A conspicuous courtship signal in the fiddler crab Uca perplexa: female choice based on display structure

The fiddler crab Uca perplexa has a conspicuous male courtship signal that is directed at females to attract them to the male’s burrow for mating. The signal involves waving the unflexed large claw up and down. To determine whether the spatiotemporal structure of the wave is under selection by female choice, we examined whether females had a preference for any particular features of the wave. Females respond to a waving display by either visiting the male’s burrow entrance or by electing to pass without visiting the burrow. We filmed mate-searching females and the waving males that they visited or passed. We documented the wave structure of these males using frame-by-frame analysis. Males produce a two-part wave with component A preceding component B. Both components have an upstroke, a pause at the apex and a downstroke. The tip of the claw was raised much higher in B than in A. Visited males had a shorter delay between the two wave components than did males that the females passed without visiting. Visited males also produced component B waves that had a slower upstroke than those of passed males. There was a significant correlation between the relative height of the raised claw and the duration of the upstroke of component B. Females were selecting males that raised their major claw to the highest position (two to three times as high as the carapace width). Passed males brought down their major claw earlier and from a lower position than did visited males. The data suggests that wave structure has evolved through female choice. Male display rate and body size were not female choice cues. An earlier study showed that display duration was also not used by females in selecting mates.     

Non-independent mate choice in a fiddler crab: a case of stimulus enhancement

Non-independent mate selection occurs when the choice behavior of a female is altered by the interactions between other females and males. In the fiddler crab Uca mjoebergi, males court mate-searching females by waving their one greatly enlarged claw. When a female approaches a male, he initiates high-intensity waving. We conducted one natural mate choice experiment and two mate choice experiments using custom-built robotic crabs. We show that the decision of one female to approach a group of males increases the probability that another female will approach and visit a male from the same group. We suggest that this behavior is best explained by the ‘stimulus enhancement’ hypothesis, where the presence of a female near a group of males makes them more likely to be detected by other females due to an increase in male display rate.     

Weapons or mating signals? Claw shape and mate choice in a fiddler crab

Males often possess traits, such as horns, claws, and tusks, which are used during male–male combat. Studies suggest that selection has fine tuned these weapons to improve their effectiveness and that the shape of these weapons provides cues for males to assess the strength of rivals. Similarly, females might also assess a male’s weaponry to determine his value as a mate. The largest weapon relative to body size is the major claw of male fiddler crabs (genus: Uca). Males use their claws both as a weapon and as a courtship signal, waving it to attract females. We used robotic crabs in two-choice experiments to test female mating preferences based on male claw shape in Uca mjoebergi. First, females did not prefer conspecific claws over any of three alternatively shaped stimuli matched for color and for the rate and pattern of waving. The alternative stimuli were two different heterospecific claws and a plain rectangular shape. Second, females significantly preferred the alternative stimuli when they were presented at a faster wave rate. We conclude that claw shape in U. mjoebergi has not evolved under sexual selection imposed by female mating preferences and is more likely to have evolved under selection for effectiveness during male–male competition.     

How do weaponless male fiddler crabs avoid aggression?

Mimicry of females enables weaker males in many species to avoid intrasexual aggression. In fiddler crabs (Uca annulipes), males use their major claw in aggressive interactions to acquire and defend a territory. Males that have autotomised their major claw will be disadvantaged in fighting, but might use their temporary resemblance to females to avoid costly aggressive encounters with other males. We investigated whether: (1) courting males mistake clawless male fiddler crabs for females; (2) clawless males are able to acquire, defend and retain territories as successfully as intact males; and (3) clawless males are more cautious than intact males. Clawless and intact males differed in burrow acquisition methods and fighting behaviour, but were equally successful at acquiring and retaining burrows. While courting males treated clawless males as female, we found no evidence that clawless males mimic the behaviour of females, or that they exploit the advantage of their mistaken identity. Clawless males further appear to avoid male aggression by altering their territorial strategies to minimise the potential for conflict.     

The effect of food availability on the semilunar courtship rhythm in the fiddler crab<Emphasis Type="Italic"> Uca lactea</Emphasis> (de Haan) (Brachyura: Ocypodidae)

Semilunar or lunar reproductive rhythms that follow tidal cycles are common in marine animals. For brachyuran crabs, an adaptive explanation for the synchronous release of larvae in phase with the tidal cycle is that females release larvae when their offspring are best able to escape predators. As a corollary to this hypothesis, the synchronous male reproductive cycle is selected to be in phase with female receptivity. As an alternative hypothesis, we propose that variation in food abundance influences the onset and intensity of the semilunar courtship cycle. We tested this hypothesis in male fiddler crabs ( Uca lactea) by experimentally manipulating food availability for 4 weeks. Food-supplemented males built more semidomes and waved for more days than did food-deprived males or untreated control males. Moreover, food-supplemented males began courtship earlier and the median courtship day advanced with an increase in food. Courtship intensity was not related to crab size. These results provide the first evidence in marine animals that changes in food abundance due to the tides, and hence in phase with the semilunar cycle, influence male reproductive rhythm.     

Synchronous waving in an ocypodid crab, Ilyoplax pusilla : analyses of response patterns to video and real crabs

In several species of ocypodid crabs, including Ilyoplax pusilla (De Haan), waving displays tend to be synchronized among neighboring individuals. However, no quantitative study has yet been conducted. In this study, responses of I. pusilla males to waving of video and real crabs were examined. Spontaneous, periodic waving was observed even when no stimulus waves were present. When single isolated waves of the video crab were randomly delivered (single pulse perturbation), crabs waved either immediately after the stimuli or after some time had elapsed. Assuming that waves immediately after the stimuli were triggered prior to those stimuli, crabs typically responded after a roughly constant delay from the onset of each stimulus. The delay (response time) was close to, but slightly shorter than, the spontaneous wave period. Dependency of response time upon timing of the stimulus (stimulus time) was at most weak. Responses of crabs to periodic waves of the video crabs and waves of␣the real crabs were basically consistent with the results of single pulse perturbation, although there was a consistent tendency for stimulus time and response time to be negatively correlated near stimulus time = 0 s. Overall, these results demonstrate that males of I. pusilla adjust signal timing relative to a neighbor in a manner of phase delay synchrony. The similar response pattern has also been noted in synchronous flashing of fireflies and the calling of katydids. Moreover, these experiments with I.␣pusilla demonstrate the value of video playback as a research tool in this species. Received: 4 April 1997 / Accepted: 17 December 1997     

The assessment of female reproductive state during courtship and scramble competition in the fiddler crab, Uca paradussumieri

Male fiddler crabs, Uca paradussumieri, mate underground during a 4- to 7-day period each full and new moon. As soon as the tide recedes, males enter the burrows of females that will ovulate the following day ('pre-ovigerous' females). Males copulate with and guard these females until they ovulate. When interrupted by an intruding male, the first male to reach the female is usually able to defend her and successfully mate with her. In fiddler crabs, females mate multiply and there is last male sperm precedence. Before each semi-lunar mating period, male U. paradussumieri were more likely to court females with whom they would later mate than other nearby females with whom they did not mate. This suggests that males collect information on female reproductive state prior to the females becoming ovigerous. In this species, aggression was common between males that courted the same female. When previously courted females were approached by other males, the initial courter attempted to forcefully disrupt the courtship. This behavior may allow males the exclusive use of information on female reproductive condition. It also suggests a type of scramble competition between males over females. Furthermore, it indicates that males are able to locate receptive females prior to their becoming ovigerous. The shorter guarding period observed in this species, as compared with other fiddler crabs, is caused by females rejecting longer guarding periods. Male ability to assess female reproductive status may therefore be advantageous because it increases male mating success within a scramble type of competitive polygyny.     

Testing function of fiddler crab claw waving by manipulating social context

Many territorial advertisement signals are thought to be dual-function signals, directed to both rival male and receptive female conspecifics. However, few studies have tested this assumption by examining whether in fact both sexes are likely to elicit signaling behavior from territorial males. In this study, I experimentally manipulated the social context of male sand fiddler crabs (Uca pugilator) to investigate the effect of different audiences on the performance of the claw-waving display, a territorial signal that is often presumed to be directed to both males and females. To test whether males perform this signal to both audiences, I measured the frequency of waving behavior by focal males when housed in field enclosures alone, with only males, with only females, or with both males and females. Focal males waved at a low frequency when alone, and the presence of males had no effect on their level of waving. However, in the presence of females, focal males showed a significantly higher level of waving, whether or not males were also present. In addition, there was no association between fighting and waving behavior. This experiment provides evidence that from the perspective of the signaling male, the claw-waving display of U. pugilator is not a dual-function signal but rather is primarily directed to receptive females. Received: 16 December 1999 / Received in revised form: 1 February 2000 / Accepted: 19 February 2000     

Fiddler crab burrowing affects growth and production of the white mangrove (<Emphasis Type="Italic">Laguncularia racemosa</Emphasis>) in a restored Florida coastal marsh

Positive plant–animal interactions are important in community ecology, but relatively little attention has been paid to their effect on the production of mangroves, dominant halophytic trees in tropical coastal marshes. Here, the role of fiddler crab (Uca spp.) burrowing on the growth and production of the white mangrove, Laguncularia racemosa (<2 years old), was examined in a restored marsh in Tampa Bay, Florida (27°41.65 N, 82°30.34 W) with manipulative experiments from June 2006 to May 2007. Fiddler crab burrowing significantly increased mangrove height by 27%, trunk diameter by 25%, and leaf production by 15%, compared to mangroves in crab exclusion enclosures. Additionally, the exclusion of fiddler crabs significantly increased interstitial water salinity from 32.4 to 44.2, and decreased the oxidation–reduction potential of the low organic sediments, but did not affect soil pH or sulfide concentration. Mangrove height, trunk diameter, and leaf production along a transect that varied in crab burrow density were positively associated with the number of crab burrows. Further, the density of sympatric Spartina alterniflora shoots was positively correlated with crab burrow density along the transect. As in temperate marshes, fiddler crabs can have significant ecological effects on mangrove communities, serving as ecological engineers by modulating the amount of resources available to marsh plants, and by altering the physical, chemical, and biological state of these soft sediment communities. In restored coastal systems that typically have very poor sediment quality, techniques such as soil amendment could be used to facilitate a more natural interaction between crabs and mangroves in ecosystem development.     

To court or not to court: reproductive decisions by male fiddler crabs in response to fluctuating food availability

For males, courting and foraging are often behavioral alternatives, which take time and consume energy. When males have a possibility of mating with receptive females, there may be a behavioral trade-off between courtship and feeding; the outcome of which may be affected by male physiological condition and food availability. Although many mathematical models and empirical studies suggest that the expression of male courtship signals are condition-dependent, decisions about courtship and mating strategies in relation to food availability have not attracted much attention. In this study, we tested whether daily changes in food availability affect males’ decisions about whether to court. We conducted experiments with the fiddler crab Uca lactea by providing males with additional food every other day. In food-supplemented enclosures, males did not increase courtship activity on the days when food was supplemented. However, they built more courtship structures (semidomes) and waved more on the days when they were not given additional food. Male size had a strong influence on the number of days the males courted. We also tested whether the frequency of surface mating, as an alternative reproductive tactic, decreased when food was supplemented. Contrary to our expectation, the number of males that exhibited the surface-mating tactic increased when food was supplemented whereas the number of mate-searching females did not change. Our findings in this field study suggest that reproductive decisions by male fiddler crabs are affected by fluctuating food availability and present body condition, and the alternative mating tactic of this species may be more frequently used by males under good condition.     

Soft song in song sparrows: response of males and females to an enigmatic signal

Low-amplitude “soft song” is used by a variety of songbirds; in some species during aggressive encounters, in others during courtship, and yet others in both these contexts. In song sparrows (Melospiza melodia), soft song has thus far been observed only in aggressive encounters, where its production is a more reliable predictor of attack than any other signaling behavior. We used song playback to test the response of both male and female song sparrows to soft song. The design of the playback experiments took into account the existence of two classes of soft song: crystallized soft song, which consists of song types also found in the broadcast repertoire, and warbled soft song, which consists of less-structured song types not found in the broadcast repertoire. Female song sparrows responded with significantly less courtship display to the playback of crystallized soft song than to that of normal broadcast song, and response to warbled soft song was if anything lower than to that of crystallized soft song. Male song sparrows responded equally aggressively to normal broadcast song as to crystallized soft song, and equally aggressively to warbled soft song as to crystallized soft song. The female results support the conclusion that neither form of soft song functions in courtship. The male results suggest that the reliability of soft song as a signal of aggressive intent is not maintained by a receiver retaliation rule.     

Influence of shells on mating behavior in the hermit crab Calcinus tibicen

Summary Populations of the intertidal hermit crab Calcinus tibicen were observed in the laboratory and reproductive behaviors recorded. Of the 218 interactions, 68 resulted in copulation(s). Male and female sizes were positively correlated. Male size affected copulation success in a non-linear fashion. In particular, the largest males did not obtain any copulations. This was largely a consequence of the shell species occupied by large individuals; males in Nerita sp and Cittarium pica shells were unsuccessful in courtship. The ability to execute precopulatory rotation of the female was negatively affected by certain shell types. Repeated pairings of individuals suggested some level of individual recognition within the reproductively active population.     

Claw allometry in green crabs, Carcinus maenas: heterochely, handedness, and sex

Claw loss and reversal of handedness during regeneration are common phenomena in heterochelous decapod crustaceans, which typically have one large ‘crusher’ claw on the right side and a smaller ‘cutter’ claw on the left. Little is known about the relative importance of claw growth vs. body growth during claw regeneration. Here the relationship between claw size and body size of green crabs, Carcinus maenas, was examined to test for differences in claw allometry as a function of handedness and sex, as there are differences in how males and females use their claws. A total of 730 crabs (range = 15.7–83.6 mm CW) were collected from Maine to New Jersey, USA from May to October 1997, 2000, and 2004–2005. Claw growth, particularly crushers, was accelerated in left-handed crabs and in males compared to right-handed crabs and females respectively. These differing growth strategies highlight the role of sexual dimorphism in claw usage and the importance of achieving heterochely after claw injury. These results imply that handedness should be an important factor to consider in future studies of crab morphology, behavior, and morphometrics. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Vocalizations and courtship intensity correlate with mounting success in marginated tortoises<Emphasis Type="Italic"> Testudo marginata</Emphasis>

The courtship and mounting behaviour of tortoises is elaborate, and based on a multiple signalling system involving visual, olfactory and acoustic signals. Vocalizations related to mounting seem to be particularly significant because tortoises vocalize mainly at this time. Vocalizations and courtship behaviour may be costly for males, and if these costs increase differentially for different males, then the potential exists for vocalizations and displays to reveal male individual quality. In this correlative study, we analysed relationships between male mounting success and morphological and behavioural traits, particularly acoustic signals, exhibited by male marginated tortoises (Testudo marginata) during courtship, in a group of 94 individuals breeding in semi-natural enclosures. For each male, we estimated general body condition, courtship intensity and mounting success; calls of mounting males were recorded and four sonagraphic features were measured. Calls differed significantly among males, and two features varied according to body condition. Male mounting success significantly increased according to the male/female size-ratio, suggesting the existence of a size-based assortative mating. Mounting success was also highly correlated with courtship intensity, measured as number of bites and rams given to females before mounting, and with number of calls emitted during mounting. Finally, mounting success was negatively related to call duration. To our knowledge, this is the first study in which features of tortoise vocalizations are shown to convey reliable information about male quality in socio-sexual contexts.Communicated by T. Czeschlik     

Endogenous swimming rhythms in estuarine crab megalopae: implications for flood-tide transport

Up-estuary migration of crab larvae to adult habitats is thought to be accomplished by selective tidal transport in which late-stage larvae enter the water column on flood tides and remain on or near the bottom on ebb tides. This study measured endogenous rhythms in swimming by the last larval stage (megalopa) of blue crabs Callinectes sapidus and fiddler crabs Uca spp. Previous field studies found that megalopae of both species were only abundant in the estuarine water column on nocturnal rising tides. Megalopae were collected from the Newport River Estuary, North Carolina (34°41N; 76°40W) during August–September 1992 and swimming activity was recorded for 4.5 to 7 d under constant conditions with a video system. Rhythms exhibited by both genera in the laboratory were not identical to those recorded in the field. Uca spp. displayed a circatidal rhythm, with maximum swimming occurring near the time of high tide in the field. Rhythm amplitude increased when crushed oyster shells were present, which suggested that megalopae bury or cling to the substrate during quiescent periods. In contrast, C. sapidus had a circadian rhythm in which maximum swimming coincided with the day phase in the field. In most trials, the activity of blue crab megalopae was unrelated to the expected tidal cycle. It was concluded that a tidal rhythm in swimming was the behavioral basis of flood-tide transport for fiddler crab larvae. The endogenous rhythm in blue crabs does not participate in transport, which probably results from behavioral responses to environmental cues associated with flood tide.     

Territorial counter-singing in male sac-winged bats (<Emphasis Type="Italic">Saccopteryx bilineata</Emphasis>): low-frequency songs trigger a stronger response

In many animal species, individuals compete for resources but avoid escalated conflicts by threat displays, i.e. a mutual signalling behaviour that enables the opponents to predict the outcome of the conflict without the necessity of actual fighting. For example, territory holders may use acoustic signals to communicate not only their own identity and the borders of their territory but also their competitive quality, fighting ability and motivation. Here, we show that male sac-winged bats, Saccopteryx bilineata, adjust their vocal territorial displays according to the fundamental frequency of territorial songs of their opponents. In playback experiments with territorial males, low-frequency stimuli elicited a higher territorial song rate and length than high-frequency stimuli. Male S. bilineata that sing more often and with lower fundamental frequencies have been shown to sire more offspring than their competitors. Fundamental frequency of territorial songs, hence, may reveal male quality and, consequently, the resulting threat posed to competing males. We argue that this is reflected in the increased response of competitors to low-frequency territorial songs shown here. Such competitive signalling behaviour has been shown in a few mammal species like red deer and baboons but, thus far, not in bats. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mound building in Ilyoplax pingi (Crustacea: Brachyura: Ocypodidae)

The ocypodid crab Ilyoplax pingi, observed in Kanghwa I., Korea in 1992, builds a mound at the burrow entrance, by piling mud dug out from the burrow. The mounds were made by both waving and non-waving males, and by both ovigerous and non-ovigerous females. The burrow diameter at the widest part tended to be larger in crabs with mounds than in crabs without mounds, whereas the burrow depth was not different between them. The mound density increased in the late exposure period, when the crabs' surface activities declined. Mound removal and rebuilding experiments revealed that the presence of the mounds has the effect of keeping neighboring crabs away.     

Interactive effects of predation pressure and insecticide (temefos) toxicity on populations of the marsh fiddler crab Uca pugnax

The interaction of predation pressure with lethal and sublethal effects of temefos (Abate^®) an organophosphorous insecticide, were studied in field populations of Uca pugnax (S.I. Smith). Changes in fiddler crab population densities were followed in open-marsh temefos-treated and untreated test plots and in treated and untreated plots which were caged over to reduce predation by marsh birds. Temefos significantly reduced the population density of U. pugnax in the open test plots but not in the caged plots. These results indicate that temefos has a primarily sublethal effect on the crabs, the effect becoming lethal only after interaction with avian predation. Evidence from the presence of a time-lag effect in the population decrease, from a calculated predation index, and from laboratory studies reported elsewhere of behavioral alteration by temefos also supports the conclusion that temefos primarily impairs the escape response of U. pugnax: this leads to increased predation and subsequently to a decreased fiddler crab population. Such studies of sublethal effects of toxicants and field studies of interactions of lethal and sublethal effects of such compounds with natural population dynamics of affected species are necessary to evaluate possible effects of toxicants on populations.     

The influence of sexual selection and predation on the mating and postcopulatory guarding behavior of stone crabs (Xanthidae,Menippe)

Summary Postcopulatory mate guarding in crustaceans traditionally has been viewed as a behavioral mechanism that prevents predation on the soft post-molt female. This study tests the effects of sexual selection and predation on the postcopulatory guarding durations of male stone crabs, Menippe mercenaria, M. adina, and their hybrid. Male stone crabs were held with a pre-molt female, and either another adult male stone crab, an intermolt female, or a male blue crab, which corresponded to intermale competition, control, and predation treatments, respectively. The mating behavior of the heterosexual pair was recorded with a time lapse video system and the durations of copulation and postcopulatory guarding were measured. Males guarded longer in the intermale competition treatment than either the control or predation treatments. In the competition treatment, agonistic encounters occurred between the males at the den containing the female and several mate takeovers occurred. Females survived the predation treatment in trials in which the guarding durations were the longest, whereas females were eaten by the blue crab in trials with the shortest guarding durations. Sexual selection appears to be important in maintaining postcopulatory mate guarding in stone crabs.     

Does mating experience of male house crickets affect their behavior to subsequent females and female choice?

Male mating experience was shown to play an important role in settling conflicts between males; however, little is known about whether and how prior access to females influences male behavior during intersexual interactions and female choice itself. Here, I experimentally test this relationship in the house cricket (Acheta domesticus) by combining one-on-one interaction between the male and female with direct comparison of males by the female, but precluding aggression between males. I found that solitary males were more active during subsequent courtship displays than paired males, suggesting the detrimental effect of mating on courtship performance. At the same time, females spent significantly more time close to solitary males or playbacks of male's natural courtship songs, and responded positively to the condition of males, ignoring body size of males. In contrast, females responded similarly to computer-modified playbacks of courtship songs of solitary and paired males with standardized rate of phrases and amplitudes; however, when females were additionally allowed to contact with anesthetized males they spent more time close to bigger males, irrespective of the acoustic parameters of courtship songs. These results show that although females were able to differentiate between many behavioral and morphological characteristics of males, including voluntary and intrinsic ones, they preferred traits conditional upon the costliness of male's displays. In addition, mating experience appeared to be a crucial factor in the choice of a particular costly mating strategy by males.