DOI: 10.5061/dryad.tmpg4f585
title: Data from: Thermal stress and availability of potential mates drive decisions related to thermoregulatory burrow retreat and emergence in fiddler crabs

description: Behavioral tradeoffs are common, as many behaviors are incompatible. In
 ectotherms, these tradeoffs often result from incompatibility between
 thermoregulatory behaviors and other critical behaviors. These trade-offs
 are resolved by choice, with decisions expected to be based on the costs
 and benefits of each behavior to optimize fitness outcomes. We examined
 the social and abiotic factors driving resolution of the trade-off between
 thermoregulatory behavior and courtship behavior in the fiddler crab
 Austruca mjoebergi. Male fiddler crabs perform a courtship display on the
 high intertidal sediment surface, where they face extreme thermal stress;
 retreat into the burrow is an effective thermoregulatory strategy but
 requires a cessation of courtship behavior. Surface duration decreased as
 environmental temperatures increased, indicated that time available for
 courtship is limited by high temperature. Yet when exposed to a stimulus
 female, males remained on the surface longer, spent less time in the
 burrow following a thermoregulatory retreat to the burrow, and spent an
 overall greater proportion of time on the sediment surface. Results
 demonstrate that behavioral decisions related to both burrow retreat and
 subsequent emergence are influenced by the abiotic and social context that
 determine the relative costs and benefits of the behavioral choices
 available to the individual.
description: # Data from: Thermal stress and availability of potential mates drive
 decisions related to thermoregulatory burrow retreat and emergence in
 fiddler crabs
 [https://doi.org/10.5061/dryad.tmpg4f585](https://doi.org/10.5061/dryad.tmpg4f585) ## Description of the data and file structure *Study Species and Site* The fiddler crab *Austruca mjoebergi* is a small (&lt;15 mm carapace width) fiddler crab that inhabits* *intertidal mudflats in the tropical Indo-Pacific. Males maintain and defend territories (~10-cm diameter) centred on a breeding burrow (Reaney and Backwell, 2007). Reproductive activity in *A. mjoebergi *occurs throughout the year. During a 6–8 d period during each neap tide, when the mudflat is exposed throughout the tidal cycle, sexually receptive females wander through the population searching for mates. Males perform a species-specific waving display using the greatly enlarged major claw on the sediment surface near their breeding burrows to attract mate-searching females. Mating takes place in the male’s burrow, after which time the male guards the female until oviposition (1–2 d after mating). The male then leaves the burrow and resumes courtship behavior, while the female remains in the burrow for the ~20 d incubation period (Reaney and Backwell, 2007). This study was conducted in December 2023 at East Point Reserve, Darwin, Northern Territory, Australia (-12.409°, 130.831°). The study site is an area approximately 100 m × 50 m and consists of open, un-shaded mudflat interspersed with areas shaded by mangroves. All experiments took place in the open, un-shaded areas of the mudflat between 08:30–13:30 h during the neap-tide mating period. The thermal environment experienced by the crabs was quantified using several methods. Operative environmental temperature (*T*e) was estimated using physical models that approximated the size and shape of a fiddler crab. Operative environmental temperatures represent the estimated steady-state body temperature of an organism in the absence of thermoregulation (Bakken, 1992; Hertz et al., 1993) and thus provides a general characterization of the thermal environment and levels of thermal stress experienced by the crabs. Models were constructed of iButton dataloggers (Thermochon DS1921G-F5#, Maxim Integrated Products Inc., Sunnyvale, CA) with an accuracy of ±1°C and resolution of 0.5°C. Each iButton was attached with acrylic caulk to a semi-circular plastic pipe strap base that held the iButton ~1 cm above the sediment. Such models have been used previously with fiddler crabs and closely approximate steady-state temperatures of live crabs (Allen and Levinton, 2014; Darnell et al., 2015). Three to five iButton models were deployed in representative areas of the open, unshaded mudflat each day (05-Dec-2024 to 10-Dec-2024) and an additional three iButton models were deployed in nearby shaded areas each day (06-Dec-2024 to 10-Dec-2024) to further characterize the broader thermal environment in the fiddler crab habitat. iButtons logged temperatures every minute. Air temperature was measured 6 cm above the sediment surface in a representative location on the open, unshaded mudflat and logged every minute throughout the experimental period using a HOBO USB Micro Station Data Logger (H21-USB, Onset Computer Corp., Bourne, MA, U.S.A.). and temperature sensor (S-TMB-M002, Onset Computer Corp.), shaded to ensure accurate air temperature measurement. *Experimental Design* Experimental methods were modified from those used by Darnell et al. (2020). An actively waving male *A. mjoebergi* (hereafter the ‘focal male’ was identified, driven into their burrow by approaching, and the burrow (mean burrow diameter = 9.3 mm, range – 7.2–11.8 mm) was temporarily capped to prevent the male from emerging during experimental setup. To prevent interference from other males, all burrows within 50 cm of the focal burrow were temporarily sealed. Each focal male was then randomly assigned to one of two treatments: female-present or female-absent. For the female-present treatment, a clear plastic cup (base diameter = 29 mm, height = 41 mm) containing a female *A. mjoebergi* (mean carapace width = 9.7 mm, range = 6.5–11.4 mm) and 3–5 mm of ambient seawater was placed 10 cm from the burrow entrance. Our previous work indicates that male fiddler crabs are able to recognize female fiddler crabs within a clear plastic cup, and respond in much the same way as they respond to an un-confined female (Darnell et al., 2020). For the female-absent treatment, a clear plastic cup containing only 3–5 mm of ambient seawater was placed 10 cm from the burrow entrance. The burrow was then un-capped the observed from a distance of ~2 m, a distance that ensures that the crabs are not disturbed by experimenter presence. After uncapping the burrow, the focal male typically emerged from the burrow in &lt; 5 min. Upon emergence of the focal male, we recorded the amount of time spent on the surface before the next retreat to the burrow (hereafter referred to as “surface duration”). Emergence was defined as the point at which the male’s carapace was fully out of the burrow, and retreat was defined as the point at which the male’s carapace had fully re-entered the burrow. After the focal male retreated to the burrow, we recorded the amount of time spent in the burrow before the focal male re-emerged onto the sediment surface (hereafter referred to as “burrow duration”). Each male was observed for a single surface duration followed by a single burrow duration. Total observation time (surface duration + burrow duration) averaged 265.3 s (range = 45–960 s). Over the 6-day experimental period, a total of 49 observations were made for the female-present treatment and 45 observations were made for the female-absent treatment. Three observations were excluded from all analyses, including two instances when female fiddler crabs wandered through the experimental area and the focal male began courting those females and one instance in which the focal male knocked over the cup containing the female while vigorously waving. Sample size after removing these observations was thus 47 in the female-present treatment and 44 in the female-absent treatment. Four additional observations where the focal male sealed the burrow entrance after retreat were excluded from analyses focused on burrow duration. Sample sizes for analyses that include burrow duration were thus 45 in the female-present treatment and 42 in the female-absent treatment. ### Files and variables #### **File: sun\_ibuttons.zip** **Description:** This archive contains data files from iButtons deployed in the open, unshaded area of the mudflat. Each file is a single iButton on a single day. Variables are as follows for all: * Date/Time: Date (MM/DD/YY) * Unit: Time (ACST) * Value: C = degrees celsius * No heading: temperature (degrees C) **File: shade_ibuttons.zip** **Description:** This archive contains data files from iButtons deployed in the area of the mudflat under the mangrove canopy. Each file is a single iButton on a single day. Variables are as follows for all: * Date/Time: Date (MM/DD/YY) * Unit: Time (ACST) * Value: C = degrees celsius * No heading: temperature (degrees C) **File: mjoebergi_2023_data.csv** **Description:** This file contains data from the field experiment. **Variables** * crab_id: unique ID for each focal male. * date: date the observation was conducted (MM/DD/YY) * treatment: F = female-present, N = female-absent * fem_cw: carapace width (mm) for the female used. NA = female-absent treatment. * surface_duration_min: minutes component of surface duration * surface_duration_sec: seconds component of surface duration * total_duration_min: minutes component of the total duration of the observation (surface duration + burrow duration). NA indicates no data.  * total_duration_sec: seconds component of the total duration of the observation (surface duration + burrow duration). NA indicates no data. * end_time: the time of day the observation ended (ACST) * burrow_diam: diameter of the focal male burrow (mm). NA indicates burrow was sealed before it could be measured.  * notes: notes recorded during the experiment * observer: initials or name indicating who conducted the observation. Asa = Asa M. Darnell, MZD = M. Zachary Darnell. * exclude: should the observation be excluded from all analyses? * exclude_burdur: should the observation be excluded from burrow duration analyses? **File: weather_station.csv** **Description:** This file contains data from the Hobo weather station. ##### Variables * Date Time, GMT+09:30: date and time (MM/DD/YY HH:MM, ACST) * Temp, °C (LGR S/N: 20661382, SEN S/N: 21129659, LBL: sediment): sediment temperature (degrees C) * Temp, °C (LGR S/N: 20661382, SEN S/N: 21390994, LBL: 6 cm air): air temperature (degrees C) ## Code/software **mjoebergi_2023_code.Rmd** contains all data wrangling and analysis code. It was created using R 4.3.3. All packages used are listed in the code. ## Access information Other publicly accessible locations of the data: * NA Data was derived from the following sources: * NA