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.     

Males are attracted by their own courtship signals

Courting male fiddler crabs Uca terpsichores (1 cm carapace width) sometimes build mounds of sand called hoods at the entrances to their burrows. Males wave their single enlarged claws to attract females to their burrows for mating. It was shown previously that burrows with hoods are more attractive to females and that females visually orient to these structures. In this study, we test whether males also use their hoods to find their burrows. We first determined the maximum distance that males can see and find a burrow opening without a hood. Males were removed from their burrows and placed on the sand at a range of distances from a burrow opening. If they were more than about 8 cm (seven units of eye-height) away, they were unable find the burrow. In contrast, males that were burrow residents used a non-visual path map to return to their burrows from much greater distances. To determine if hoods help males find their burrows when there are errors in their path maps, we moved residents 1–49 cm on sliding platforms producing errors equal to the distances they were moved. Males with self-made hoods or hood models at their burrows relocated their burrows at significantly greater distances than did males with unadorned burrows. Hood builders also relocated their burrows faster. Hence, hoods have two functions: they attract females and they provide a visual cue that males use to find their burrows quickly and reliably when their path maps fail. An erratum to this article can be found at     

Density affects female and male mate searching in the fiddler crab, <Emphasis Type="Italic">Uca beebei</Emphasis>

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 density 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 perceived 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 predators, decreases their costs of searching and because the abundance of males and male burrows facilitates comparisons and thus may increase their benefits from searching. Males at high densities decrease their mate-searching rate perhaps in response to the increase in female searching and to the corresponding increase in the intensity of their competitors' mate-attraction signals.     

Temporal constraints and female preference for burrow width in the fiddler crab, <Emphasis Type="Italic">Uca mjoebergi</Emphasis>

We studied sampling behaviour and mate choice in the fiddler crab Uca mjoebergi. Once a female selects a mate, she copulates in his burrow and remains there until releasing her aquatic larvae. U. mjoebergi occurs in habitats that are inundated only by the highest amplitude spring tides. Females can only release their larvae during these tides, and release before or after will result in complete failure of reproductive effort. Matings occur over a 5-day period near the end of neap tides. Our results suggest that within the mating period, females adjust their larval developmental rates by selecting specific burrows in which to incubate their clutches. We found that at the start of the mating period, females chose larger males as mates. Since male size was positively correlated to burrow width, females were selecting wide burrows and effectively incubating at lower temperatures. This would slow down the developmental rates of larvae. In contrast, females that mated late in the mating period selectively chose small males. By incubating in narrower, warmer burrows, these females may increase the developmental rates of larvae. We propose that females are selecting burrows to influence incubation rate and ensure timely release of their larvae. Female U. mjoebergi appear to adjust their preference for the direct benefits of mate choice to increase their reproductive success.     

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.     

Paternity analysis of two male mating tactics in the fiddler crab, Uca mjoebergi

The fiddler crab Uca mjoebergi mates both underground in male-defended burrows and on the surface near female-defended burrows. The reproductive tract of Uca species facilitates last-male precedence, suggesting that males that do not guard-mated females are likely to gain very little paternity if the female re-mates with another male. Here, we test the reproductive success of burrow and surface matings using paternity analysis. We found that 100?% of the females that mated in burrows extruded a clutch of eggs. Furthermore, we show conclusively, for the first time in a fiddler crab species, that last-male sperm precedence results in the majority of the female’s eggs being fertilised by the burrow-mated male. In contrast, surface matings resulted in significantly fewer females extruding eggs (5.6?%). Paternity analysis also revealed that more than half of the clutches from burrow-mated females showed low levels of extra-pair paternity from previous matings. Although multiple matings appear common in U. mjoebergi, burrow-mated males that guard females are guaranteed a successful mating with extremely high rates of assured paternity. Surface matings therefore appear to be an opportunistic tactic that may increase male reproductive success in a highly competitive environment.     

Patterns,correlates, and paternity consequences of extraterritorial foray behavior in the field sparrow (Spizella pusilla): an automated telemetry approach

Behavioral Ecology and Sociobiology - ᅟMales of fiddler crabs (genus: Uca) construct courtship structures using mud or sand to attract mate-searching females for underground mating. A sensory...     

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.     

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.     

Some beavioral characteristics related to food supply and soil texture of burrowing habitats observed on Uca vocans vocans and U. lactea perplexa

From the observations on two species of fiddler crabs, droving is conspicuous in Uca vocans vocans but does not occur in U. lactea perplexa. U. vocans come down to feed at the water's edge with a richer food supply and fine substrate particles. Droving is advantageous to increase feeding efficiency in this species which holds burrows on a relatively sandy habitat where the food supply is less rich. In U. lactea, which burrows at higher levels, on the other hand, the food supply and soil texture of their burrow area are not significantly different from those at the water's edge. Home range relationships of both species are consistent with the occurrence or not of droving behavior and depend on food productivity of burrowing habitats.     

Time constraints and multiple choice criteria in the sampling behaviour and mate choice of the fiddler crab,Uca annulipes

Active female sampling occurs in the fiddler crab Uca annulipes. Females sample the burrows of several males before remaining to mate in the burrow of the chosen partner. Females time larval release to coincide with the following nocturnal spring tide and must therefore leave sufficient time for embryonic development after mating. Here we show how this temporal constraint on search time affects female choosiness. We found that, at the start of the sampling period (when time constraints are minimal), females selectively sample the larger males in the population. Towards the end of the sampling period (when the temporal constraints increase the costs of sampling), females are less selective. Furthermore, we suggest that the number of males sampled (and other indices of ‘‘sampling effort’’) may not be reliable indicators of female choosiness and may not reflect the strength of female mating preferences under certain conditions. Burrow quality also emerged as an important criterion in final mate choice. Burrow structure potentially influences reproductive success, and mate acceptance based on burrow structure appears to involve a relatively invariant threshold criterion. Since there is no relationship between male size and burrow quality, females are using at least two independent criteria when choosing potential mates. We envisage mate choice as a two-stage process. First, females select which males to sample based on male size. They then decide whether or not to mate with a male based on burrow features. This sampling process explains how two unrelated variables can both predict male mating success. Received: 23 March 1995/Accepted after revision: 14 January 1996     

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.     

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.     

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.     

Adaptive droving behavior observed in the fiddler crab Uca vocans vocans

Droving is conspicuous in Uca vocans vocans in summer. The crabs burrowing on the upper habitat have a higher tendency to wander compared to the ones burrowing on the lower habitat. Most of the wandering crabs captured on the low tide levels are relatively large and male. Larger crabs and males prefer to burrow on the upper zones of U. vocans habitat, but the smaller ones and females prefer the lower habitat. The upper level of the U. vocans habitat has relatively low N-content compared to the water's edge. Therefore droving is advantageous in crabs that have burrows on the upper level. Female and smaller resident crabs have faster feeding motions than male and larger ones, and can satisfy their feeding demands more rapidly. Therefore, for males and larger crabs it is advantageous to move away from the burrow area and forage in areas of higher food content near the low tide level where the number of feeding motions increases.     

Differences in context and function of two distinct waving displays in the fiddler crab, <Emphasis Type="Italic">Uca perplexa</Emphasis> (Decapoda: Ocypodidae)

To respond appropriately to communication signals, animals must have the ability to decipher signal meaning. At a basic level, interpreting the difference between territorial and courtship signals can be vital for the survival and reproduction of social animals. Male and female fiddler crabs communicate with claw-waving displays, but the function of these waves remains uncertain. Species differ in the context in which they wave: Some wave during courtship, some during territorial defence and some during both. In this paper, we provide evidence that males of an Australian species of fiddler crab, Uca perplexa, use two different types of claw waving display, lateral and vertical. Lateral waves are employed solely in a courtship context, whilst vertical waves are employed during courtship as well as territorial interactions. Using video recordings, we show that lateral waves were produced spontaneously (broadcast), and their frequency increased only in the presence of female wanderers. Vertical waves were not broadcast but were elicited by male wanderers during agonistic interactions and female wanderers during close range courtship. Male resident U. perplexa were able to discriminate the sex of wandering crabs on the mudflat at distances of 32 cm. During all resident–wanderer interactions, residents attempted to maintain a position directly between the wanderer and the home burrow and orient themselves to face females and to present the major claw towards males. To our knowledge, this is the first demonstration of the multiple use of waving displays in a fiddler crab species.     

Repeatable female preferences, mating order and mating success in the poeciliid fish, Heterandria formosa

Sexual selection via female choice can afford preferred males comparably higher mating success than those males that lack preferred traits. In addition, many models of sexual selection assume that both male traits and female preferences are heritable. In this study we test whether females of the poeciliid fish, Heterandria formosa, have repeatable pre-copulatory preferences for larger males. We also test whether female pre-copulatory preferences are always reliable indicators of male mating success. When given a choice between a large and a small male, females prefer larger males, and the repeatability of this preference is high. Although there are no overall differences in male mating success between large and small males, large males have a higher mating success when they are the first to mate than when they are the second to mate. Likewise, preferred males also have higher mating success when they are the first to mate than when they mate second. Therefore, the repeatable female preferences observed in this study only predict male mating success when the preferred male mates first. These results illustrate that even significantly repeatable female preferences do not translate into male mating success, which is an assumption of many examinations of the importance of female choice in sexual selection.     

Parental care and mate attraction in the Florida flagfish, <Emphasis Type="Italic">Jordanella floridae</Emphasis>

Theory suggests that there is a trade-off between mate attraction and investment in parental care. Yet, under some circumstances, such as in species with uniparental male care, it is conceivable that males who provide more care also attract more females, and hence avoid this cost. We tested these two alternative hypotheses using the Florida flagfish, Jordanella floridae, which is a highly sexually dimorphic species with male parental care. We predicted that if components of male care behavior primarily serve offspring survival, they should display these behaviors more in the presence of eggs than without eggs. We also predicted that males should show a trade-off between display, and other activities directed to other fish, and care behaviors. Alternatively, if care has a positive effect on mating success, we expected males to show care even in the absence of eggs, and that they should display more care in the presence of females. We found that care behavior mostly depended on the presence of eggs, males increased care when they guarded eggs, and there was a negative relationship between rates of interaction and care behavior. Most care was exhibited in the absence of other fish while the presence of males and females both had a negative effect on care behavior. There was no difference in care behavior with males or females present. Only nest-visitation rates were highest in the presence of females, suggesting that this behavior is used as part of the courtship. These results support the idea that male care in this species is primarily influenced by natural selection. Males did, however, display fanning and other egg-directed behaviors even in the absence of eggs. The level of these behaviors was constant and independent of display behavior. Hence, it seems that care may be used in mate attraction before the male has received his first clutches.     

Reproductive biology of a small isopod symbiont living on a large isopod host: from the maternal marsupium to the protective grip of guarding males

Mating systems of many symbiotic crustaceans are characterised by a high degree of mate guarding. A peculiar case of mate guarding has been reported for small symbiotic janirid isopods where males mate with immature females. Field samples of individual hosts and laboratory experiments were conducted to reveal the mating behaviour of the symbiont in a natural environment, that is, on their hosts. Along the coast of the Magellan Strait, Chile, the janirid isopod Iais pubescens was frequently found on the shore-living isopod Exosphaeroma gigas. Symbiont prevalence (percent hosts occupied) was high at eight of the nine sampling sites. Mean symbiont intensity was very low at one site (<<1 individual host^-1), intermediate at two sites (1-10 individuals host^-1) and high at the other sites (10-40 individuals host^-1). The mean sex ratio (males:females) was male biased at most sampling sites (n=7). Females of I. pubescens reached substantially larger sizes (1.5-3.0 mm body length, BL) than males (1.1-1.9 mm BL). The majority of males were carrying small juveniles (66.15%), and males with juveniles were significantly larger than males without juveniles - this suggests that males prefer virgin juveniles to adult females and that they compete for small juveniles. In laboratory observations, males were seen to manipulate the marsupium of adult females that were about to release small juveniles. Males obtained virgin juveniles in this manner. Juveniles were carried for ~7 days, and they moulted shortly before being fertilised and released by males. The high proportion of juveniles carried by males in the field (68.2%) supports previous observations that males initially are not able to distinguish male and female juveniles. It is suggested that the mating system of symbiotic janirid isopods with long-term sperm storage and continuous receptivity in females and male mating with virgin females has evolved in response to highly unpredictable encounter probabilities between the sexes. Mate guarding and manipulation of small virgin juveniles may be favoured on the highly mobile hosts of symbiotic janirid isopods. Furthermore, adult females may gain by leaving their emerging offspring in the protective grip of guarding males, thereby reinforcing the maintenance of this peculiar mating system.     

Mating behaviour of Macrophthalmus hirtipes (Brachyura: Ocypodidae)

 We examined the mating behaviour of the New Zealand ocypodid crab Macrophthalmus hirtipes in the laboratory between February and June 1998. This species has a discrete breeding season. Mating and moulting were not linked and only intermoult females with mobile gonopore opercula were attractive to males. Allometry and compatibility of gonopods and gonopores of different-sized crabs was investigated. Under laboratory conditions, the opercula of intermoult females remained mobile on average for 11.4 d, but the duration of receptivity did not appear to be under female control. The operational sex ratio in the laboratory fluctuated greatly, but was always male-dominated. During the period of opercular mobility, females mated many times with several different males. Matings in the absence of burrows were relatively short (mean duration = 23 min, max. = 122 min) and the mating behaviour of M. hirtipes lacked courtship and mate-guarding. Males used a search-intercept method to acquire mates, with very low levels of intrasexual competition. There was no evidence of mate preference in M. hirtipes, and males spent just as long mating with ovigerous females as with non-ovigerous ones. Although M. hirtipes has ventral-type spermathecae, as do several other ocypodid crabs, it is unclear whether this promotes last-male sperm precedence. The role of burrows in modifying the mating behaviour of M. hirtipes in the field remains to be established. Received: 7 January 2000 / Accepted: 5 June 2000