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Thermal constraints on microhabitat selection and mating opportunities
Animal Behaviour 123 (2017) 259e265
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Thermal constraints on microhabitat selection and mating opportunities Pablo Munguia a, *, Patricia R. Y. Backwell b, M. Zachary Darnell c, 1 a
School of Biological Sciences, The University of Adelaide, Adelaide, Australia Research School of Biology, The Australian National University, Canberra, Australia c Department of Biological Sciences, Nicholls State University, Thibodaux, LA, U.S.A. b
a r t i c l e i n f o Article history: Received 15 April 2016 Initial acceptance 15 June 2016 Final acceptance 6 October 2016 MS. number: 16-00329R Keywords: behavioural thermoregulation fiddler crab sexual dimorphism sexual selection Uca mjoebergi
Hot tropical environments constrain ectotherm mating opportunities when mate selection occurs on the surface. Thus, microhabitats and refugia can become a qualitative trait in mate selection. In fiddler crabs, the enlarged claw of males can act as a heat sink, which becomes advantageous when surface temperatures reach 50 C during the day and crabs are actively seeking to mate. Uca mjoebergi females prefer male burrows found in the shade; therefore, we investigated the thermal constraints imposed on males and females in shaded and unshaded habitats. Crab surface activity decreased and body temperature increased as the day progressed, with more crabs active in shaded than sunny microhabitats. Body temperature was lower in male crabs found in burrows relative to crabs on the surface. Male claw size explained 10% of body temperature. Our results add further support to the hypothesis that thermal constraints imposed on males can be overcome by the large claw acting as a heat sink and the burrow acting as a refuge from heat. Classic sexually selected traits, including ornaments and behaviours, can have a secondary purpose in thermoregulation. © 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
The tropics are hot, yet many tropical species exhibit diurnal activity patterns and are active in direct sunlight, exposed to extreme heat stress (Sinervo et al., 2010). Species that court and mate during the hottest part of the day would be particularly stressed; selection therefore favours individuals that endure adverse environmental conditions in exchange for mating opportunities. Sexual selection via endurance rivalry should favour extended presence at the breeding site, which is often associated with an increase in the number of mating opportunities (Banks & Thompson, 1985; Darnell, Fowler, & Munguia, 2013; Salvador, Díaz, Veiga, Bloor, & Brown, 2008). Across both tropical and temperate zones, high temperature, solar radiation and desiccation risk can limit time spent in these stressful breeding areas, as has been shown for both endotherms (Campagna & Leboeuf, 1988) and ectotherms (Darnell et al., 2013; Monaco, Wethey, Gulledge, & Helmuth, 2015), and thus environmental conditions may modulate mating behaviour and mating opportunities. This is expected to
* Correspondence: P. Munguia, School of Biological Sciences, The University of Adelaide, Adelaide, SA 5005, Australia. E-mail address: [email protected] (P. Munguia). 1 Present address: Division of Coastal Sciences, School of Ocean Science and Technology, The University of Southern Mississippi, Ocean Springs, MS, U.S.A.
be of greater importance in the tropics, where temperature and solar radiation are at a maximum. In ectotherms, behavioural thermoregulation can buffer individuals from thermal stress associated with extreme or rapidly changing temperatures (Huey et al., 2012; Kearney, Shine, & Porter, 2009; Smith & Miller, 1973). Mobile species can exploit the thermal heterogeneity of the environment to regulate body temperatures and reduce exposure to thermal extremes. Yet because behavioural thermoregulation often involves a retreat from breeding or display sites to more thermally innocuous habitats, it may come at a cost to mating opportunities. Thermoregulatory microhabitat selection can thus limit activity periods involved in mate searching and display (Darnell et al., 2013). Sexually dimorphic traits can also influence thermoregulatory ability and body temperature, as morphology alters rates of heat transfer between the organism and the environment. A number of sexually dimorphic traits have thermoregulatory effects including bills (Greenberg & Danner, 2013; Luther & Greenberg, 2014), horns (Shepherd, Prange, & Moczek, 2008) and claws (Darnell & Munguia, 2011). Fiddler crabs are often studied in the context of sexual selection and sexual dimorphism. Male fiddler crabs have an enlarged claw that is used in courtship (Detto, 2007; Murai & Backwell, 2006; Pope, 2000), intrasexual combat (Hyatt & Salmon, 1978) and thermoregulation, transferring heat away from the body and
http://dx.doi.org/10.1016/j.anbehav.2016.11.004 0003-3472/© 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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dissipating it to the environment (Darnell & Munguia, 2011). Differential selective pressures on males and females due to sexual selection via endurance rivalry on males results in sex-specific thermal constraints on behaviour (Darnell et al., 2013). Male fiddler crabs perform a waving display on the surface to attract females and because signal competition occurs during this time, endurance rivalry favours increased time on the surface. Yet while on the surface performing the waving display, males endure extreme thermal conditions, with operative environmental temperatures often exceeding preferred and even lethal temperatures (Allen & Levinton, 2014; Darnell, Nicholson, & Munguia, 2015). Surface activity of female fiddler crabs does not seem to be constrained by temperature (Darnell et al., 2013; Milner, Detto, Jennions, & Backwell, 2010), but female mate choice can be influenced by temperature, as mate-searching females preferentially approach males that are in the shade regardless of whether the females are released in the sun or shade (Kerr & Backwell, 2016). The challenge is to understand how mating strategies and structures (i.e. claws and burrows) aid in thermoregulation. The extent to which claw size affects body temperature and burrows can serve as thermal refugia is still unknown. Here, we were interested in understanding the thermal constraints imposed on a highly dimorphic intertidal species. We asked (1) how does crab activity and body temperature differ between microhabitats? We defined shaded and unshaded sections of mangrove forests where crabs are mostly active. (2) Is male surface activity constrained by temperature? (3) How does the large male claw affect body temperature and thermoregulation? (4) How does use of the burrow help buffer temperatures? If burrow depth or claw size affects body temperature, then these structures, coupled with shifts in behaviour, can help explain how organisms can reconcile mating strategies in adverse environments.
the burrow for the ca. 20 day incubation period until she reemerges on a nocturnal spring tide to release larvae (Reaney & Backwell, 2007). This study was conducted over 6 days in a single mating cycle during November 2015 at East Point Reserve, Darwin, Australia (12 24.530 S, 130 49.850 E). This population of U. mjoebergi resides in the high intertidal zone where the mudflat represents a heterogeneous matrix of microhabitats, with open unshaded areas interspersed with areas shaded by mangroves. To characterize the temperature profile, we placed five iButton dataloggers (Thermochron, Maxim Integrated Products, Inc., Sunnyvale, CA, U.S.A.) 1 cm below the surface in shaded and unshaded habitats of the mudflat and recorded temperature every 10 min for the duration of the study. Three Hobo pendant temperature dataloggers (Onset Computer Corporation, Bourne, MA, U.S.A.) were suspended from mangrove branches and recorded air temperature every 5 min for the duration of the study. Population Level Surface Activity in Different Microhabitats Surface activity was quantified in both unshaded and shaded microhabitats throughout the day as the number of crabs active on the surface within 0.33 m2 plots. Each plot was monitored for 5 min every hour from 0900 to 1300 hours and the maximum number of male and female crabs active on the surface during the 5 min observation time was recorded. A total of six plots in each microhabitat were monitored each day, and the experiment was conducted over 4 days. Plots were moved each day to a new random location, for a total sample size of 36 plots (2 microhabitats 6 plots 4 days). Data were analysed using a linear mixed-effects REML-fitted model with number of crabs active on the surface as the response variable. Microhabitat, sex and time were included as fixed effects, and date was included as a random effect.
METHODS Surface Activity, Body Temperatures and Thermal Limits Study Site and Species Uca mjoebergi is a small (<15 mm carapace width, <1.2 g wet mass) fiddler crab that is endemic to Australian intertidal mudflats. As in other fiddler crab species, sexual dimorphism is extreme. Females possess two small feeding claws, while males possess a single small (minor) feeding claw and a greatly enlarged major claw. In male U. mjoebergi, the major claw accounts for 35.6 ± 0.007% (mean ± SE) of total body mass (we measured N ¼ 28 males using a balance). Individual male and female U. mjoebergi defend burrows and surrounding territories (ca. 10 cm diameter) on the mudflat surface (Crane, 1975). The burrow is used as a refuge from predation (Reaney, 2007), heat stress (Darnell et al., 2013; Smith & Miller, 1973) and desiccation stress, as well as a mating site (Reaney & Backwell, 2007). The surrounding territory is used for feeding and courting (Reaney, 2007). Time spent on the surface also influences feeding, and females can generally feed faster than males (Weis & Weis, 2004); yet the number of males feeding on the surface does not differ between shaded and unshaded habitats (Kerr & Backwell, 2016). Mating in U. mjoebergi occurs during a 7e9-day period around neap tides, when the mudflat is exposed throughout the tidal cycle. Sexually receptive females leave their own burrows and wander the mudflat surface searching for a mate. Males perform a species-specific waving display on the surface near their burrows to attract the attention of nearby females. Females typically sample multiple burrows before selecting a mate (Clark & Backwell, 2015). Mating occurs in the male's burrow, and males then guard females until oviposition (1e2 days). The male return to the surface to resume courting, feeding and mating, while the female remains in
The duration of surface activity was measured for individual male and female crabs. Individual focal crabs were observed and timed from the time they emerged from the burrow until the next retreat to the burrow. Observations were made from 1e2 m away to reduce the risk of disturbance, and data were discarded if crabs were startled into the burrow. Durations of surface activity were measured throughout the day, from ca. 0800 to 1400 hours, in both shaded and open microhabitats. A total of 172 individual crabs were observed for the duration of surface activity. Body temperatures of surface-active fiddler crabs were measured using a hypodermic copper-constantan thermocouple (Model MT-29/1, 0.1 C accuracy, Physitemp Instruments Inc., Clifton, NJ, U.S.A.) connected to a thermocouple thermometer (Microtherma 2T, 0.2 C accuracy, ThermoWorks, American Fork, UT, U.S.A.) inserted into the body cavity through the posterior margin of the carapace. Body temperatures were only measured if the crab could be captured on the surface or within 30 s of entering the burrow, and were taken within 5 s of capture. We measured body temperature for 205 individual surface-active crabs, both male and female, in both shaded and unshaded microhabitats, from ca. 0800 to 1400 hours. The critical thermal maximum (CTmax), which represents the maximum temperature at which performance is possible, was estimated under natural conditions using the loss of righting response as the endpoint (Allen, Rodgers, Tuan, & Levinton, 2012; Cuculescu, Hyde, & Bowler, 1998; Lutterschmidt & Hutchison, 1997). Prior to experimentation, crabs were held in the shade in a shallow plastic pan containing ca. 1 cm of water to ensure full hydration. Male crabs were then tethered on the mudflat surface
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using cotton thread glued to the carapace, and allowed to heat under direct sunlight. This procedure approximated the natural rate at which crabs would heat when exiting the burrow and moving onto the mudflat surface. At least every minute (more frequently as temperature increased), the righting response of each crab was tested by flipping the crab onto its dorsal side. Fiddler crabs typically right themselves immediately. When a crab could no longer right itself within 1 min, body temperature was measured using a hypodermic copper-constantan thermocouple connected to a thermocouple thermometer and recorded as CTmax. A total of 12 male crabs were tested for CTmax (see Ethical Note). Burrows as Thermal Refugia Fiddler crabs retreat into burrows in response to thermal stress. Burrows that are accepted by females have a volume of approximately 51 ml (Clark & Backwell, 2016), yet burrow shape can differ depending on underground structures such as mangrove roots. We examined body temperatures of male fiddler crabs confined to the surface and crabs confined within the burrow, to assess the value of the burrow as a thermal refuge. Experiments were conducted in two microhabitats, full sun and shade, and during two times of the day, morning (0800e1000 hours) and afternoon (1300e1400 hours). Prior to all experiments, crabs were held in the shade in a shallow plastic pan containing ca. 1 cm of water to ensure full hydration. Each crab was confined individually either on the surface of the mudflat or within a natural burrow, with crabs randomly assigned to treatments. Crabs assigned to the surface were confined in rectangular plastic containers (17 12 cm and 8 cm high) with a mesh bottom and containing a thin layer of moist sediment taken from the nearby mudflat surface. Crabs assigned to burrows were placed at a burrow entrance and quickly entered the burrow. To ensure the crab descended completely, a rubber catheter was inserted into the burrow to its maximum depth, where the crab was confined. Following 20 min of confinement in the appropriate microhabitat, body temperatures were measured using a hypodermic copper-constantan thermocouple connected to a thermocouple thermometer and inserted into the body cavity through the posterior margin of the carapace (for crabs on the surface) or a flexible fine-wire (0.23 mm diameter) copperconstantan thermocouple (Model IT-23 or IT-24P, 0.1 C accuracy, Physitemp Instruments Inc.) connected to a thermocouple thermometer (Microtherma 2T, 0.2 C accuracy, ThermoWorks) and implanted ca. 2 mm into the body cavity prior to the start of the experiment through the posterior margin of the carapace (for crabs in burrows). Following measurements of body temperature, carapace width and claw length (measured as propodus length) were measured and burrow depth was determined by measuring catheter insertion length. Three to four crabs were confined in each microhabitatelocation combination (shaded versus unshaded, surface versus burrow) simultaneously during the morning and afternoon of each of 3 days. Data were first analysed to test for an effect of claw size (relative to body size) on body temperature. An REML-fitted linear mixedeffects model was fitted with body temperature (at the end of the experiment) as the response variable, date as a random effect and relative claw size (calculated as residuals from a regression of claw length against carapace width) as a fixed effect. Residuals from this analysis, representing body temperature not explained by claw size, were used as the response variable in a second analysis, with microhabitat (shaded versus unshaded) and crab location (surface versus burrow) as fixed effects and date as a random effect for early and late experiments. Finally, a third analysis examined the effect of burrow depth on body temperature for the two microhabitats (shaded versus unshaded) at the two times of day (early versus
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late). All analyses were performed in JMP (SAS Institute, Cary, NC, U.S.A.). Ethical Note All the methods in this study have progressed from previous laboratory trials and have been designed to minimize mortality and stress. All crabs were collected by hand the same day as, and released at the collection site following, experimentation. Between collection and experimentation, all crabs were held in the shade plastic containers containing ca. 1 cm of water to minimize thermal or desiccation stress. Preliminary experiments indicate minimal mortality (<5%) associated with thermocouple insertion. Although we did not directly quantify recovery rates after field-based CTmax experiments here, our past research has shown high recovery rates (96% alive 12 h after experimentation) in laboratory tests of CTmax (Darnell & Darnell, 2016). RESULTS Population-level Surface Activity in Different Microhabitats The mangrove forests in this study presented strong daily fluctuations in air and surface temperatures. During the morning (0800e1000 hours) air temperatures averaged 34.09 C, shaded surface temperatures were 30.95 C and unshaded surface temperatures were 34.11 C. Late afternoon (1300e1400 hours) temperatures were higher in air (37.83 C), shaded surface (35.43 C) and unshaded surface (43.69 C). The number of crabs active on the surface decreased as the day progressed, with crabs becoming less active in unshaded, open areas than in shaded areas (Fig. 1). The number of females decayed similarly in both shaded and unshaded microhabitats as the day progressed (Fig. 1a, Table 1). However, the number of males diverged between microhabitats with time (Fig. 1b, Table 1) with males in the unshaded habitat rapidly becoming less active than males in the shade; while the activity rates were marginally significant there were significantly fewer males in unshaded than shaded habitats. Surface Activity, Body Temperatures and Thermal Limits As the day progressed, individual crabs spent less time on the surface (Fig. 2). Day of observation accounted for 10.57% of the variance and surface time was strongly affected by microhabitat (F ¼ 45.74, P < 0.0001) and time of day (F ¼ 17.276, P < 0.0001) but there was no effect of either sex (F ¼ 2.386 P ¼ 0.124) or the microhabitat)sex interaction (F ¼ 0.0002, P ¼ 0.9878). Female surface time averaged 618 ± 34.56 s (mean ± SE) in shaded and 223 ± 30.86 s in unshaded microhabitats (Fig. 2a). The duration of male surface activity averaged 696 ± 123.56 s and 253 ± 34.56 in shaded and unshaded microhabitats, respectively (Fig. 2b). The large variation in male surface time was likely to be driven by the hour of the day and body temperature. In the shade, crab body temperatures increased as the day progressed (F ¼ 16.66, P < 0.0001; Fig. 3); however, there were no differences between male and female temperatures (F ¼ 0.636, P ¼ 0.427) or the interaction between time of day and sex (F ¼ 0.708, P ¼ 0.400). In open, unshaded spaces with direct sunlight, body temperatures also increased with time of day (F ¼ 34.76 P < 0.0001) with females having slightly higher temperatures than males (F ¼ 4.35, P ¼ 0.039) but there was no interaction between time of day and sex (F ¼ 0.000, P ¼ 0.995). The upper critical thermal limit, CTmax, measured in the field under natural heating rates, averaged (±SE) 40.2 ± 0.42 C.
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3000
3 (a)
Unshaded Shaded
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Figure 2. Time spent on the surface after emerging from a burrow as a function of time of day for (a) females and (b) males in shaded and unshaded microhabitats.
Time Figure 1. The number of (a) female and (b) male U. mjoebergi fiddler crabs active on the surface as a function of time of day and microhabitat. Means ± SE.
Table 1 Results of mixed model testing population level activity of U. mjoebergi throughout the day in different habitats Source All Hour Microhab Hour)microhab Sex Hour)sex Microhab)sex Hour)microhab)sex Male Hour Microhab Hour)microhab Female Hour Microhab Hour)microhab
df
F ratio
P>F
h2partial
4 1 4 1 4 1 4
83.164 227.526 9.037 534.795 0.497 103.785 7.580
<0.001 <0.001 <0.001 <0.001 0.738 <0.001 <0.001
0.404 0.316 0.0691 0.521 0.004 0.174 0.058
4 1 4
25.983 180.899 2.031
<0.001 <0.001 0.091
0.394 0.542 0.190
4 1 4
31.843 8.578 0.498
<0.001 0.004 0.737
0.418 0.051 0.015
Time of day, microhabitat (microhab, shaded versus unshaded) and sex were fixed effects. Sampling day was considered a random effect and accounted for 8.04% of the variance. To tease apart the three-way interaction, we split the analysis by sex and performed a mixed model with time of day and microhabitat. Random effect of day accounted for 4.57% and 7.63% of the variance in the male and female models (N ¼ 480 crabs). Partial effect size (h2partial) was calculated for a linear model not taking the random effects into account.
Burrows as Thermal Refugia The morning residual body temperature was explained by microhabitat (F ¼ 12.528, P ¼ 0.012) with animals in the shade being cooler than animals in the sun (Fig. 4a). Location (surface or burrow) also had an effect (F ¼ 31.36, P < 0.0001): crabs in burrows were cooler than crabs on the surface (Fig. 4a) but there was no interaction between location and microhabitat (F ¼ 0.638, P ¼ 0.43). With morning crabs, date accounted for 12.42% of the variance. In contrast, afternoon crab temperatures were still affected by microhabitat (F ¼ 36.1, P < 0.0001) and the interaction between location and microhabitat (F ¼ 6.743, P ¼ 0.015) but not by location alone (F ¼ 1.428, P ¼ 0.242; Fig. 4b). In direct sunlight, body temperatures differed between crabs in burrows and crabs on the surface if these crabs were exposed to direct sunlight, while in the shade body temperatures did not differ between burrow and surface crabs. In the afternoon experiments, date accounted for 26.41% of the variance. Burrow depth affected Tb, with crabs in deeper burrows having lower temperatures (Table 2). Relative claw size was negatively correlated with body temperature and accounted for 9.67% of the variation in body temperature (F ¼ 7.71, P ¼ 0.007; Fig. 5). When the two x-axis outliers are removed, claw size explains 11% of the variation in body temperature (F ¼ 8.80, P ¼ 0.004). DISCUSSION Hot tropical environments constrain ectotherm mating opportunities when mate selection occurs on the surface. Thus,
P. Munguia et al. / Animal Behaviour 123 (2017) 259e265
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40 (a)
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Figure 3. Body temperature (Tb) regressed against time of day for (a) females and (b) males in shaded and unshaded microhabitats. Dashed line in (b) represents CTmax.
microhabitats and refugia can become a qualitative trait in mate selection and in fiddler crabs it pays to be in the shade. Uca mjoebergi has a narrow thermal window: males must be on the surface to actively display to females and females must be on the surface to assess mates. The thermal niche can have a strong effect on feeding and mating opportunities and those individuals that can sustain longer periods actively seeking mates can directly reap benefits through increased matings. Sex-specific response to temperature can arise from sex-specific mating behaviours. In this study, both sexes were most active during the cooler part of the day (Fig. 1) and females in the choosy role can afford to spend less time on the surface (Fig. 2). If males retreat to a burrow it may prove costly as they will not be able to display to potential mates. However, male size does not differ between shaded and unshaded microhabitats (Kerr & Backwell, 2016). Sexually selected characters such as the large claw in males can confer a thermal buffer to males (Darnell & Munguia, 2011), perhaps allowing them to remain on the surface longer than females, as we saw in this study (Figs 1 and 2). In Uca pugilator and Uca panacea, males and females have different colour-changing abilities, with males being more labile when changing colour in response to temperature (Kronstadt, Darnell, & Munguia, 2013; Munguia, Levinton, & Silbiger, 2013; Silbiger & Munguia, 2008), suggesting that colour change is another strategy that can allow males to remain on the surface for longer periods. This body of work suggests that while sexual selection is a powerful force affecting sex-specific morphologies and behaviours, temperature
Shaded
Unshaded
Figure 4. Body temperature (Tb) of male fiddler crabs in (a) the morning and (b) the afternoon as a function of microhabitat (shaded versus unshaded) and location (burrow versus surface). Figure shows full Tb to show temperature in degrees, not taking claw size into account. Different letters represent statistically different temperatures in a Tukey HSD test.
Table 2 Effect of burrow depth on total male crab body temperature Source
df
F ratio
Prob>F
h2partial
Time Microhab Time)microhab Depth Time)depth Microhab)depth Time)microhab)depth
1 1 1 1 1 1 1
67.255 22.739 1.407 6.026 2.60 0.402 0.125
<0.001 <0.001 0.244 0.019 0.117 0.531 0.727
0.671 0.400 0.047 0.146 0.079 0.012 0.003
The mixed model included time of day (early versus late), microhabitat (microhab, shaded versus unshaded) and log-transformed burrow depth (depth). Date as a random effect accounted for 6.37% of the variance (N ¼ 41 samples). Partial effect size (h2partial) was calculated for a linear model not taking the random effects into account.
can modulate these traits with a feedback to mating strategies. In this study, almost 10% of the variation in male body temperature was explained by the size of claw (taking body size into account), which is not trivial given that claw size driving mate choice has an effect size ranging from 34% to 79% in U. mjoebergi (Milner et al., 2010) and in Uca tangeri the number of female visits had a 38% correlation with male claw size (Latruffe, McGregor, & Oliveira, 1999). Endurance rivalry brings greater mating success to individuals outlasting direct competitors in the breeding area (Banks & Thompson, 1985). The endurance rivalry hypothesis is a useful framework to examine direct and indirect links between sexual and natural selection (Campagna & Leboeuf, 1988; Darnell et al., 2013; Judge & Brooks, 2001; Salvador et al., 2008). In many species of fiddler crabs, mating occurs in the burrow but mate attraction
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microhabitats can play a critical role modulating mating opportunities. Uca mjoebergi individuals, and particularly males, are constrained by hot tropical climates and their structural adaptations have synergistically evolved with the mating behaviours. High temperature is an imposing barrier to ectotherms. Individuals attempt to push their limits to gain advantage over conspecifics. Classic sexually selected traits, including ornaments and behaviours, can have a secondary purpose in thermoregulation.
Body temperature (Tb, °C)
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Acknowledgments
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–6 –4 0 2 –2 Claw size independent of body size
Figure 5. Body temperature (Tb) as a function of claw size independent of body size. Claw size was regressed against body size and the residuals are used in this analysis to explain body temperature (N ¼ 74).
occurs on the surface (Crane, 1975). The burrow has received a lot of attention given its role in mating (Christy, 1983; Pratt, McLain, & Lathrop, 2003; Reaney & Backwell, 2007), and females select mates based on burrow quality (Backwell & Passmore, 1996; Christy, 1982). Yet, understanding the role of burrows as thermal refugia helps us explain more of the variance in mating success (Fig. 4). The longer a male remains on the surface, the greater its mating probability, but high temperatures limit time on the surface later in the day. Burrows are used for mating, but also offer protection from predators and serve as thermal refugia particularly in sun-exposed habitats. With rising temperatures, males must balance time spent in the burrow cooling off (at the expense of mating opportunities) with time spent on the surface attracting potential mates (at the cost of increased temperature stress and risk of desiccation). The large claw helps males remain cooler (Darnell & Munguia, 2011), modulating this trade-off and this study shows the claw contributes to increased time on the surface. Thus, endurance rivalry is enhanced by structures that serve different functions, and understanding their links can provide more accurate models that quantify selection. Mating strategies are often affected by habitat quality (Myhre, Forsgren, & Amundsen, 2013). Uca mjoebergi male sizes do not differ between shaded and unshaded habitats. However, females prefer males in shaded areas (Kerr & Backwell, 2016). Courtship activity, mate choice and parental care are all adjusted to factors like predation risk (Lima & Dill, 1990), visibility in dense vegetation (Candolin, Salesto, & Evers, 2007), water colour (Endler & Houde, 1995), food availability (Allen & Levinton, 2014) and temperature (Bashey & Dunham, 1997). The effect of thermal constraints on mating behaviour is particularly important due to its potential consequences. In sceloporus lizards, for example, individuals need to retreat into cooler refuges during the hottest parts of the day since they risk death by overheating. Retreating, however, limits foraging and reproductive behaviour which increases the risk of extinction (Sinervo et al., 2010). It is predicted that 39% of all lizard species will be extinct by 2080 due to the effects of temperature increases (Sinervo et al., 2010). In this study, shaded areas under mangrove trees were highly sought after by males, presumably because shaded individuals do not need to retreat into their burrows during the hottest part of the day, and females chose males in shaded areas (Kerr & Backwell, 2016). Mating behaviour and secondary sexual characters are often the focus behind sexual selection; however, thermal environments and
This project was funded by University of Adelaide Mid-Career Fellowship to P.M., ARC DP to P.B., and an Early-Career Research Fellowship from the Gulf Research Program of the National Academies of Sciences, Engineering, and Medicine to M.Z.D. The content is solely the responsibility of the authors and does not necessarily represent the official views of the Gulf Research Program of the National Academies of Sciences, Engineering, and Medicine. DATA ACCESSIBILITY Data are available in Dryad http://dx.doi.org/10.5061/dryad. ph416. References Allen, B. J., & Levinton, J. S. (2014). Sexual selection and the physiological consequences of habitat choice by a fiddler crab. Oecologia, 176, 25e34. Allen, B. J., Rodgers, B., Tuan, Y., & Levinton, J. S. (2012). Size-dependent temperature and desiccation constraints on performance capacity: Implications for sexual selection in a fiddler crab. Journal of Experimental Marine Biology and Ecology, 438, 93e99. Backwell, P. R. Y., & Passmore, N. I. (1996). Time constraints and multiple choice criteria in the sampling behaviour and mate choice of the fiddler crab, Uca annulipes. Behavioral Ecology and Sociobiology, 38, 407e416. Banks, M. J., & Thompson, D. J. (1985). Lifetime mating success in the damselfly Coenagrion puella. Animal Behaviour, 33, 1175e1183. Bashey, F., & Dunham, A. E. (1997). Elevational variation in the thermal constraints on and microhabitat preferences of the greater earless lizard Cophosaurus texanus. Copeia, 725e737. Campagna, C., & Leboeuf, B. J. (1988). Reproductive behavior of southern sea lions. Behaviour, 104, 233e261. Candolin, U., Salesto, T., & Evers, M. (2007). Changed environmental conditions weaken sexual selection in sticklebacks. Journal of Evolutionary Biology, 20, 233e239. Christy, J. H. (1982). Burrow structure and use in the sand fiddler crab Uca pugilator (Bosc). Animal Behaviour, 30, 687e694. Christy, J. H. (1983). Female choice in the resource-defense mating system of the sand fiddler crab, Uca pugilator. Behavioral Ecology and Sociobiology, 12, 169e180. Clark, H. L., & Backwell, P. R. Y. (2015). Temporal and spatial variation in female mating preferences in a fiddler crab. Behavioral Ecology and Sociobiology, 69, 1779e1784. Clark, H. L., & Backwell, P. R. Y. (2016). Male mating success in a fiddler crab: A lesson in sample sizes. Journal of Ethology, 34, 119e126. Crane, J. (1975). Fiddler crabs of the world (Ocypodidae: genus Uca). Princeton, NJ: Princeton University Press. Cuculescu, M., Hyde, D., & Bowler, K. (1998). Thermal tolerance of two species of marine crab, Cancer pagurus and Carcinus maenas. Journal of Thermal Biology, 23, 107e110. Darnell, M. Z., & Darnell, K. (2016). Latitudinal gradients in thermal tolerance and morphology in a fiddler crab sister species pair. Manuscript in preparation. Darnell, M. Z., Fowler, K. K., & Munguia, P. (2013). Sex-specific thermal constraints on fiddler crab behavior. Behavioral Ecology, 24, 997e1003. Darnell, M. Z., & Munguia, P. (2011). Thermoregulation as an alternate function of the sexually dimorphic fiddler crab claw. American Naturalist, 178, 419e428. Darnell, M. Z., Nicholson, H. S., & Munguia, P. (2015). Thermal ecology of the fiddler crab Uca panacea: Thermal constraints and organismal responses. Journal of Thermal Biology, 52, 157e165. Detto, T. (2007). The fiddler crab Uca mjoebergi uses colour vision in mate choice. Proceedings of the Royal Society B: Biological Sciences, 274, 2785e2790. Endler, J. A., & Houde, A. E. (1995). Geographic variation in female preferences for male traits in Poecilia reticulata. Evolution, 49, 456e468. Greenberg, R., & Danner, R. M. (2013). Climate, ecological release and bill dimorphism in an island songbird. Biology Letters, 9, 20130118. Huey, R. B., Kearney, M. R., Krockenberger, A., Holtum, J. A. M., Jess, M., & Williams, S. E. (2012). Predicting organismal vulnerability to climate warming:
P. Munguia et al. / Animal Behaviour 123 (2017) 259e265 Roles of behaviour, physiology and adaptation. Philosophical Transactions of the Royal Society B: Biological Sciences, 367, 1665e1679. Hyatt, G. W., & Salmon, M. (1978). Combat in fiddler crabs Uca pugilator and Uca pugnax: Quantitative analysis. Behaviour, 65, 182e211. Judge, K. A., & Brooks, R. J. (2001). Chorus participation by male bullfrogs, Rana catesbeiana: A test of the energetic constraint hypothesis. Animal Behaviour, 62, 849e861. Kearney, M., Shine, R., & Porter, W. P. (2009). The potential for behavioral thermoregulation to buffer “cold-blooded” animals against climate warming. Proceedings of the National Academy of Sciences of the United States of America, 106, 3835e3840. Kerr, K., & Backwell, P. R. Y. (2016). The effect of shading on female fiddler crab mate preferences. Manuscript in preparation. Kronstadt, S. M., Darnell, M. Z., & Munguia, P. (2013). Background and temperature effects on Uca panacea color change. Marine Biology, 160, 1373e1381. Latruffe, C., McGregor, P. K., & Oliveira, R. F. (1999). Visual signalling and sexual selection in male fiddler crabs Uca tangeri. Marine Ecology Progress Series, 189, 233e240. Lima, S. L., & Dill, L. M. (1990). Behavioral decisions made under the risk of predation: A review and prospectus. Canadian Journal of Zoology, 68, 619e640. Luther, D., & Greenberg, R. (2014). Habitat type and ambient temperature contribute to bill morphology. Ecology and Evolution, 4, 699e705. Lutterschmidt, W. I., & Hutchison, V. H. (1997). The critical thermal maximum: History and critique. Canadian Journal of Zoology, 75, 1561e1574. Milner, R. N. C., Detto, T., Jennions, M. D., & Backwell, P. R. Y. (2010). Experimental evidence for a seasonal shift in the strength of a female mating preference. Behavioral Ecology, 21, 311e316. Monaco, C. J., Wethey, D. S., Gulledge, S., & Helmuth, B. (2015). Shore-level size gradients and thermal refuge use in the predatory sea star Pisaster ochraceus: The role of environmental stressors. Marine Ecology Progress Series, 539, 191e205. Munguia, P., Levinton, J. S., & Silbiger, N. J. (2013). Latitudinal differences in thermoregulatory color change in Uca pugilator. Journal of Experimental Marine Biology and Ecology, 440, 8e14.
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Murai, M., & Backwell, P. R. Y. (2006). A conspicuous courtship signal in the fiddler crab Uca perplexa: Female choice based on display structure. Behavioral Ecology and Sociobiology, 60, 736e741. Myhre, L. C., Forsgren, E., & Amundsen, T. (2013). Effects of habitat complexity on mating behavior and mating success in a marine fish. Behavioral Ecology, 24, 553e563. Pope, D. S. (2000). Testing function of fiddler crab claw waving by manipulating social context. Behavioral Ecology and Sociobiology, 47, 432e437. Pratt, A. E., McLain, D. K., & Lathrop, G. R. (2003). The assessment game in sand fiddler crab contests for breeding burrows. Animal Behaviour, 65, 945e955. Reaney, L. T. (2007). Foraging and mating opportunities influence refuge use in the fiddler crab, Uca mjoebergi. Animal Behaviour, 73, 711e716. Reaney, L. T., & Backwell, P. R. Y. (2007). Temporal constrains and female preference for burrow width in the fiddler crab, Uca mjoebergi. Behavioral Ecology and Sociobiology, 61, 1515e1521. Salvador, A., Díaz, J. A., Veiga, J. P., Bloor, P., & Brown, R. P. (2008). Correlates of reproductive success in male lizards of the alpine species Iberolacerta cyreni. Behavioral Ecology, 19, 169e176. Shepherd, B. L., Prange, H. D., & Moczek, A. P. (2008). Some like it hot: Body and weapon size affect thermoregulation in horned beetles. Journal of Insect Physiology, 54, 604e611. Silbiger, N., & Munguia, P. (2008). Carapace color change in Uca pugilator as a response to temperature. Journal of Experimental Marine Biology and Ecology, 355, 41e46. Sinervo, B., Mendez-de-la-Cruz, F., Miles, D. B., Heulin, B., Bastiaans, E., VillagranSanta Cruz, M., et al. (2010). Erosion of lizard diversity by climate change and altered thermal niches. Science, 328, 894e899. Smith, W. K., & Miller, P. C. (1973). The thermal ecology of two South Florida fiddler crabs: Uca rapax Smith and U. pugilator Bosc. Physiological Zoology, 46, 186e207. Weis, J. S., & Weis, P. (2004). Behavior of four species of fiddler crabs, genus Uca, in southeast Sulawesi, Indonesia. Hydrobiologia, 523, 47e58.
Contents lists available at ScienceDirect
Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav
Thermal constraints on microhabitat selection and mating opportunities Pablo Munguia a, *, Patricia R. Y. Backwell b, M. Zachary Darnell c, 1 a
School of Biological Sciences, The University of Adelaide, Adelaide, Australia Research School of Biology, The Australian National University, Canberra, Australia c Department of Biological Sciences, Nicholls State University, Thibodaux, LA, U.S.A. b
a r t i c l e i n f o Article history: Received 15 April 2016 Initial acceptance 15 June 2016 Final acceptance 6 October 2016 MS. number: 16-00329R Keywords: behavioural thermoregulation fiddler crab sexual dimorphism sexual selection Uca mjoebergi
Hot tropical environments constrain ectotherm mating opportunities when mate selection occurs on the surface. Thus, microhabitats and refugia can become a qualitative trait in mate selection. In fiddler crabs, the enlarged claw of males can act as a heat sink, which becomes advantageous when surface temperatures reach 50 C during the day and crabs are actively seeking to mate. Uca mjoebergi females prefer male burrows found in the shade; therefore, we investigated the thermal constraints imposed on males and females in shaded and unshaded habitats. Crab surface activity decreased and body temperature increased as the day progressed, with more crabs active in shaded than sunny microhabitats. Body temperature was lower in male crabs found in burrows relative to crabs on the surface. Male claw size explained 10% of body temperature. Our results add further support to the hypothesis that thermal constraints imposed on males can be overcome by the large claw acting as a heat sink and the burrow acting as a refuge from heat. Classic sexually selected traits, including ornaments and behaviours, can have a secondary purpose in thermoregulation. © 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
The tropics are hot, yet many tropical species exhibit diurnal activity patterns and are active in direct sunlight, exposed to extreme heat stress (Sinervo et al., 2010). Species that court and mate during the hottest part of the day would be particularly stressed; selection therefore favours individuals that endure adverse environmental conditions in exchange for mating opportunities. Sexual selection via endurance rivalry should favour extended presence at the breeding site, which is often associated with an increase in the number of mating opportunities (Banks & Thompson, 1985; Darnell, Fowler, & Munguia, 2013; Salvador, Díaz, Veiga, Bloor, & Brown, 2008). Across both tropical and temperate zones, high temperature, solar radiation and desiccation risk can limit time spent in these stressful breeding areas, as has been shown for both endotherms (Campagna & Leboeuf, 1988) and ectotherms (Darnell et al., 2013; Monaco, Wethey, Gulledge, & Helmuth, 2015), and thus environmental conditions may modulate mating behaviour and mating opportunities. This is expected to
* Correspondence: P. Munguia, School of Biological Sciences, The University of Adelaide, Adelaide, SA 5005, Australia. E-mail address: [email protected] (P. Munguia). 1 Present address: Division of Coastal Sciences, School of Ocean Science and Technology, The University of Southern Mississippi, Ocean Springs, MS, U.S.A.
be of greater importance in the tropics, where temperature and solar radiation are at a maximum. In ectotherms, behavioural thermoregulation can buffer individuals from thermal stress associated with extreme or rapidly changing temperatures (Huey et al., 2012; Kearney, Shine, & Porter, 2009; Smith & Miller, 1973). Mobile species can exploit the thermal heterogeneity of the environment to regulate body temperatures and reduce exposure to thermal extremes. Yet because behavioural thermoregulation often involves a retreat from breeding or display sites to more thermally innocuous habitats, it may come at a cost to mating opportunities. Thermoregulatory microhabitat selection can thus limit activity periods involved in mate searching and display (Darnell et al., 2013). Sexually dimorphic traits can also influence thermoregulatory ability and body temperature, as morphology alters rates of heat transfer between the organism and the environment. A number of sexually dimorphic traits have thermoregulatory effects including bills (Greenberg & Danner, 2013; Luther & Greenberg, 2014), horns (Shepherd, Prange, & Moczek, 2008) and claws (Darnell & Munguia, 2011). Fiddler crabs are often studied in the context of sexual selection and sexual dimorphism. Male fiddler crabs have an enlarged claw that is used in courtship (Detto, 2007; Murai & Backwell, 2006; Pope, 2000), intrasexual combat (Hyatt & Salmon, 1978) and thermoregulation, transferring heat away from the body and
http://dx.doi.org/10.1016/j.anbehav.2016.11.004 0003-3472/© 2016 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
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dissipating it to the environment (Darnell & Munguia, 2011). Differential selective pressures on males and females due to sexual selection via endurance rivalry on males results in sex-specific thermal constraints on behaviour (Darnell et al., 2013). Male fiddler crabs perform a waving display on the surface to attract females and because signal competition occurs during this time, endurance rivalry favours increased time on the surface. Yet while on the surface performing the waving display, males endure extreme thermal conditions, with operative environmental temperatures often exceeding preferred and even lethal temperatures (Allen & Levinton, 2014; Darnell, Nicholson, & Munguia, 2015). Surface activity of female fiddler crabs does not seem to be constrained by temperature (Darnell et al., 2013; Milner, Detto, Jennions, & Backwell, 2010), but female mate choice can be influenced by temperature, as mate-searching females preferentially approach males that are in the shade regardless of whether the females are released in the sun or shade (Kerr & Backwell, 2016). The challenge is to understand how mating strategies and structures (i.e. claws and burrows) aid in thermoregulation. The extent to which claw size affects body temperature and burrows can serve as thermal refugia is still unknown. Here, we were interested in understanding the thermal constraints imposed on a highly dimorphic intertidal species. We asked (1) how does crab activity and body temperature differ between microhabitats? We defined shaded and unshaded sections of mangrove forests where crabs are mostly active. (2) Is male surface activity constrained by temperature? (3) How does the large male claw affect body temperature and thermoregulation? (4) How does use of the burrow help buffer temperatures? If burrow depth or claw size affects body temperature, then these structures, coupled with shifts in behaviour, can help explain how organisms can reconcile mating strategies in adverse environments.
the burrow for the ca. 20 day incubation period until she reemerges on a nocturnal spring tide to release larvae (Reaney & Backwell, 2007). This study was conducted over 6 days in a single mating cycle during November 2015 at East Point Reserve, Darwin, Australia (12 24.530 S, 130 49.850 E). This population of U. mjoebergi resides in the high intertidal zone where the mudflat represents a heterogeneous matrix of microhabitats, with open unshaded areas interspersed with areas shaded by mangroves. To characterize the temperature profile, we placed five iButton dataloggers (Thermochron, Maxim Integrated Products, Inc., Sunnyvale, CA, U.S.A.) 1 cm below the surface in shaded and unshaded habitats of the mudflat and recorded temperature every 10 min for the duration of the study. Three Hobo pendant temperature dataloggers (Onset Computer Corporation, Bourne, MA, U.S.A.) were suspended from mangrove branches and recorded air temperature every 5 min for the duration of the study. Population Level Surface Activity in Different Microhabitats Surface activity was quantified in both unshaded and shaded microhabitats throughout the day as the number of crabs active on the surface within 0.33 m2 plots. Each plot was monitored for 5 min every hour from 0900 to 1300 hours and the maximum number of male and female crabs active on the surface during the 5 min observation time was recorded. A total of six plots in each microhabitat were monitored each day, and the experiment was conducted over 4 days. Plots were moved each day to a new random location, for a total sample size of 36 plots (2 microhabitats 6 plots 4 days). Data were analysed using a linear mixed-effects REML-fitted model with number of crabs active on the surface as the response variable. Microhabitat, sex and time were included as fixed effects, and date was included as a random effect.
METHODS Surface Activity, Body Temperatures and Thermal Limits Study Site and Species Uca mjoebergi is a small (<15 mm carapace width, <1.2 g wet mass) fiddler crab that is endemic to Australian intertidal mudflats. As in other fiddler crab species, sexual dimorphism is extreme. Females possess two small feeding claws, while males possess a single small (minor) feeding claw and a greatly enlarged major claw. In male U. mjoebergi, the major claw accounts for 35.6 ± 0.007% (mean ± SE) of total body mass (we measured N ¼ 28 males using a balance). Individual male and female U. mjoebergi defend burrows and surrounding territories (ca. 10 cm diameter) on the mudflat surface (Crane, 1975). The burrow is used as a refuge from predation (Reaney, 2007), heat stress (Darnell et al., 2013; Smith & Miller, 1973) and desiccation stress, as well as a mating site (Reaney & Backwell, 2007). The surrounding territory is used for feeding and courting (Reaney, 2007). Time spent on the surface also influences feeding, and females can generally feed faster than males (Weis & Weis, 2004); yet the number of males feeding on the surface does not differ between shaded and unshaded habitats (Kerr & Backwell, 2016). Mating in U. mjoebergi occurs during a 7e9-day period around neap tides, when the mudflat is exposed throughout the tidal cycle. Sexually receptive females leave their own burrows and wander the mudflat surface searching for a mate. Males perform a species-specific waving display on the surface near their burrows to attract the attention of nearby females. Females typically sample multiple burrows before selecting a mate (Clark & Backwell, 2015). Mating occurs in the male's burrow, and males then guard females until oviposition (1e2 days). The male return to the surface to resume courting, feeding and mating, while the female remains in
The duration of surface activity was measured for individual male and female crabs. Individual focal crabs were observed and timed from the time they emerged from the burrow until the next retreat to the burrow. Observations were made from 1e2 m away to reduce the risk of disturbance, and data were discarded if crabs were startled into the burrow. Durations of surface activity were measured throughout the day, from ca. 0800 to 1400 hours, in both shaded and open microhabitats. A total of 172 individual crabs were observed for the duration of surface activity. Body temperatures of surface-active fiddler crabs were measured using a hypodermic copper-constantan thermocouple (Model MT-29/1, 0.1 C accuracy, Physitemp Instruments Inc., Clifton, NJ, U.S.A.) connected to a thermocouple thermometer (Microtherma 2T, 0.2 C accuracy, ThermoWorks, American Fork, UT, U.S.A.) inserted into the body cavity through the posterior margin of the carapace. Body temperatures were only measured if the crab could be captured on the surface or within 30 s of entering the burrow, and were taken within 5 s of capture. We measured body temperature for 205 individual surface-active crabs, both male and female, in both shaded and unshaded microhabitats, from ca. 0800 to 1400 hours. The critical thermal maximum (CTmax), which represents the maximum temperature at which performance is possible, was estimated under natural conditions using the loss of righting response as the endpoint (Allen, Rodgers, Tuan, & Levinton, 2012; Cuculescu, Hyde, & Bowler, 1998; Lutterschmidt & Hutchison, 1997). Prior to experimentation, crabs were held in the shade in a shallow plastic pan containing ca. 1 cm of water to ensure full hydration. Male crabs were then tethered on the mudflat surface
P. Munguia et al. / Animal Behaviour 123 (2017) 259e265
using cotton thread glued to the carapace, and allowed to heat under direct sunlight. This procedure approximated the natural rate at which crabs would heat when exiting the burrow and moving onto the mudflat surface. At least every minute (more frequently as temperature increased), the righting response of each crab was tested by flipping the crab onto its dorsal side. Fiddler crabs typically right themselves immediately. When a crab could no longer right itself within 1 min, body temperature was measured using a hypodermic copper-constantan thermocouple connected to a thermocouple thermometer and recorded as CTmax. A total of 12 male crabs were tested for CTmax (see Ethical Note). Burrows as Thermal Refugia Fiddler crabs retreat into burrows in response to thermal stress. Burrows that are accepted by females have a volume of approximately 51 ml (Clark & Backwell, 2016), yet burrow shape can differ depending on underground structures such as mangrove roots. We examined body temperatures of male fiddler crabs confined to the surface and crabs confined within the burrow, to assess the value of the burrow as a thermal refuge. Experiments were conducted in two microhabitats, full sun and shade, and during two times of the day, morning (0800e1000 hours) and afternoon (1300e1400 hours). Prior to all experiments, crabs were held in the shade in a shallow plastic pan containing ca. 1 cm of water to ensure full hydration. Each crab was confined individually either on the surface of the mudflat or within a natural burrow, with crabs randomly assigned to treatments. Crabs assigned to the surface were confined in rectangular plastic containers (17 12 cm and 8 cm high) with a mesh bottom and containing a thin layer of moist sediment taken from the nearby mudflat surface. Crabs assigned to burrows were placed at a burrow entrance and quickly entered the burrow. To ensure the crab descended completely, a rubber catheter was inserted into the burrow to its maximum depth, where the crab was confined. Following 20 min of confinement in the appropriate microhabitat, body temperatures were measured using a hypodermic copper-constantan thermocouple connected to a thermocouple thermometer and inserted into the body cavity through the posterior margin of the carapace (for crabs on the surface) or a flexible fine-wire (0.23 mm diameter) copperconstantan thermocouple (Model IT-23 or IT-24P, 0.1 C accuracy, Physitemp Instruments Inc.) connected to a thermocouple thermometer (Microtherma 2T, 0.2 C accuracy, ThermoWorks) and implanted ca. 2 mm into the body cavity prior to the start of the experiment through the posterior margin of the carapace (for crabs in burrows). Following measurements of body temperature, carapace width and claw length (measured as propodus length) were measured and burrow depth was determined by measuring catheter insertion length. Three to four crabs were confined in each microhabitatelocation combination (shaded versus unshaded, surface versus burrow) simultaneously during the morning and afternoon of each of 3 days. Data were first analysed to test for an effect of claw size (relative to body size) on body temperature. An REML-fitted linear mixedeffects model was fitted with body temperature (at the end of the experiment) as the response variable, date as a random effect and relative claw size (calculated as residuals from a regression of claw length against carapace width) as a fixed effect. Residuals from this analysis, representing body temperature not explained by claw size, were used as the response variable in a second analysis, with microhabitat (shaded versus unshaded) and crab location (surface versus burrow) as fixed effects and date as a random effect for early and late experiments. Finally, a third analysis examined the effect of burrow depth on body temperature for the two microhabitats (shaded versus unshaded) at the two times of day (early versus
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late). All analyses were performed in JMP (SAS Institute, Cary, NC, U.S.A.). Ethical Note All the methods in this study have progressed from previous laboratory trials and have been designed to minimize mortality and stress. All crabs were collected by hand the same day as, and released at the collection site following, experimentation. Between collection and experimentation, all crabs were held in the shade plastic containers containing ca. 1 cm of water to minimize thermal or desiccation stress. Preliminary experiments indicate minimal mortality (<5%) associated with thermocouple insertion. Although we did not directly quantify recovery rates after field-based CTmax experiments here, our past research has shown high recovery rates (96% alive 12 h after experimentation) in laboratory tests of CTmax (Darnell & Darnell, 2016). RESULTS Population-level Surface Activity in Different Microhabitats The mangrove forests in this study presented strong daily fluctuations in air and surface temperatures. During the morning (0800e1000 hours) air temperatures averaged 34.09 C, shaded surface temperatures were 30.95 C and unshaded surface temperatures were 34.11 C. Late afternoon (1300e1400 hours) temperatures were higher in air (37.83 C), shaded surface (35.43 C) and unshaded surface (43.69 C). The number of crabs active on the surface decreased as the day progressed, with crabs becoming less active in unshaded, open areas than in shaded areas (Fig. 1). The number of females decayed similarly in both shaded and unshaded microhabitats as the day progressed (Fig. 1a, Table 1). However, the number of males diverged between microhabitats with time (Fig. 1b, Table 1) with males in the unshaded habitat rapidly becoming less active than males in the shade; while the activity rates were marginally significant there were significantly fewer males in unshaded than shaded habitats. Surface Activity, Body Temperatures and Thermal Limits As the day progressed, individual crabs spent less time on the surface (Fig. 2). Day of observation accounted for 10.57% of the variance and surface time was strongly affected by microhabitat (F ¼ 45.74, P < 0.0001) and time of day (F ¼ 17.276, P < 0.0001) but there was no effect of either sex (F ¼ 2.386 P ¼ 0.124) or the microhabitat)sex interaction (F ¼ 0.0002, P ¼ 0.9878). Female surface time averaged 618 ± 34.56 s (mean ± SE) in shaded and 223 ± 30.86 s in unshaded microhabitats (Fig. 2a). The duration of male surface activity averaged 696 ± 123.56 s and 253 ± 34.56 in shaded and unshaded microhabitats, respectively (Fig. 2b). The large variation in male surface time was likely to be driven by the hour of the day and body temperature. In the shade, crab body temperatures increased as the day progressed (F ¼ 16.66, P < 0.0001; Fig. 3); however, there were no differences between male and female temperatures (F ¼ 0.636, P ¼ 0.427) or the interaction between time of day and sex (F ¼ 0.708, P ¼ 0.400). In open, unshaded spaces with direct sunlight, body temperatures also increased with time of day (F ¼ 34.76 P < 0.0001) with females having slightly higher temperatures than males (F ¼ 4.35, P ¼ 0.039) but there was no interaction between time of day and sex (F ¼ 0.000, P ¼ 0.995). The upper critical thermal limit, CTmax, measured in the field under natural heating rates, averaged (±SE) 40.2 ± 0.42 C.
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3000
3 (a)
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Figure 2. Time spent on the surface after emerging from a burrow as a function of time of day for (a) females and (b) males in shaded and unshaded microhabitats.
Time Figure 1. The number of (a) female and (b) male U. mjoebergi fiddler crabs active on the surface as a function of time of day and microhabitat. Means ± SE.
Table 1 Results of mixed model testing population level activity of U. mjoebergi throughout the day in different habitats Source All Hour Microhab Hour)microhab Sex Hour)sex Microhab)sex Hour)microhab)sex Male Hour Microhab Hour)microhab Female Hour Microhab Hour)microhab
df
F ratio
P>F
h2partial
4 1 4 1 4 1 4
83.164 227.526 9.037 534.795 0.497 103.785 7.580
<0.001 <0.001 <0.001 <0.001 0.738 <0.001 <0.001
0.404 0.316 0.0691 0.521 0.004 0.174 0.058
4 1 4
25.983 180.899 2.031
<0.001 <0.001 0.091
0.394 0.542 0.190
4 1 4
31.843 8.578 0.498
<0.001 0.004 0.737
0.418 0.051 0.015
Time of day, microhabitat (microhab, shaded versus unshaded) and sex were fixed effects. Sampling day was considered a random effect and accounted for 8.04% of the variance. To tease apart the three-way interaction, we split the analysis by sex and performed a mixed model with time of day and microhabitat. Random effect of day accounted for 4.57% and 7.63% of the variance in the male and female models (N ¼ 480 crabs). Partial effect size (h2partial) was calculated for a linear model not taking the random effects into account.
Burrows as Thermal Refugia The morning residual body temperature was explained by microhabitat (F ¼ 12.528, P ¼ 0.012) with animals in the shade being cooler than animals in the sun (Fig. 4a). Location (surface or burrow) also had an effect (F ¼ 31.36, P < 0.0001): crabs in burrows were cooler than crabs on the surface (Fig. 4a) but there was no interaction between location and microhabitat (F ¼ 0.638, P ¼ 0.43). With morning crabs, date accounted for 12.42% of the variance. In contrast, afternoon crab temperatures were still affected by microhabitat (F ¼ 36.1, P < 0.0001) and the interaction between location and microhabitat (F ¼ 6.743, P ¼ 0.015) but not by location alone (F ¼ 1.428, P ¼ 0.242; Fig. 4b). In direct sunlight, body temperatures differed between crabs in burrows and crabs on the surface if these crabs were exposed to direct sunlight, while in the shade body temperatures did not differ between burrow and surface crabs. In the afternoon experiments, date accounted for 26.41% of the variance. Burrow depth affected Tb, with crabs in deeper burrows having lower temperatures (Table 2). Relative claw size was negatively correlated with body temperature and accounted for 9.67% of the variation in body temperature (F ¼ 7.71, P ¼ 0.007; Fig. 5). When the two x-axis outliers are removed, claw size explains 11% of the variation in body temperature (F ¼ 8.80, P ¼ 0.004). DISCUSSION Hot tropical environments constrain ectotherm mating opportunities when mate selection occurs on the surface. Thus,
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Figure 3. Body temperature (Tb) regressed against time of day for (a) females and (b) males in shaded and unshaded microhabitats. Dashed line in (b) represents CTmax.
microhabitats and refugia can become a qualitative trait in mate selection and in fiddler crabs it pays to be in the shade. Uca mjoebergi has a narrow thermal window: males must be on the surface to actively display to females and females must be on the surface to assess mates. The thermal niche can have a strong effect on feeding and mating opportunities and those individuals that can sustain longer periods actively seeking mates can directly reap benefits through increased matings. Sex-specific response to temperature can arise from sex-specific mating behaviours. In this study, both sexes were most active during the cooler part of the day (Fig. 1) and females in the choosy role can afford to spend less time on the surface (Fig. 2). If males retreat to a burrow it may prove costly as they will not be able to display to potential mates. However, male size does not differ between shaded and unshaded microhabitats (Kerr & Backwell, 2016). Sexually selected characters such as the large claw in males can confer a thermal buffer to males (Darnell & Munguia, 2011), perhaps allowing them to remain on the surface longer than females, as we saw in this study (Figs 1 and 2). In Uca pugilator and Uca panacea, males and females have different colour-changing abilities, with males being more labile when changing colour in response to temperature (Kronstadt, Darnell, & Munguia, 2013; Munguia, Levinton, & Silbiger, 2013; Silbiger & Munguia, 2008), suggesting that colour change is another strategy that can allow males to remain on the surface for longer periods. This body of work suggests that while sexual selection is a powerful force affecting sex-specific morphologies and behaviours, temperature
Shaded
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Figure 4. Body temperature (Tb) of male fiddler crabs in (a) the morning and (b) the afternoon as a function of microhabitat (shaded versus unshaded) and location (burrow versus surface). Figure shows full Tb to show temperature in degrees, not taking claw size into account. Different letters represent statistically different temperatures in a Tukey HSD test.
Table 2 Effect of burrow depth on total male crab body temperature Source
df
F ratio
Prob>F
h2partial
Time Microhab Time)microhab Depth Time)depth Microhab)depth Time)microhab)depth
1 1 1 1 1 1 1
67.255 22.739 1.407 6.026 2.60 0.402 0.125
<0.001 <0.001 0.244 0.019 0.117 0.531 0.727
0.671 0.400 0.047 0.146 0.079 0.012 0.003
The mixed model included time of day (early versus late), microhabitat (microhab, shaded versus unshaded) and log-transformed burrow depth (depth). Date as a random effect accounted for 6.37% of the variance (N ¼ 41 samples). Partial effect size (h2partial) was calculated for a linear model not taking the random effects into account.
can modulate these traits with a feedback to mating strategies. In this study, almost 10% of the variation in male body temperature was explained by the size of claw (taking body size into account), which is not trivial given that claw size driving mate choice has an effect size ranging from 34% to 79% in U. mjoebergi (Milner et al., 2010) and in Uca tangeri the number of female visits had a 38% correlation with male claw size (Latruffe, McGregor, & Oliveira, 1999). Endurance rivalry brings greater mating success to individuals outlasting direct competitors in the breeding area (Banks & Thompson, 1985). The endurance rivalry hypothesis is a useful framework to examine direct and indirect links between sexual and natural selection (Campagna & Leboeuf, 1988; Darnell et al., 2013; Judge & Brooks, 2001; Salvador et al., 2008). In many species of fiddler crabs, mating occurs in the burrow but mate attraction
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microhabitats can play a critical role modulating mating opportunities. Uca mjoebergi individuals, and particularly males, are constrained by hot tropical climates and their structural adaptations have synergistically evolved with the mating behaviours. High temperature is an imposing barrier to ectotherms. Individuals attempt to push their limits to gain advantage over conspecifics. Classic sexually selected traits, including ornaments and behaviours, can have a secondary purpose in thermoregulation.
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Figure 5. Body temperature (Tb) as a function of claw size independent of body size. Claw size was regressed against body size and the residuals are used in this analysis to explain body temperature (N ¼ 74).
occurs on the surface (Crane, 1975). The burrow has received a lot of attention given its role in mating (Christy, 1983; Pratt, McLain, & Lathrop, 2003; Reaney & Backwell, 2007), and females select mates based on burrow quality (Backwell & Passmore, 1996; Christy, 1982). Yet, understanding the role of burrows as thermal refugia helps us explain more of the variance in mating success (Fig. 4). The longer a male remains on the surface, the greater its mating probability, but high temperatures limit time on the surface later in the day. Burrows are used for mating, but also offer protection from predators and serve as thermal refugia particularly in sun-exposed habitats. With rising temperatures, males must balance time spent in the burrow cooling off (at the expense of mating opportunities) with time spent on the surface attracting potential mates (at the cost of increased temperature stress and risk of desiccation). The large claw helps males remain cooler (Darnell & Munguia, 2011), modulating this trade-off and this study shows the claw contributes to increased time on the surface. Thus, endurance rivalry is enhanced by structures that serve different functions, and understanding their links can provide more accurate models that quantify selection. Mating strategies are often affected by habitat quality (Myhre, Forsgren, & Amundsen, 2013). Uca mjoebergi male sizes do not differ between shaded and unshaded habitats. However, females prefer males in shaded areas (Kerr & Backwell, 2016). Courtship activity, mate choice and parental care are all adjusted to factors like predation risk (Lima & Dill, 1990), visibility in dense vegetation (Candolin, Salesto, & Evers, 2007), water colour (Endler & Houde, 1995), food availability (Allen & Levinton, 2014) and temperature (Bashey & Dunham, 1997). The effect of thermal constraints on mating behaviour is particularly important due to its potential consequences. In sceloporus lizards, for example, individuals need to retreat into cooler refuges during the hottest parts of the day since they risk death by overheating. Retreating, however, limits foraging and reproductive behaviour which increases the risk of extinction (Sinervo et al., 2010). It is predicted that 39% of all lizard species will be extinct by 2080 due to the effects of temperature increases (Sinervo et al., 2010). In this study, shaded areas under mangrove trees were highly sought after by males, presumably because shaded individuals do not need to retreat into their burrows during the hottest part of the day, and females chose males in shaded areas (Kerr & Backwell, 2016). Mating behaviour and secondary sexual characters are often the focus behind sexual selection; however, thermal environments and
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