal amplitude. Unlike most crabs, larvae of these three species will encounter, not avoid, planktivorous fishes. The predator avoidance hypothesis predicts that these larvae should be relatively well protected from predation compared to larvae of species that are released when predators are avoided. We tested this prediction experimentally by exposing larvae of these species and the sympatric crab, Cataleptodius taboganus (Rathbun, 1912), which exhibits the common hatching pattern, to silversides, Membras gilberti an abundant diurnal planktivore that feeds where all species release larvae. The rank order of silversides' prey preferences matched that predicted from the timing of larval release. Larval vulnerabilities to predation were related to the apparent conspicuousness of larvae, which is determined by the color of yolk and chromatophores. A survey of 30 other species of crabs further indicates that conspicuously colored larvae are released when they best avoid predators and that inconspicuously colored larvae may be released at other times.

Pillar building in the fiddler crab Uca beebei : evidence for a condition-dependent ornament

Pillar building in the fiddler crab Uca beebei : evidence for a condition-dependent ornament

Pillar building in the fiddler crab Uca beebei: evidence for a condition-dependent ornament

In the fiddler crab (Uca beebei) males build a small mud pillar next to their burrow which increases their attractiveness to females. Three hypotheses were tested to explain inter-male variation in pillar-building. (1) The benefits of pillar-building are density dependent. The experimental addition of vertical structures did not support this hypothesis as there was no change in the level of pillar-building. (2) There are two classes of males (pillar-builders and non-pillar-builders). This could either be due to an alternative mating strategy, or because pillar building is age or size-dependent. There was also no support for this hypothesis. (3) Pillar-building is an honest signal of male quality dependent on body condition. A food supplementation experiment was performed. Addition of food affected several aspects of male behaviour and resulted in a two fold increase in the number of pillars built between control and food treatments (P < 0.001). However, the percentage of males building pillars did not increase significantly. Pillar building in this species has been attributed to sensory exploitation. Our results indicate that a trait which may well have evolved through sensory exploitation also appears to be condition-dependent. We emphasise that showing that an ornament or behaviour is condition-dependent does not necessarily mean that it evolved through “good gene” processes. However, in terms of its current selective value, pillar building may be maintained through female choice because it acts as a signal of male condition.

A test for resource-defence mating in the fiddler crab Uca beebei

Abstract. Males of the fiddler crab Uca beebei defend, court from and attract females to burrows in intertidal mud flats. Females sequentially enter and leave several male burrows before they stay in one, mate and breed. This reproductive pattern is common among fiddler crabs and suggests that males may compete for females by competing for high-quality breeding sites and females may choose mates based on burrow quality. These ideas were tested for U. beebei by comparing structural features of (1) male and female burrows and (2) the male burrows females entered and left and those in which they mated. Male burrows had narrower openings and shafts than female burrows, but neither these nor 10 other burrow features differed between the male burrows females left and those in which they mated. Thus, there was no evidence of resource-defence mating in U. beebei. This pattern of mating behaviour may be rare in the genus because burrows suitable for breeding generally may be abundant and available to both sexes. Male fiddler crabs may compete for burrows due to their value for post-copulatory mate guarding and females may choose mates on the basis of courtship displays, not the quality of burrows for breeding.

Comparative Studies of Reproductive Behavior in Mantis Shrimps and Fiddler Crabs

Comparative ethological studies of closely related species can place into clear perspective the functions of behavioral traits as species diverge and adapt to different environments. In this paper we contrast the reproductive behavior of species within two crustacean groups: the mantis shrimps (stomatopods) and the fiddler crabs (genus Uca ). For the stomatopods, we identify probable selective relationships between features of species' ecology and their mating systems. Population density, the kind and availability of shelters in which these shrimps live, the intensity of predation and spawning cycles all play important roles in molding reproductive behavior in this group. Associations between the ecology and mating systems of fiddler crabs have been discussed recently elsewhere. Here we focus on a comparative analysis of sexual communication in these crabs. A study of competitive courtship signaling and mate choice in the fiddler crab Uca beebei has shown that certain male courtship signals are highly attractive probably because they exploit female sensory-response systems that have been molded by selection for escape from predators. Interspecific comparison of male courtship displays and the responses of females to these displays suggest that sensory exploitation may play an important role in the evolution of sexual signals in the genus. Comparative studies have advanced our understanding of how natural and sexual selection affect the reproductive behavior of both stomatopods and fiddler crabs.

Pillar Function in the Fiddler Crab Uca beebei (I): Effects on Male Spacing and Aggression

AbstractMales of 14 species of fiddler crabs (genus Uca) are known to build structures out of mud or sand at the entrances to burrows they court from and defend. A study of spacing, space use and aggression between courting male Uca musica (Zucker 1974, 1981) suggested that the hoods males build reduce territorial overlap and rates of aggression between neighboring males. Thus, each male may have more time to court females during limited lunar, diurnal and tidal mating periods.I studied the courtship and aggressive behavior of male Uca beebei in the field to determine if the pillars males of this species build affect male behavior as do the hoods of U. musica. U. beebei occurs sympatrically with U. musica on the Pacific coast of Panama and is broadly similar in its ecology and mating behavior. Unlike the hoods of U. musica, pillars did not focus a male's activity space away from its closest neighbor nor did they reduce either overlap with neighbors' activity spaces or rates of aggressive interaction among neighbors. Pillar builders courted more but also fought their neighbors more than did males that did not build pillars. The pillars of U. beebei and the hoods of U. musica affect male behavior differently and probably have different functions.

Pillar Function in the Fiddler Crab Uca beebei (II): Competitive Courtship Signaling

AbstractMale Uca beebei court and attract females into burrows they defend on muddy sand flats in the intertidal zone on the Pacific coast of the tropical Americas. Mating, oviposition and incubation (breeding) occur underground in males' burrows. Some courting males build mud pillars (2 cm high) at the entrance of their burrow. The purpose of this field study was to assess the role of pillars in competitive courtship signaling among males.I studied the effect of pillars on female behavior by recording the responses of wandering females to courtship from males resident at burrows with and without pillars. I also caught females, released them individually in a circular arena with an equal number of empty burrows with and without pillars around its circumference, and chased the females with a simulated avian predator. Females moved to burrows of both types more often when they were courted (82%) than when they were chased (67%). Receptive females were attracted to the burrows of the males that courted them significantly more often (97%) when these burrows had pillars than when they lacked pillars (66%). However, once females entered males' burrows they were equally likely to remain, mate and breed in both types of burrows. Females also more often moved to burrows with pillars (66%) than to burrows without pillars when they ran from the simulated predator. Both male courtship displays and pillars probably provide cues females use to locate males' burrows. The visual similarity between pillars and a display courting males give immediately before they enter their burrows suggests that pillars are icons of the display. The effect of pillars on female behavior, the timing of pillar building relative to when females choose mates, and contrasts in the behavior of males that do and those that do not build pillars suggest that pillar building has evolved due to competition among males to attract females into their burrows.

Female choice and the breeding behavior of the fiddler crab Uca beebei: Christy, J.H., 1987. J. crustacean Biol., 7(4):624–635. Smithsonian Tropical Res. Inst., AP 2072, Balboa, Republic of Panama

Female choice and the breeding behavior of the fiddler crab Uca beebei: Christy, J.H., 1987. J. crustacean Biol., 7(4):624–635. Smithsonian Tropical Res. Inst., AP 2072, Balboa, Republic of Panama

Female Choice and the Breeding Behavior of the Fiddler Crab Uca beebei

Female Choice and the Breeding Behavior of the Fiddler Crab Uca beebei

FEMALE CHOICE AND THE BREEDING BEHAVIOR OF THE FIDDLER CRAB UCA BEEBEI

ABSTRACT Fiddler crabs have been divided into species that mate and breed in burrows that males court from and defend (e.g., Uca pugilator), and species in which crabs pair on the surface and females breed in their own burrows (e.g., Uca vocans). I studied the reproductive behavior of the fidder crab Uca beebei at a site on the Pacific coast of Panama and found that this species exhibits both of these major modes of mating and breeding. Some females mated on the surface near burrows which they defended and used for oviposition and incubation. This may occur when females are large enough to defend their burrows successfully, when their burrows are suitable breeding sites, and when food is abundant nearby. Other females left their burrows, sampled the burrows of several courting males, then chose mates and breeding sites by remaining in males' burrows. These females did not select mates on the basis of size, nor did they prefer males that built pillars by their burrows. They consistently chose males whose burrows were longer (by 8%) and deeper (by 12%) than those they rejected. Long deep burrows may provide better thermal environments for incubation than short shallow ones.