Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw

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1 Behavioral Ecology Vol. 14 No. 5: DOI: /beheco/arg065 Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw Denson K. McLain, Ann E. Pratt, and Allison S. Berry Biology Department, PO Box 8042, Georgia Southern University, Statesboro, GA 30460, USA The enlarged (major) claw of male fiddler crabs is used in contests over breeding burrows and is waved to attract females. We recently discovered that males of the red-jointed fiddler crab, Uca minax, also use the claw to kill smaller-sized fiddler crabs, U. pugnax and U. pugilator, with which they co-occur in Atlantic coast salt marshes. Large U. minax males use walking legs or the enlarged claw to capture prey feeding on moist sand flats. On sand flats, small U. minax males and females are much less common than large males, suggesting that large males move onto sand flats to seek prey. Males of prey species use the major claw against attacking predators and, consequently, are more likely than females to escape. In laboratory experiments, large U. minax males were more likely to attack and kill small-clawed males and females than large-clawed males, consistent with a preference for more vulnerable, less threatening prey. The size of the major claw is a positive allometric function of body size. The allometric function varies little among species. Also, the mechanical advantage and indices of closing speed and closing force of the major claw, when corrected for body size, are not consistently greater in U. minax relative to prey species. Thus, predation by U. minax males may reflect the opportunity afforded by larger body size and positive allometric growth, which result in a major claw that is more massive than the prey it is directed against. Key words: allometry, body size, defense, fiddler crabs, predation, Uca. [Behav Ecol 14: (2003)] Sexual selection on male fiddler crabs has favored the evolution of an enlarged (major) claw that is, perhaps, the most exaggerated sexually selected structure of animals (Crane, 1975). Within a population, claw size correlates with mating success (Pratt and McLain, 2002), indicating continued selection for enlargement. The claw is waved to attract females to breeding burrows (Crane, 1975; Pope, 2000) as well as used as a weapon in agonistic contests between males for possession of breeding burrows (Crane, 1975; Hyatt and Salmon, 1978; Jennions and Backwell, 1996; Salmon and Atsaides, 1968). In some species, large males may use their major claw to kill smaller crabs. The Pacific fiddler crab, Uca tetragonon, kills conspecifics and congeneric fiddler crabs that it encounters near its intertidal burrows (Koga et al., 1995). Large males of U. minax leave their supratidal burrows in maritime forests and stalk individuals of U. pugilator and U. pugnax on intertidal sand flats of Atlantic coast salt marshes (Pratt et al., 2002). We examined (1) the distribution of U. minax in time and space with respect to the distribution of its prey, (2) prey susceptibility and defensive use of the claw by potential prey, (3) predatory tactics and prey choice in the field and laboratory, (4) the allometry of claw size, and (5) mechanical indices of claw function in predator and prey species. Predation by fiddler crabs is of interest because they are considered quintessential deposit feeders (Crane, 1975; Genoni, 1991; Murai et al., 1982; Robertson et al., 1980; Weissburg, 1992). Females, using small, paired feeding chelae, and males, using their single small chela, scoop moist sediment that is ingested, then separated into organic and inorganic fractions (Miller, 1961). Chelae and mouthparts of U. minax appear adapted to utilize fine, muddy sediment containing detritus (Miller, 1961). Predatory fiddler crabs kill prey with the sexually selected claw that evolved its substantial closing force (Levinton and Judge, 1993) in the context of ritualized fights for possession of breeding burrows (Crane, 1975). This contrasts with the claws of other kinds of predatory crabs, which, though sexually selected in males, are not sex limited and are used by both sexes in predation (Vermeij, 1977). METHODS Study dates and field site Observations were made on 25 dates from 19 February 2000 to 21 October 2001 on Cat Island, South Carolina, USA, in a smooth cordgrass (Spartina alterniflora) marsh adjacent to the Beaufort River, an Atlantic coast saltwater estuary. Here, three species of Uca co-occur along a north-facing sand flat measuring approximately m. U. pugilator burrows primarily occur along the up-slope margin of the sand flat where black needlerush ( Juncus roemerianus) grows, while U. pugnax burrows primarily occur in muddy down-slope sediments supporting glasswort (Salicornia virginica) and smooth cordgrass. U. minax burrows occur m up-slope of black needlerush in a lowland maritime forest (Johnson et al., 1974). U. pugilator (Salmon and Hyatt, 1983) and U. pugnax (Pratt and McLain, 2002) deposit-feed in mixed-species droves containing hundreds to thousands of individuals. Droving occurs on sand flats when the sediment is moist (Miller, 1961; Murai et al., 1982). Address correspondence to D. K. McLain. dk_mclain@ gsvms2.cc.gasou.edu. Received 12 February 2002; revised 18 December 2002; accepted accepted 22 January Ó 2003 International Society for Behavioral Ecology Field observation and collection To observe predatory attacks by U. minax, we sat on sand flats among droves. Using binoculars or an auto-focus video camera with a zoom lens, we observed the movement and activity of U. minax individuals in 1 2 h sessions. Each attack

2 742 Behavioral Ecology Vol. 14 No. 5 chi-square values and degrees of freedom of the two models compared. Figure 1 Claw of male fiddler crabs. Measurements taken of the dactyl and propodus, consisting of the manus and pollex, are indicated. we observed was permitted to continue until the prey was killed or escaped. In every case, we captured the predator and retrieved the prey. We assessed the pool of potential prey by collecting droving U. pugnax and U. pugilator. Droves were charged from opposite directions, preventing differential escape by size, sex, or running speed, and individuals were scooped into plastic pails. Prior use of pit traps indicated that this technique yields representative subsamples. We sexed individuals by asymmetry of the claws and by the shape of the abdomen (much wider in females). We also collected U. minax from the marsh margin as encountered. The composition of sand flat droves changes over the lunar cycle. Therefore, to examine prey choice in the field, we compared prey of a given date to the mean size (carapace width) or mean propodus (claw) length of other droving crabs on the same date. Carapace difference is mean width of drovers minus the width of a given prey individual. Claw difference is the mean size of the major claw (¼ product of the mean among males and the proportion of males) minus the major claw size of a given prey individual. Females were assigned a major claw size of 0, as were males who had previously autotomized but not yet regenerated their claws. We tested prey choice by determining if the confidence interval for carapace or claw difference contained 0. Mantel-Haenszel chi-square tests were used with categorical data when it was necessary to control for sampling date. We used log linear models (Hinckle et al., 1989) to simultaneously test for an effect of relative claw or carapace size and sex on the risk of predation in the field. All individuals collected on a sampling date were assigned values of þ1 or 1 to indicate if they were attacked, had a smaller carapace than the mean on that date, or had a smaller claw than the mean on that date. We determined the significance of an effect, such as the interaction between relative claw size and attack status, by comparing a model with all interactions to models missing specific interaction terms. The chi-square value and appropriate degrees of freedom associated with the interaction were determined from the differences in Laboratory experiments We conducted five experiments in the laboratory using crabs collected at the field site on sampling dates in March April and September Experiments were conducted in 35-cm tall, round plastic buckets with a bottom diameter of 25 cm. Each bucket contained one predator, usually a large U. minax with a propodus (claw) length of at least 40 mm. Buckets contained saltwater, mixed to 20 ppt (Instant Ocean synthetic sea salt, Aquarium Systems, Mentor, Ohio), at a depth of 0.75 cm. We placed predators in buckets upon return to the laboratory and gave them 1 day to habituate before an experiment began. We kept prey in a 65-l cooler containing saltwater and fish food (TetraMin tropical crisps, Tetra Sales, Blacksburg, Virginia) for 1 day before the start of an experiment. Once an experiment began, buckets were examined three times daily to determine if predation had occurred. We used two types of potential prey: (1) large-clawed males, with a major claw propodus length (see Figure 1).20 mm, and (2) small- or nonclawed individuals, which included females and males with a major claw of propodus length,14 mm. Males with claws of intermediate propodus length (14 20 mm) were not used in experiments. We conducted a pilot study to determine necessary sample sizes. Fifteen U. minax males were each presented with a single small-clawed U. pugilator male, and nine attacked their prey. Assuming that small- or nonclawed fiddler crabs would be twice as vulnerable as large-clawed males, which available field data suggested was reasonable, we calculated that sample sizes would need to be to detect a significant difference with a chi-square test at a levels between 0.05 and In the first experiment, we provided half of 43 U. minax predators with a small- or nonclawed U. pugilator and the other half with a large-clawed U. pugilator. After 2 days, each bucket was cleaned and the predator was provided with fresh saltwater and a prey of the opposite type. The second part of the experiment ran for another 2 days. We conducted a second experiment, using 64 U. minax males, in the same manner except that the prey species was U. pugnax. In both of these experiments, we monitored buckets continuously for the first hour that U. minax were exposed to prey to determine if prey types were attacked at the same rate that they were consumed. In the third experiment, we simultaneously presented two U. pugilator prey to the U. minax predator (n ¼ 53). The two prey were either one large-clawed and one small- or nonclawed, or two small- or nonclawed. As described above, half the predators initially encountered one prey combination and half the other. After 2 days, we reversed combinations among predators for another 2 days. The fourth and fifth experiments used either U. pugilator (n ¼ 32) or U. pugnax (n ¼ 46) as predators exposed to a single female of the other species for 2 days. Males in these trials were habituated to buckets in the same manner as U. minax. We ran all replicates of each experiment concurrently. The spatial distribution of predator prey combinations was mixed to minimize any effect of minor spatial variation in noise, lighting, or air drafts. In experiments where a predator was sequentially offered types of prey, the order in which prey were encountered did not affect the probability of being killed (v 2 ¼ 1.106, df ¼ 1, p ¼.293). The probability of predation on the first prey encountered was 45.9% (83 of 181), while for the second prey it was 51.4% (93 of 181). Nevertheless, we used Mantel-Haenszel chi-square tests to control for the order of encounter of prey types.

3 McLain et al. Predation by fiddler crabs 743 Claw measurements We measured predators and prey in the field. Carapace width was measured as an index of body size. We also measured the length of the propodus of the major claw in males. Measurements were made to the nearest 0.1 mm with electronic digital calipers. In the laboratory, we determined propodus length, mass of the major claw, carapace width, and body mass for collections of males of U. minax (N ¼ 200), U. pugnax (N ¼ 159) and U. pugilator (N ¼ 144). We measured body masses with an electronic balance to the nearest milligram. Each claw was removed by applying pressure with forceps at its proximal segments, the basis and coxa, until the claw was autotomized. Crabs readily autotomize and regenerate claws. Nearly half of all males in natural populations possess a regenerated claw (Blackwell et al., 2000). We preserved a subset of claws of U. minax (n ¼ 33), U. pugnax (n ¼ 59), and U. pugilator (n ¼ 71) at 20 C. These claws came from individuals with carapace widths of at least 60% that of the largest conspecific within the original sample (widths: U. pugilator. 12 mm, U. pugnax mm, U. minax. 15). Measurements on these, made to the nearest 0.01 mm with an ocular micrometer on a dissecting scope, permitted analyses of claw mechanics. Measures included the height (dorsal-ventral axis), depth (medial-lateral axis), and length (distal-proximal axis) of the manus (Figure 1). In addition, we measured the length of the dactyl (see L 2 of Figure 1) and the distance between the hinge point of the movable dactyl and the point of insertion of the apodeme at the base of the dactyl (see L 1 of Figure 1) to permit calculation of mechanical advantage at the dactyl tip, L 1 /L 2 (Warner and Jones, 1976). The closer muscle within the manus pulls the apodeme proximally, causing rotation about the hinge that swings the dactyl tip toward the tip of the pollex (Alexander, 1968). The force exerted by the closer muscle is a function of its crosssectional area, which correlates with both the height and depth of the manus (Vermeij, 1977). Empirical tests revealed that the interior volume (ll) of the manus is closely approximated by the product height 3 length 3 depth, where all linear measurements are in millimeters. Therefore, the cross-sectional area of the manus interior is approximated by the product, manus height 3 manus depth This product is used to index the force applied by the closer muscle at the juncture of the dactyl and apodeme. The index of closing force at the dactyl tip is the product of mechanical advantage and the index of the force applied at the apodeme (Warner and Jones, 1976). The speed and distance traveled by the dactyl tip are inversely proportional to mechanical advantage but proportional to the length of the closer muscle (Warner and Jones, 1976). To index this length, we measured the length of the apodeme (Figure 1). We tested interspecific differences in the mechanics of claw closure by using species as the categorical variable and propodus length as the covariate indexing claw size. The length of the propodus was used because this length is not a component of any measure of mechanical advantage, muscle size, or force applied. RESULTS Predatory behavior Uca minax males moved into droves of their prey by using a gait that was both slower and lower in profile than the gait used otherwise. U. pugilator and U. pugnax typically moved away from a close-approaching predator, maintaining a distance of cm. Some U. minax sat submerged in water-filled depressions, with only their eye stalks peering above the surface of the water, until droving prey moved near. A predator would lunge or jump at a potential prey when the distance between them was about 5 cm, attempting to enwrap the prey in the walking legs. Other prey were chased for up to 1 m. A successful chase resulted in the prey being caught in the major claw of the predator. Predators used their anterior walking legs to manipulate prey into a position where the claw could be used to pierce or crack the carapace of the captured crab. With larger prey, the predator often rolled onto his dorsal surface to position prey, which could take as long as 10 min. In almost all cases, death appeared to immediately follow the breaking of the carapace. About 5 min was required for U. minax to consume its prey. The test of the carapace and the major claw, if present, were discarded. Before being consumed, prey were sometimes carried to the marsh margin, impaled on the claw, or cradled in walking legs. Size and sex of U. minax on the sand flat No male or female U. minax with a carapace width of less than mm was observed on the sand flat. The mean carapace width of males was mm (SD ¼ 2.00), whereas that of females was mm (SD ¼ 1.66). Males and females with carapace widths as small as 5.8 mm were observed in needlerush and under racks of smooth cordgrass along the marsh margin. There was a significant difference in the size of U. minax on the sand flat versus along the marsh margin (effect for place, F ¼ ; effect for sex, F ¼ ; interaction effect, F ¼ 60.37; df ¼ 1,1270; p,.001 in all cases). The proportion of females on the sand flat (70/836 ¼ 8.4%) was much less than the proportion along the marsh margin (126/368 ¼ 34.2%; v 2 ¼ , df ¼ 1, p,.001). We observed 166 captures of U. pugilator or U. pugnax by U. minax males. Numerous chases were observed that did not result in prey capture. Females of U. minax were never observed to attack other fiddler crabs. The smallest U. minax male observed to capture a droving fiddler crab had a carapace width of 18.0 mm. The smallest U. minax male observed to capture, kill, and consume another fiddler crab had a carapace width of 19.9 mm. The average size of male U. minax capturing prey (24.07 mm, SD ¼ 1.82, n ¼ 166) was slightly but significantly larger (F ¼ 11.97, df ¼ 1,834, p ¼.001) than the mean of males without prey (23.45 mm, SD ¼ 2.02, n ¼ 689). Predators with captured prey that managed to escape were smaller than those with prey that did not escape (escaped: mm, SD ¼ 2.00, n ¼ 8; did not escape: mm, SD ¼ 1.76, n ¼ 143; F ¼ 9.12, df ¼ 1,149, p ¼.003). Prey choice in the field The composition of droves varied by species, sex, body size, and claw size. The species composition of prey was not different from that of droves (v 2 ¼ 2.23, df ¼ 1, p ¼.135). The carapace width of captured prey ranged from 8.2 to 19.3 mm (mean ¼ 13.19; SD ¼ 2.24), while that of drovers ranged from mm (mean ¼ 13.84; SD ¼ 2.31). The difference between the mean carapace width of drovers and that of individual prey on the same date was significantly greater than 0 (mean carapace difference ¼ 0.88 mm, SD ¼ 2.22; t ¼ 7.73, df ¼ 165, p,.001; Figure 2). Claw (propodus) length of captured prey ranged from 0 to 25.7 mm (mean ¼ 7.20; SD ¼ 8.39) and that of drovers ranged from mm (mean ¼ 11.20; SD ¼ 2.31). The difference between the mean claw length of drovers and that of individual prey on the same date was significantly greater than 0 (mean claw difference ¼ 5.16 mm, SD ¼ 8.58; t ¼ 12.85, df ¼ 165, p,.001).

4 744 Behavioral Ecology Vol. 14 No. 5 result was obtained when prey were U. pugnax (48 of 64 smallor non-clawed eaten ¼ 75%; 6 of 64 large-clawed eaten ¼ 9%; Mantel-Haenszel v 2 ¼ 56.09, df ¼ 1, p,.001). Large-clawed males constituted 5 of 28 (¼ 18%) kills during the first hour of continuous monitoring of these experiments. This is similar to the frequency killed during the remainder of the experiments, 6 of 67 (¼ 9%; Kruskal-Wallis test, v 2 ¼ 1.51, df ¼ 1, p ¼.219). Figure 2 Carapace width of prey compared to the mean of droving fiddler crabs on the same date. Note that 112 of 166 prey widths are smaller than the corresponding mean. Droves were typically male biased (64% overall). Prey, however, were not male biased (51% overall). Thus, females were more likely to be attacked and captured relative to their frequency in droves (Mantel-Haenszel v 2 ¼ 18.27, df ¼ 1, p,.001). Log linear models revealed a significant interaction between claw length and the probability of being captured by U. minax (v 2 ¼ 25.70, df ¼ 1, p,.001). However, neither sex (v 2 ¼ 3.32, df ¼ 1, p ¼.057) nor carapace width (v 2 ¼ 2.73, df ¼ 1, p ¼.070) had a significant effect on the probability of capture in models containing the claw length interaction. Claw length was smaller relative to carapace width for captured U. pugilator males than for droving U. pugilator males (ANCOVA, effect for carapace width: F ¼ , df ¼ 1, 1434, p,.001; effect for carapace width-by-prey status interaction: F ¼ 28.70, df ¼ 1, 1434, p,.001). This was also the case for U. pugnax (carapace width: F ¼ , df ¼ 1, 187, p,.001; interaction: F ¼ 8.91, df ¼ 1, 187, p ¼.003). Defensive use of claws At the field site, we observed four instances of captured prey using their claws defensively to pinch the major or minor claws of the predator. In every instance, the prey autotomized its claw and escaped. On 10 other occasions, major claws were autotomized by captured prey who escaped in eight instances. One large male (propodus length.20 mm) of each prey species was grabbed, then immediately released. No other prey were released. Males were significantly more likely than females to escape (17/84 ¼ 0.20 vs. 3/82 ¼ 0.04; v 2 ¼ 10.77, df ¼ 1, p,.001). Laboratory experiments Experiments 1 and 2 U. minax males sequentially exposed to U. pugilator were significantly more likely to kill small- or nonclawed individuals (36 of 43 ¼ 84%) rather than large-clawed males (5 of 43 ¼ 12%; Mantel-Haenszel v 2 ¼ 41.07, df ¼ 1, p,.001). A similar Experiment 3 U. minax males simultaneously exposed to one large-clawed male and one female of U. pugilator attacked and consumed the female first 81% of the time (26 of 32 cases) and ate a total of 29 females to 11 males (v 2 ¼ 26.50, df ¼ 1, p,.001). Each predator afforded the choice between a male and female was also exposed to two females together. Predators consumed males after first consuming a female less frequently than they consumed females after first consuming a male or female (v 2 ¼ 15.56, df ¼ 1, p,.001). Thus, predators eating one member of a prey combination were more likely to eat the second member if the combination was two females (21 of 29 cases ¼ 72%) rather than a male and a female (13 of 37 cases ¼ 35%; v 2 ¼ 8.99, df ¼ 1, p ¼.003). The vulnerability of females was not reduced by the presence of a large-clawed conspecific male. U. minax males were as likely to eat a female when exposed to two U. pugilator females as when exposed to one female and one large-clawed male (Mantel-Haenszel v 2 ¼ 1.41, df ¼ 1, p ¼.24). Experiments 4 and 5 No females of U. pugilator (n ¼ 46) or U. pugnax (n ¼ 32) were attacked by large-clawed males of the other species. Claw size and operational indices Allometry of claw to body size Claw length increased as a positive allometric function of carapace width in all three species (Table 1). Controlling for body size, U. pugnax claws were longer than those of either U. pugilator or U. minax. U. pugilator and U. minax did not differ significantly (Table 1). Claw mass also increased as a positive allometric function of body mass in all three species (Table 1). Controlling for body size, U. pugnax claws were more massive than those of either U. pugilator or U. minax. U. pugilator and U. minax did not differ significantly (Table 1). However, controlling for propodus length, the major claws of U. minax were more massive than those of U. pugnax but similar in mass to those of U. pugilator (Table 1). Indices of mechanical function The mechanical advantage of the major claw decreased with propodus length in all three species (Table 1). Controlling for propodus length, the mechanical advantage for U. minax was greater than for U. pugnax but did not differ significantly from U. pugilator (Table 1). The index of closing force was a positive linear function of propodus length in all three species (Table 1). Controlling for propodus length, the index was greater in U. minax than U. pugnax but did not differ significantly between U. minax and U. pugilator (Table 1). The length of the apodeme, which indexes the speed of action of the closer muscle, was a positive linear function of propodus length in all three species (Table 1). The index of closing speed, with propodus length controlled, was significantly greater in U. minax than U. pugnax but did not differ significantly between U. minax and U. pugilator (Table 1).

5 McLain et al. Predation by fiddler crabs 745 Table 1 Comparison of allometric relationships and mechanical indices for the major claw of 3 fiddler crab species U. minax U. pugnax U. pugilator U. minax vs. U. pugnax U. minax vs. U. pugilator Propodus (mm) ¼ b 3 [carapace width] a a b F df 2,195 2,157 2,142 1,353 1,338 p,.001,.001,.001, R Claw mass (mg) ¼ b 3 [body mass] a a b F df 2,195 2,157 2,142 1,353 1,338 p,.001,.001, R Class mass (mg) ¼ b þ a 3 [propodus] 3 a b F df 1,195 1,157 1,142 1,353 1,338 p,.001,.001,.001, R Mechanical advantage ¼ b [a(propodus)] a b F df 1,31 1,57 1,69 1,89 1,101 p.009,.001,.001, R Closing force ¼ b þ [a(propodus)] a b F df 1,31 1,57 1,69 1,89 1,101 p,.001,.001,.001, R Closing speed ¼ b þ [a(propodus)] a b F df 1,26 1,52 1,67 1,89 1,94 p,.001,.001,.001, R DISCUSSION Behavior of predators At the study site, U. minax individuals must move m from their up-marsh burrows to reach sand flats. Three lines of evidence suggest that this movement is a consequence of an active predatory lifestyle. First, the individuals of U. minax that venture onto sand flats are predominantly those capable of capturing and killing members of prey species. Second, the gastric mill and maxillipeds are adapted for deposit-feeding on fine-grained silty and muddy sediments, not the coarse sand of flats (Miller, 1961). Third, predatory acts are readily observed in the field and laboratory. Tissue of congeneric prey is energy rich compared to sediment. Therefore, predation may permit males to reallocate some deposit-feeding time to mating-related activities (see Caravello and Cameron, 1987; Genoni, 1991; Weissburg, 1993). Movement of U. minax onto the sand flat only follows spring tides or rains, which is also when the number of females and small males of U. pugilator and U. pugnax in droves is especially high (Pratt et al., 2002). This results in a high rate of encounter between large U. minax males and vulnerable prey. The major claw of fiddler crabs can be an effective deterrent against some avian predators (e.g., Bildstein et al., 1989). Our field and laboratory data also indicate an effective defensive role against U. minax. Continuous monitoring of laboratory experiments for the first hour revealed that failure of predators to kill large-clawed prey was largely a consequence of their failure to attack such prey. Claw size and operational indices The major cheliped of fiddler crabs bears an enlarged claw that makes it the most elaborate appendage in crustaceans and

6 746 Behavioral Ecology Vol. 14 No. 5 little, if anything, compares among all animals to the degree of exaggeration of the major claw (Crane, 1975). Sexual selection appears responsible for evolution of the major claw which functions in visual displays (waving), acoustic signaling (drumming), and ritualized combat over breeding burrows (Crane, 1975; Hyatt and Salmon, 1978; Salmon and Atsaides, 1968). In the three species examined here, the length of the claw approaches twice the width of the body, whereas the mass of the major cheliped exceeds half the mass of the rest of the body. Fiddler crab claws possess more than 80 morphological features, including ridges, mounds, grooves, pits, depressions, and associated tubercles, most of which appear to be adaptations for use of the claw in specific elements of ritualized fighting (Crane, 1975). The claws of different species of fiddler crabs are highly specialized with respect to these features, reflecting how ritualized fights are conducted (Crane, 1975). However, the biomechanical characteristics of major claws, such as closing force, which increases with overall claw length, and mechanical advantage, which decreases with overall claw length, are similar between species of all six subgenera (Levinton and Judge, 1993; Levinton et al., 1995). This suggests that selective forces on claw morphology have been consistent among species because of similar use in intersexual signaling and intrasexual fighting. Perhaps as a consequence, the claws of the predatory fiddler crab U. minax do not appear to be specially adapted for predation. U. minax claws are neither longer nor more massive, as allometric functions of body size, than those of the prey species, U. pugilator. Also, when claw length is controlled for, the claws of these species do not differ in mechanical advantage or the index of closing force at the dactyl tip. The claws of U. minax do have a pronounced tuberculate tooth on the pollex approximately mid-way out from the manus. Analogous to similar teeth in molluscivorous crabs (Vermeij, 1977), this tooth may be used to deliver an especially strong carapace-puncturing force to fiddler crab prey due to its location nearer the manus, resulting in increased mechanical advantage. Pronounced tuberculate teeth are found in many species of fiddler crabs (Crane, 1975), including U. pugnax. Thus, the presence of such a structure in U. minax is not suggestive of selection for increased efficiency as a predator, although it may, in fact, facilitate predation. The larger overall body size of U. minax coupled with positive allometry in the size of the major claw results in a claw that is enormous relative to the body size of individuals of the prey species, U. pugnax and U. pugilator. This, rather than any enhancement of biomechanical characteristics such as closing force and the speed of the dactyl-apodeme lever system, appears to have preadapted large males of U. minax for predation on smaller fiddler crab species. Preadaptation Alternative use of sexually selected traits is not uncommon. For instance, sexually selected horns, tusks, and fangs that evolved for use in agonistic contests between males are sometimes used for defense against predators (Andersson, 1994). The major claw preadapts fiddler crabs for predation. Large males possess grabbing and pinching structures that have increased in size disproportionately with the growth of their bodies and that have a crushing power proportional to their length. Use of the claw as an instrument of predation may only require that some agonistic behavioral elements already used in burrow defense or burrow acquisition be used in a new context. Even mating behaviors may be used in the context of predation. Males of directing or herding fiddler crab species push and pull struggling females into burrows after first enveloping them between their flexed major claw and body (Zucker, 1986), much like U. minax secure their prey. Even the action of a U. minax male on his back, grappling with a prey that he holds in his walking legs, is reminiscent of mating in fiddler crabs. Males of U. vocans and U. thayeri may chase females, then maneuver them into a ventral-to-ventral position with their walking legs (Salmon, 1984, 1987). Thus, the capture and manipulation of prey into a position where they can be killed bears similarity to behaviors used in other contexts. Large relative body size favors predation by members of one species on congeners (e.g., Hurd, 1988; Griffiths et al., 1994; Snyder and Hurd, 1995; Southerland, 1986). Uca tetragonon, the only other fiddler crab known to prey on other crabs (Koga et al., 1995) kills congeners, Uca vocans and Uca annulipes, that are much smaller in maximum body size (Crane, 1975). The relatively large size of U. minax compared to prey species, U. pugnax and U. pugilator, enhances its ability to secure and kill individuals. Yet, large males of one prey species do not kill small members of the other species. This suggests that the predatory habitat of U. minax is not just a consequence of its greater body size but is also an adaptive response to opportunity. As relative body size of one species increases, the opportunity to prey on congeners is enhanced because (1) the proportion of congeners encountered that can be captured and killed will increase, and (2) exposure and vulnerability to prey defenses will decrease as prey can be more quickly secured and killed. Thus, the relatively large body and claw size of U. minax afforded males more opportunity to prey on congeners. Selection has favored the predatory habit in concert with increased opportunity. We have observed U. minax males kill and eat fiddler crabs in Carolina Beach, North Carolina, and therefore, suspect that they prey on other fiddler crabs across their range. We predict that other species of fiddler crabs will also behave as predators on congeners when differences in body size bring increased opportunity. In fact, we observed three large males of Uca rapax actively stalk, capture, and kill smaller U. pugilator individuals on a sand flat near the Matanzas River in Crescent Beach, Florida. At this locality, Uca rapax males attain a maximum body size similar to that of U. minax males. As the claw of fiddler crabs can grow to 40% of total body mass (Weissburg, 1992), economic considerations favor its additional use outside of mating competition if fitness or energy gains more than compensate for the cost of any alternative use. This in turn might permit even greater response to sexual selection (McLain, 1993). Given that breeding-associated activities such as burrow defense and waving limit feeding opportunities for male fiddler crabs (Caravello and Cameron, 1987; Genoni, 1991; Pratt and McLain, 2002), the use of claws for predation on congeners may be especially favored when there is sufficient opportunity to acquire prey. REFERENCES Alexander RM, Animal mechanics. 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