Many sexually selected ornaments and weapons are elaborations of an animal’s outer body surface, including long feathers, colorful skin, and rigid outgrowths. The time and energy required to keep these traits clean, attractive, and in good condition for signaling may represent an important, but understudied cost of bearing a sexually selected trait. Male fiddler crabs possess an enlarged and brightly colored claw that is used both as a weapon to fight with rival males and also as an ornament to court females. Here, we demonstrate that males benefit from grooming because females prefer males with clean claws over dirty claws, but also that the time spent grooming detracts from the amount of time available for courting females. Males therefore face a temporal tradeoff between attracting the attention of females and maintaining a clean claw. Our study provides rare evidence of the importance of grooming for mediating sexual interactions in an invertebrate, indicating that sexual selection has likely shaped the evolution of self-maintenance behaviors across a broad range of taxa.

Time budgets

We conducted 10-minute focal observations on surface-active individuals (n = 50) to estimate the amount of time males spend grooming their claw. Observations were conducted between 10 am and 3 pm when crabs are most active. We measured the total amount of time a male spent grooming his major claw during the 10-minute sample using a stopwatch. Focal males were haphazardly selected from surface-active individuals on the mudflat, and we alternated between individuals initially identified in the sun and shade to account for potential differences in time budgets between microhabitats. Because crabs were not individually marked, we sampled individuals from different locations on the mudflat to avoid repeat sampling. Males that were not surface-active for the entire 10-minute focal sample (e.g., because they retreated into their burrow) were excluded from the analyses (<10% of males). At the end of the sample, we captured the focal male and measured his claw length to the nearest 0.1 mm using dial calipers. 

Male-male competition

To evaluate whether males benefit from having clean claws in the context of male-male fights over burrow ownership, we staged fights between unmanipulated resident males and focal intruder males with either clean (n = 51) or dirty claws (n = 49). To stage fights, we captured males that were defending their territories and released them back into the colony. Captured males were released several meters from their burrow to avoid encounters with familiar neighbors and observed through binoculars as they wandered across the mudflat. Males were measured for claw length and then their claw was painted either with mud from the mudflat (“dirty” claw) or a clean brush (“clean” claw). To minimize handling disturbance, experimenters wore latex gloves and placed a plastic sheet between the crab’s claw and body during the painting treatment. After being released, most males took several seconds before resuming normal activity. We therefore measured a male’s fighting latency as the time elapsed between becoming active and initiating a fight with another resident male. We also recorded whether a male spent any time grooming his claw before initiating a fight (yes/no), and whether the male won or lost the fight. Males were defined as winners if they displaced the resident from his burrow and as losers if they were unable to displace the resident and continued wandering. 

Female choice

We conducted female mate choice trials (n = 40) to evaluate whether males benefit from having clean claws in the context of attracting mates. We captured mate-searching females wandering on the mudflat and tested their preference for males with clean versus dirty claws using custom-built robotic crabs. Each robot consisted of a plaster claw replica (21.0 mm long) molded from a real male claw and painted with yellow enamel paint. Because natural A. mjoebergi claws reflect UV light, and females prefer UV-reflecting claws over claws that do not reflect UV light, we also painted the entire surface of the claw replicas with a UV marker. Each claw was attached to a metal arm that was driven by a small motor to mimic the species’ claw waving pattern at a rate of 16.8 waves/min. A pair of identical robots were placed in a 60 x 60 cm level test arena that was covered with a smooth layer of mudflat sediment and positioned 15 cm away from each other and 25 cm away from the choosing female. Once positioned, the claws were painted either with mud from the mudflat (“dirty” claw) or a clean brush (“clean” claw). At the start of each mate-choice trial, a female was placed under a transparent cup in front of the robotic crabs, allowed to observe the robotic crabs for two complete wave cycles, and then released from the cup using a remote trigger. Females were counted as having chosen a male if they moved directly towards and stopped at the base of one of the robotic crab units. Females that gave a startle response (i.e., dashed on release to the perimeter of the arena) or were motionless for > 3 min after release were retested a maximum of three times before being excluded from the data set and released into the population. 

Temporal tradeoffs

We tested for a potential cost of grooming due to a tradeoff in the time spent maintaining a clean claw and courting females. Specifically, we conducted two-minute focal observations on males with either clean or dirty claws to measure the amount of time spent grooming versus waving. For each sample, we captured a resident male from his burrow and placed a plastic fence around the entrance to prevent other individuals from entering. Captured males were measured for claw length, painted either with mud (“dirty” claw, n = 23) or a clean brush (“clean” claw, n = 23) as described above, and then returned to their territory. We widened the burrow entrance slightly with a stick to minimize the chances of a “dirty” claw being wiped clean as the male entered or exited his burrow. To encourage the males to emerge from their burrow and engage in courtship activity, we tethered a mate-searching female to a short (~5 cm) piece of cotton thread and anchored the tether near the male’s territory. Focal males were recorded with a video camera (Sony Handycam HDR-PJ410 or DCR-SR45) and observed for two minutes following the initiation of his first wave. We measured the amount of time a male spent either grooming his major claw or waving using a stopwatch. 

Table 1: time budget estimates

Final dataset on the ten-minute focal observations used in the grooming time budget analyses

Table1_time_budget.csv

Table 2: Male-male competition

Final dataset on the fight outcomes between unmanipulated resident males and focal intruder males used in the male-male competition analyses

Table2_male_fights.csv

Table 3: Female choice

Final dataset on the outcome of the female choice trials

Table3_mate_choice.csv

Table 4: Temporal tradeoffs

Final dataset on the two-minute focal observations used in the temporal tradeoff analyses

Table4_temporal_tradeoff.csv