The alteration of signals of animals in response to changes in environmental factors is a common phenomenon. In male fiddler crabs, waving major claws towards females is energetically costly; thus, males need to adjust their waving in a way that increases the chance of potential mate attraction while reducing the waving cost. In this study, I examined how Austruca perplexa males adjusted their waving rate based on male-male competition (male numbers in a cluster [Austruca perplexa males make groups and wave synchronously towards females]), female body size, and the distances of the receiver females from the signaller males. Forty clusters were selected randomly; from each cluster, I randomly selected one male, video recorded his waving behaviour and calculated waving rate (waves/min). Body size (carapace width) and distances of receiver females were measured. To analyze the effects of competition, female size, and their distances on male waving rate through binary logistic regression analysis, all variables were divided into two categories (male waving rate: low and high, competition: low and high, female size: small and large, and female distances: short and long) based on a median split method. Afterwards a series of binary logistic regression models were built and the relative supports of various models were assessed based on the corrected Akaike information criterion. Results showed that competition, female body size and their distances affected the male waving rate in an additive manner, but their interactions did not show any effect. Further research can be conducted to investigate how breeding season and predation risk along with competition, female size and female distances affect the claw-waving display of male fiddler crabs.
This study determines the winning status of regenerated-clawed males (regenerated males) in contests with original-clawed males (original males) in Austruca perplexa (H. Milne Edwards, 1852), and how they deal with original opponents. We observed that the numbers of regenerated and original winners were not different. The regenerated contestants, who were either burrow owners, or larger than their opponents, or avoided escalated contests (claw-interlock/fling), had the same chance of winning in comparison to their original opponents, and had a higher winning chance than regenerated contestants, who were either intruders, or smaller than their opponents, or did engage in escalated contests. Regenerated intruders tended to challenge opposite-handed opponents for avoiding claw-interlock (the risk of injury is high during claw-interlock), because claw-interlock is mechanically difficult between opposite-handed contestants. This study shows that regenerated contestants, who are either residents, or larger than opponents, or avoid fighting escalation, are able to compensate for the detrimental effect of regenerated claws during contests.
Females often choose mates based on their courtship signals. Males may signal their heritable genetic quality, defended resources, or parental care efforts; however, the reasons why females choose males based on their signals are often not clear. Here, we show that, in the fiddler crab Austruca perplexa, male signals (major-claw waving rates) were correlated with important characteristics of their defended resources (width and depth of breeding burrows). By using the male signals, females may be able to roughly predict the burrow quality and decide whether to enter and check the burrow characteristics. The signals are predicted to be honest because the female’s final decision is based on burrow quality. Since females can reject males if their burrow quality is insufficient for breeding, the courtship efforts of deceptive males will be dismissed. The honesty of the signals is beneficial for both sexes and thus easily evolved in their signalling system.
We examined the effects of population density on body size and burrow characteristics of Uca bengali Crane, . We predicted that (1) males in high-density areas (HD) should be larger in size and build higher quality burrows than males in low-density areas (LD), and (2) HD females should be larger in size, but build lower quality burrows than LD females, as HD females can find higher numbers of good quality male burrows around them for breeding and egg incubation. Our results showed that males and females in HD were larger in size than those in LD. Since HD males were larger in size, they built higher quality burrows than males in LD. On the other hand, even though LD females were smaller in size than HD ones, they built higher quality burrows than HD females. Our results thus indicate that density effects both body size and burrow characteristics.
We investigated the amount of time that large and small, male and female fiddler crabs Uca annulipes (H. Milne Edwards, 1837) spent on feeding, walking, standing, grooming, burrowing, inside burrows, fighting, and courtship waving. We video-recorded the activities of 45 males (22 small and 23 large), and 39 females (19 small and 20 large) each for 5 min, and calculated the percentage of time spent on each activity/crab. Our results showed that both sexes spent more time on feeding than on other activities. Males spent more time on building burrows, walking, and grooming than females, and females spent more time inside burrows than males. Smaller males spent more time on feeding, and less time on building burrows and on waving than larger ones. There were no relations between female body size and activities. Feeding rate/feeding claw was higher in males than in females, and crab body size was negatively associated with feeding rate/min.
We investigated whether male Uca vocans (Linnaeus, 1758) have behaviourally or morphologically compensated for having only one feeding claw while females have two. We predicted that (1) females will have a higher feeding rate/crab per min, (2) males will have a higher feeding rate/claw per min, (3) males will have larger feeding claws (dactyl length and width) than comparable-sized females, and (4) smaller crabs will feed faster than larger ones and thereby satisfy their feeding demand more rapidly than larger crabs. Our results supported all four predictions. Males compensated for having one feeding claw by feeding faster/claw per min with larger claws.
We investigated how male Uca rosea (Tweedie, 1937) have behaviourally or morphologically compensated for having only one functional feeding claw while females have two. We found that male U. rosea used four compensatory mechanisms: (1) larger feeding claws (dactyl length and width), (2) higher feeding rate/claw per min, (3) higher numbers of pinches/feeding claw per min than similar sized females, and (4) higher numbers of pinches/feeding claw lift than females of similar feeding rate/feeding claw per min. This study is the first one to demonstrate that taking higher numbers of pinches/feeding claw per min than comparable sized females, and taking higher numbers of pinches/feeding claw lift than females of similar feeding rate/claw per min are used as additional compensatory mechanisms for male fiddler crabs to compensate for having only one feeding claw.
We studied the population structure of Uca bengali Crane, (crab density, sex ratio, and body size (i.e., carapace length and width, major claw length and width)), along with burrow characteristics (diameter, length, depth, and volume) as well as sediment characteristics (percent moisture content and organic matter) along a bank gradient at three distances (2, 4 and 6 m) from the edge of a tidal river. The results showed no differences in crab density, sex ratio, and body size among the three distances. The percent moisture content at 2 m distance was higher than at 4 and 6 m, and the percentage organic matter at 2 m distance was higher than at 6 m distance. Burrows were shorter in length, shallower in depth, and smaller in volume at 2 m distance, but these measures increased with increasing distance from the river edge. Both males and females made three distinct burrow shapes: I, J and U; the J-shaped burrows were the most numerous at all distances.
We tested the effects of body size, resident status, handedness, and claw originality on Uca rosea (Tweedie, 1937) male fights. We observed 67 pairs of combats between residents and intruders, recorded fighting duration and winner identity (resident/intruder, larger/smaller), identified handedness (right/left claw) and claw types (original/regenerated claw), and measured carapace width and major claw length. Larger/smaller males were determined based on major claw length instead of carapace width. Results showed that (1) body size and resident status jointly and (2) body size, resident status, handedness and claw types together, affected fighting success. Residents won more combats than intruders regardless of handedness. Original clawed residents won more combats than regenerated clawed intruders. Fighting duration was longer in same claw type (original-original and regenerated-regenerated) combats than in different claw type combats. Carapace width was more strongly correlated with original claw length than with regenerated claw length.
We examined the effects of mudballs around burrows and of sex on burrow characteristics (shape, diameter, length, depth and volume) in an underground mating fiddler crab species, Austruca annulipes (H. Milne Edwards, 1837). We investigated 35 burrows with mudballs (males: 20; females: 15) and 34 burrows without mudballs (males: 16; females: 18), and measured their burrow characteristics. Results showed that burrow characteristics did not differ between burrows with and without mudballs. Males built larger-sized burrows with greater volumes, and had more mudballs than females. Crabs built J-, I-, L-, S- and U-shaped burrows, with higher numbers of J-shaped burrows in males, and higher numbers of I-shaped burrows in females. J-shaped burrows were larger with greater volumes, and had more mudballs than I-shaped burrows. For burrows with mudballs, mudball numbers were positively associated with burrow characteristics. This indicates that fiddler crab burrow characteristics are affected by sex, not by the production and presence of mudballs.