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Data from: Balancing risk and reward: mating opportunity influences thermal refuge use in fiddler crabs

Citation

Darnell, M. Zachary; Yeghissian, Talene; Lane, Zachary M. (2020), Data from: Balancing risk and reward: mating opportunity influences thermal refuge use in fiddler crabs, Dryad, Dataset, https://doi.org/10.5061/dryad.wm37pvmjb

Abstract

Behavioural thermoregulation can ameliorate thermal stress but is costly. For species that court in a thermally stressful microhabitat, sexual selection via endurance rivalry favours individuals that are able and willing to endure harsh conditions in the courtship habitat, as retreats to a thermal refuge will reduce mating opportunities. The relative costs and benefits of refuge use versus continued courtship in the face of thermal risk, which vary across abiotic and biotic contexts, determine the optimal behavioural strategy. We examined the social and abiotic factors driving behavioural decisions related to thermoregulatory retreat in the fiddler crab Austruca mjoebergi. Male fiddler crabs perform a courtship display on the thermally stressful intertidal sediment surface. Time on the surface, and thus time available for display, was limited by high temperatures; as temperature increased, surface time decreased. Yet when presented with a stimulus female, males were more likely to perform the courtship display, displayed at a higher rate and increased time spent on the surface. These results demonstrate that behavioural decisions related to thermal retreat depend both on the abiotic conditions that influence the degree of thermal stress and on the social conditions that influence the reproductive prospects of the individual.

Methods

Study Site

This study was conducted in December 2019 at East Point Reserve, Darwin, Northern Territory, Australia (12°24.53′S, 130°49.85′E). The study site covered an area of approximately 200 × 50 m and consisted of a matrix of open, unshaded mudflat interspersed with areas shaded by mangroves. All experiments took place on the open, unshaded mudflat between 0830 and 1330 hours during the neap tide mating period, when the mudflat is exposed throughout the tidal cycle and mating activity peaks. 

To quantify the thermal environment, multiple sensors were deployed in a single representative location on the open, unshaded mudflat and logged data every minute throughout the 5-day experimental period using a HOBO USB Micro Station Data Logger (H21-USB, Onset Computer Corp., Bourne, MA, U.S.A.). Air temperature and relative humidity were measured at a height of 193 cm (S-THB-M002, Onset Computer Corp.), and wind speed and solar irradiance were measured at 200 cm (wind speed: S-WSB-M003, irradiance: S-LIB-M003, Onset Computer Corp.). Air temperature was additionally measured 6 cm above the sediment surface (S-TMB-M002, Onset Computer Corp.), and sediment temperature was measured ~2 mm below the surface (S-TMB-M006, Onset Computer Corp.). Air temperature sensors were shaded to ensure accurate air temperature measurements.

Experimental Design

To initiate the experiment, an adult male crab (hereafter the ‘focal male’) that was actively waving was identified, driven into their burrow (by approaching the burrow), and haphazardly assigned to one of two treatments: female present or female absent. The burrow was then temporarily capped to prevent the male from emerging. To prevent interference by other males, all burrows within 50 cm of the focal burrow were temporarily sealed. For males assigned to the female-present treatment, a clear plastic cup (base diameter = 29 mm, height = 41 mm) containing a female A. mjoebergi (mean ± SE carapace width = 9.72 ± 0.21 mm) and 3–5 mm of water was placed 10 cm from the burrow entrance (distance from centre of the burrow to centre of the cup). For males assigned to the female-absent treatment, a clear plastic cup containing only 3­–5 mm of water was placed 10 cm from the burrow entrance. The focal burrow was then uncapped and observed from >2 m away. When the focal male emerged from the burrow, the time spent on the surface before the next retreat to the burrow (hereafter referred to as ‘surface duration’) and the number of waving displays performed during that period (Fig. 1) were recorded. Over the 5-day experimental period, a total of 31 observations were made for the female-present treatment and 30 observations were made for the female-absent treatment. Two observations were excluded from analyses – one male that was scared into the burrow by a predator, and a second where the female died during observation – leaving a sample size of 29 in the female-present treatment and 30 in the female-absent treatment. 

Data Analyses

To assess whether males increased courtship behaviour in the presence of a female, we used Fisher’s exact test to compare the proportion of crabs that waved at least once during their time on the surface between the two treatments. We then calculated waving rate (waves/s) for each focal male that did wave and used a Mann–Whitney U test to compare waving rate between the two treatments.

All temperature measurements were strongly correlated with each other (> 0.901), and all were also strongly correlated with solar irradiance (r > 0.829). Sediment surface temperature was chosen as a representative temperature metric for use in analyses as sediment surface temperatures represent the temperature measurement closest to the location of crab activity. To examine the effect of temperature and female presence on the duration of surface activity, surface duration was first log-transformed to meet assumptions of normality. A linear model was fit using log-transformed surface duration as the response variable and including treatment (female present or female absent), sediment surface temperature and a treatment × temperature interaction as predictor variables. Sediment temperatures used in the analysis represent the average sediment temperature over the time when the focal male was on the surface. For crabs with surface durations <1 min where there was not a temperature measurement during the observation period, the nearest sediment temperature measurement to the end time of the observation (i.e. when the crab retreated to the burrow) was used. 

To test for an effect of female size on the duration of surface activity, we fitted a linear model using log-transformed surface duration as the response variable and included female carapace width, sediment surface temperature and a female carapace width × temperature interaction as predictor variables. Only data from the female-present treatment were included. 

Funding

National Geographic Society, Award: NGS-59787R-19

National Science Foundation, Award: IOS-1755389

University of Southern Mississippi, Award: Fund for Academic Excellence