Journal of Thermal Biology 51 (2015) 15–22

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Core and body surface temperatures of nesting leatherback turtles (Dermochelys coriacea) Thomas J. Burns n, Dominic J. McCafferty, Malcolm W. Kennedy n Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, Scotland, UK

art ic l e i nf o

a b s t r a c t

Article history: Received 27 November 2014 Received in revised form 27 February 2015 Accepted 1 March 2015 Available online 3 March 2015

Leatherback turtles (Dermochelys coriacea) are the largest species of marine turtle and the fourth most massive extant reptile. In temperate waters they maintain body temperatures higher than surrounding seawater through a combination of insulation, physiological, and behavioural adaptations. Nesting involves physical activity in addition to contact with warm sand and air, potentially presenting thermal challenges in the absence of the cooling effect of water, and data are lacking with which to understand their nesting thermal biology. Using non-contact methods (thermal imaging and infrared thermometry) to avoid any stress-related effects, we investigated core and surface temperature during nesting. The mean7 SE core temperature was 31.47 0.05 °C (newly emerged eggs) and was not correlated with environmental conditions on the nesting beach. Core temperature of leatherbacks was greater than that of hawksbill turtles (Eretmochelys imbricata) nesting at a nearby colony, 30.0 7 0.13 °C. Body surface temperatures of leatherbacks showed regional variation, the lateral and dorsal regions of the head were warmest while the carapace was the coolest surface. Surface temperature increased during the early nesting phases, then levelled off or decreased during later phases with the rates of change varying between body regions. Body region, behavioural phase of nesting and air temperature were found to be the best predictors of surface temperature. Regional variation in surface temperature were likely due to alterations in blood supply, and temporal changes in local muscular activity of flippers during the different phases of nesting. Heat exchange from the upper surface of the turtle was dominated by radiative heat loss from all body regions and small convective heat gains to the carapace and front flippers. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Thermal biology Thermography Non-invasive techniques Core temperature Dermochelys coriacea Eretmochelys imbricata

1. Introduction Leatherback turtles (Dermochelys coriacea) are the largest species of marine turtle, with adult females having a mean mass of around 400 kg (Georges and Fossette, 2006). As adults they inhabit a broad range of water temperatures, migrating between high latitude, prey-rich temperate waters and the tropics or subtropics, where beaches provide the conditions for laying and development of eggs. Leatherbacks exhibit a range of physiological and behavioural adaptations to cope with different environmental temperatures, allowing them to remain active in temperate waters and prevent overheating in the tropics. Their thermoregulatory strategy likely involves aspects of both gigantothermy due to their large body mass, extensive fatty insulating tissue layers and adaptable blood circulation system (Paladino et al., 1990), and endothermy due to internal heat production (Bostrom et al., 2010; n

Corresponding authors. E-mail addresses: [email protected] (T.J. Burns), [email protected] (M.W. Kennedy). http://dx.doi.org/10.1016/j.jtherbio.2015.03.001 0306-4565/& 2015 Elsevier Ltd. All rights reserved.

Casey et al., 2014). Thermoregulation is aided by counter-current heat exchangers within the front and rear flippers (Greer et al., 1973), a vascular plexus lining the trachea to reduce respiratory heat loss, analogous to that of nasal turbinates found in birds and mammals (Davenport et al., 2009a), extensive adipose tissues in the head and neck, and major blood vessels buried deep within the insulated neck (Davenport et al., 2009b). In addition to these physiological adaptations for controlling heat loss, behaviour plays a key role in temperature control. Swimming (and the consequent metabolic heating) maintains a high body to water temperature differential (Bostrom and Jones, 2007) and turtles change flipper stroke rate in response to different water temperatures (Bostrom et al., 2010). Furthermore, leatherbacks may dive to cooler waters to lose heat in tropical waters (Southwood et al., 2005; Bostrom and Jones, 2007) and, conversely, in temperate seas may bring prey items to the surface to warm them before ingestion (James and Mrosovsky, 2004). Nesting is the only time when female leatherbacks are known to return to land, the process may last over two hours, and requires, with the exception of egg laying, extensive use of the flippers

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T.J. Burns et al. / Journal of Thermal Biology 51 (2015) 15–22

Table 1 Published values for leatherback body temperature shown alongside values derived from this study. Mean TB 7 SE (range) oC

Method of measurement

Location (turtle activity)

n

Source

31.4 7 0.05 (30.6–32.5) 29.17 0.23 (27.3–30.6) 30.2 7 0.35 (27.1–33.2) 28.3 7 0.07 (28.1–28.7) 31.1n (30.5–33.5) 30.6 (29.8–31.4) 337 0.45 (32–34) 31.6 7 0.3 30.8 7 0.2 31.4 7 0.4 31.5 31.8 26.4 7 0.23 (25.4–27.3) 24.337 0.94 (21.6–25.8)

Egg surface temperature (corrected) Sub-carapace Gastrointestinal Gastrointestinal Egg temperature Egg temperature Not specified Inserted 25–30 cm into body through drilled hole Inserted 25–30 cm into body through drilled hole Inserted 25–30 cm into body through drilled hole Gastrointestinal (juvenile- 37 kg) Gastrointestinal (juvenile- 16 kg) Gastrointestinal Cloacal

Trinidad (nesting-laying) Costa Rica (inter-nesting) Costa Rica (inter-nesting) USVI (inter-nesting) Mexico (nesting) French Guiana /Suriname (nesting) Costa Rica (nesting-restrained post laying) Costa Rica (nesting-restrained post sand scattering) Costa Rica (nesting-laying) Costa Rica (nesting-‘exercising’) Laboratory (in water tank) Laboratory (in water tank) Northwest Atlantic (foraging) Nova Scotia (restrained)

65 3 4 8 10 24 5 10 3 10 1 1 7 4

This study Southwood et al. (2005) Southwood et al. (2005) Casey et al. (2010) Mrosovsky (1980) Mrosovsky, Pritchard (1971) Lutcavage et al. (1992) Paladino et al. (1996) Paladino et al. (1996) Paladino et al. (1996) Bostrom et al. (2010) Bostrom et al. (2010) Casey et al. (2014) James and Mrosovsky (2004)

n

Represents a median value, values from this study are shown in bold.

(Eckert et al., 2012). Flipper movement has been suggested as the primary source of heat production at sea (Bostrom and Jones, 2007; Bostrom et al., 2010) and it is likely that flipper activity during nesting will result in substantial heat production. Biophysical modelling suggests that core temperature of leatherbacks increases throughout the nesting process and rises in core temperature during nesting are predicted as a result of future rising temperatures in their breeding range (Dudley and Porter, 2014). However, measurements on land of core temperature in this species are, surprisingly, relatively scarce (Table 1). The aims of this study were, first, to estimate the core temperature of nesting leatherbacks, second, to examine spatial and temporal variation in surface temperature through all phases of nesting behaviour, and, third, to estimate the relationships between both core and surface body temperatures and the environmental conditions on the nesting beaches. An important and novel aspect of this study was that all measurements were made with minimal or no disturbance using non-contact thermometry and thermal imaging. For comparison we also estimated the core temperatures of ectothermic nesting hawksbill turtles (Eretmochelys imbricata).

2. Materials and methods 2.1. Study area and discrimination of nesting phases Fieldwork on leatherback turtles was carried out at Fishing Pond Beach (approximately 10.58° N, 61.02° W) on the East coast of Trinidad, West Indies. Fishing Pond is one of three protected beaches on the island which receive high densities of nesting female leatherbacks during the nesting season which runs from March to the end of August, peak nesting occurring during April and May (Bacon, 1970). Beach visits were made on a regular basis from mid-June through to mid-August 2013 and 2014 during the hours 8 pm–1 am (local time). The beach was regularly patrolled by rangers but rarely by tourist groups, so observations could be made with minimal disturbance to the nesting females. Discrimination of the different behavioural phases of the nesting process were modified from those in previous publications (Hendrickson, 1958; Carr and Ogren, 1959, 1960; Pritchard, 1971). Seven behavioural phases were defined in this study, with the prospecting and body pitting phases being combined for practical purposes: 1-approach (movement from surf to upper beach); 2 – prospecting/body-pitting (selection of a nest site once in nesting area and preparing the nesting site for excavation); 3-excavation of

the nest hole; 4-laying; 5-refilling the nest hole; 6-sand scattering (also termed ‘camouflaging’ and ‘disguise’ elsewhere in the literature); 7-return to the sea (for a full description of nesting phases see Table S1). 2.2. Body core temperature The surface temperature of freshly laid eggs was used as a noncontact proxy of core body temperature. Measurements were taken using a Fluke 62 Max þ portable infrared thermometer (spectral range¼8–14 mm, accuracy¼ 1 °C or 1%, thermal sensitivity ¼ 0.1 °C) set to an emissivity of 0.98, which is within the range of previously used values for emissivity of living tissues (McCafferty et al., 2013; Rowe et al., 2013; Mortola, 2013). Infrared thermometers were calibrated against a thermocouple, which had itself been calibrated against a mercury thermometer of 0.1 °C sensitivity. The recorder lay in the sand with one arm extended into the nest hole (some sand was removed from one side of the nest hole to allow better access) and observed the eggs as they were laid. Measurements were taken only on eggs that were freshly laid (within one or two seconds of emergence) and had no sand attached to the side which was being measured. As measurements of surface temperature were taken so soon after the eggs emerged from the female the effects of variation in cooling rates between eggs will be negligible. Measurements were also taken for nesting hawksbill turtles (to provide a smaller ectothermic comparator) at Hermitage Bay, Tobago (approximately 11.31° N, 60.57° W) using the same methods. To correct for potential post-laying cooling and/or inaccuracy due to imprecise emissivity setting, a correction factor for egg surface temperature was estimated and applied to all measurements of both study species. The internal temperatures of up to seven of the small shelled albumen gobs (Sotherland et al., 2003), often referred to as ‘yolkless eggs’ (Eckert et al., 2012), produced by 20 leatherbacks were measured using a K-type thermocouple (calibrated against a mercury thermometer 70.1 °C) connected to a digital thermometer (RS 206-3750); leatherbacks are a vulnerable species (Wallace et al., 2013) so only these sterile ‘eggs’ were used for this purpose. The linear regression of mean surface against the mean internal temperature was

Internal egg temperature = (0.8262 × egg surface temperature) + 5.9378 (r2 ¼0.62, F1,18 ¼29.8, p¼ o0.01), (see Fig. S1)

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Fig. 1. Example of thermal images collected. From left to right: top, full body profile and from posterior; middle, front and rear flippers; bottom, head lateral and dorsal. False colour palette setting is ‘ironbow’ and emissivity (ε)¼ 0.98 (turtle surface). At this setting the sand temperature (ε¼ 0.76) is approximately 0.7–2.0 °C warmer than shown. The full colour images are available in the online version of this paper.

2.3. Thermal imaging and body surface temperature All images were collected using a Fluke Ti20 thermal camera (spectral range¼7.5–14 mm, accuracy¼2 °C or 2%, thermal sensitivity ¼0.1 °C). The thermal camera was calibrated against a thermocouple, which had been calibrated against a mercury thermometer 70.1 °C. A series of six images were taken once and on occasion twice (excavation and camouflaging phase only) for each of the behavioural phases of the nesting process (Fig. 1). Images were taken from only one side of a turtle, with the same side being used consistently throughout nesting unless access limitations made it necessary to image the opposite side, the side used varied between individuals (for a description of images taken see Table S2). Variation in the side of the turtle which was thermally imaged could potentially affect recorded surface temperatures due to asymmetrical arrangement of major organs such as the liver, stomach and intestines. However, we expect that any influence of this asymmetry will be dampened due to insulation of the organs from the skin surface; Davenport et al. (1990) reported the major organs to be within a continuous blubber capsule enclosed within the thick oily skin, with the points of penetration of the capsule by the limbs and tail having a thick cuff of blubber. Emissivity was set at 0.98 (as explained in Section 2.2). Images were analysed using InsideIR™ image analysis software version 4.0 (http://www.fluke.com/fluke/uken/support/software/ ti-update.htm). Mean temperature values were extracted from

each body region using polygons fitted around the outline of the dorsal surface of the front flipper, the dorsal surface of the head and the lateral surface of the head. Rear flipper mean temperature was estimated by a straight line running through the proximal two thirds of the centre of the dorsal surface of the rear flipper and carapace temperature was estimated from the mean of ten point measurements from sand free areas. In cases where multiple images had been taken for a body region during a phase the average of the two extracted values was used. 2.4. Environmental measurements Multiple recordings were taken for each phase of the nesting process for each of the following (unless stated otherwise). Wind speed was measured at the highest point of the carapace using an Explorer Skywatch 2 handheld anemometer (accuracy¼ 73%, sensitivity ¼0.1 ms  1, range¼ 0–42 ms  1). Substrate temperature at a depth of 6, 8, 10, 12, 14 and 16 cm depth below the sand surface was recorded using a probe which consisted of several K type thermocouples glued to the side of a plastic rod, read by a digital thermometer (RS 206-3744). It was placed into the sand adjacent to the turtle at equal beach height. A mean value of substrate temperature across the depths was calculated. Air temperature and relative humidity were measured at ground level using a UTI Hygrometer (accuracy¼ 71 °C and 5% relative humidity (r.h.), resolution¼ 0.1 °C and 1% r.h., range¼0–49.9 °C and

T.J. Burns et al. / Journal of Thermal Biology 51 (2015) 15–22

20–95% r.h.). Cloud cover was estimated for the area of sky directly above the nesting female using a quadrat (16 squares of 8  8 cm2) held at arm’s length above an observer lying on the sand adjacent to the turtle (recorded three times during the nesting process). Sea surface temperature (SST) was estimated from daytime satellite derived data (MODIS aqua Level 2, http://oceancolor.gsfc. nasa.gov/). Data was gathered for every available day during fieldwork periods in 2013 and 2014 from seven to 20 point measurements in areas within 30 km from the coastline of both Trinidad and Tobago. Within the Matura Bay and Galera Point area in Trinidad, and in areas east of Roxborough and Castara Bay, on the south and north coast of Tobago, respectively. Areas sampled in Trinidad have been shown to be regularly used by inter-nesting female leatherbacks (Eckert, 2006). Data availability was limited by satellite coverage of the area and quality, only data from the highest quality retrieval category were extracted for analysis. 2.5. Statistical analysis All statistical procedures were carried out using R software (http://www.r-project.org), unless otherwise stated. Spearman Rank Correlations were used to examine relationships between body core temperature and the environmental conditions individual turtles experienced during the period leading up to and including egg laying. A Bonferroni Correction was applied to account for the use of multiple testing and to account for ties in the test data, a sample size of 10,000 was simulated under the null hypothesis of no correlation and the p-value was defined as the probability that the observed value is greater than or equal to the null distribution. Power analysis (pwr package) was also carried out to assess the power of this dataset has to test significance, using what may be considered conventional significance and power levels (Stefano, 2003). Linear mixed effects models (lme4 package) were used to examine factors affecting body surface temperature. Behavioural phase of nesting and region of body surface were categorical fixed effects, the environmental conditions during each phase of nesting: air temperature, relative humidity, sand temperature and wind speed-were continuous fixed effects and individual was a random effect. Cloud cover was not included as a factor because sampling was insufficient across all phases of nesting. Models were compared using second order Akaike Information Criterion values (AICc), AICc differences and Akaike weights.

3. Results 3.1. Environmental conditions Air and sand temperatures were found to decrease and relative humidity increase during the nesting process (Fig. S3). Mean (7SE) wind speed was 3.5 70.13 ms  1 and mean cloud cover was 387 0.05%, with no discernible pattern of change. Mean sea surface temperature was significantly higher during 2013 than 2014 (GLM, F1, 72 ¼17.29, p o0.001) (Fig. S4). There was no significant difference in SST between locations for either year (GLM, F1, 70 ¼0.12, p¼ 0.73).

35 Leatherback Hawksbill 30 25

Frequency

18

20 15 10 5 0 28

29

30 31 o Egg temperature ( C)

32

33

Fig. 2. Distribution of egg temperatures, used as an estimation of core temperature, of leatherback and hawksbill turtles in Trinidad and Tobago, respectively, estimated from infrared thermometry of freshly laid eggs and calibrated as described in Section 2 (see also Fig. S1). Curves show fitted normal distributions. A mean of 14.17 5.19 and 17.9 7 8.87 eggs, during a total of 65 and 29 nestings were sampled for leatherbacks and hawksbills, respectively.

of eggs (see Section 2 and Fig. S1). The mean core body temperature (Tb) of our nesting leatherbacks was thereby estimated at 31.470.05 °C (7SE). For comparison with a species not considered to be an endothermic/gigantothermic mesotherm like leatherbacks, we carried out similar measurements on hawksbill turtles from a geographically nearby colony. Estimated hawksbill core temperatures were significantly lower, 30.0 70.13 °C (GLM, F 1, 92 ¼229.4, po 0.001) (Fig. 2). Moreover, Tb was greater during 2013 compared to 2014 for both species, (GLM, F 1, 91 ¼21.7, po 0.001) with the differential between years depended on species (GLM, interaction F 1, 90 ¼11.3, p ¼0.001), hawksbills showing a greater differential than leatherbacks (Fig. S2). Leatherbacks also clearly exhibit a narrower distribution of temperatures than hawksbills (Fig. 2). Tb was not correlated with curved carapace lengths (CCL) of either species (leatherbacks, r2 ¼ 0.03, n ¼53, F1, 51 ¼1.66, p 40.05; hawksbills, r2 ¼0.04, n ¼29, F1,27 ¼ 1.19, p4 0.05 ). There was no significant correlation between Tb of the leatherbacks and beach environmental conditions- air temperature, relative humidity, substrate temperature, wind speed and cloud cover (Spearmann Rank pZ 0.01 in all cases, n ¼14 individuals). Power analysis, however, indicated that the sample size was insufficient for detection of significance in factors having a relatively weak effect on Tb (r2 ¼0.78, when n¼ 14, significance level ¼0.01, power¼0.8); observed correlation coefficients suggest a sample size of 21 to 59 individuals is necessary to detect a significant correlation. 3.3. Surface temperature

3.2. Core temperature In order to obtain an estimate of core temperatures of the population under study at nesting, we measured surface temperatures of between five and 25 eggs laid by each of 65 leatherback turtles using infrared thermometers. We assumed an emissivity of 0.98 for this and then applied a correction factor based upon parallel thermocouple readings from the centres of a sample

Turtles emerging from the sea should briefly cool by evaporation before exposure to sources of heating (day-warmed sand, air, physical exercise) and loss of cooling by water. Compensatory mechanisms for heat loss would be detectable at the body surface, and may be regional. Thermographic images show regional differences in surface temperatures (Fig. 1), and sequential measurements taken throughout several nestings showed a consistent

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4. Discussion

28.5

4.1. Core temperatures

o

Surface temperature ( C)

28.0

27.5

27.0

26.5

ce pa a r Ca

r r al ral pe pe ors ate flip flip d l t r a ad ad on Re Fr He He

Fig. 3. Surface temperature of different body regions of nesting leatherback turtles (n¼ 17 individuals). Mean (7 SE) of all measurements taken throughout all the nestings reported, of turtles emerging from the sea between approximately 8 pm and midnight.

rank order in which the head was consistently hottest, followed by the flippers and the carapace (Fig. 3). Detailed breakdowns of these data (Fig. 4) show that different surface regions increased in temperatures at different rates, the rear flippers showing the largest and most rapid increase up to the laying phase. Body region, behavioural phase of nesting, and air temperature (Ta) were found to be the best predictors of surface temperature (linear mixed effect models, Table 2). There was a positive relationship between leatherback surface temperature and air temperature. The 95% confidence intervals for these factors: body region (maximum range across all levels; 0.0586–1.2661); behavioural phase of nesting (0.1828–2.1878); and air temperature (0.1196–0.5247), exclude zero indicating a significant influence upon Ts. It is worth noting that there is only a small difference in fit between this model and one that also includes humidity; confidence intervals for the model including humidity also exclude zero, suggesting a significant effect (Table 2). Surface temperature (7SE) was greatest on lateral and dorsal regions of the head (28.0 7 0.09 and 27.87 0.1 °C, respectively), the carapace had the lowest temperature (26.8 70.1 °C) and the temperature of the rear flipper was greater than front flipper (27.670.08 °C and 27.070.07 °C, respectively) (Figs. 1 and 3). Surface temperatures increased during the early nesting phases, remained relatively constant in mid-phases and then decreased or remained constant following egg laying, with rates of change differing between body regions (Fig. 4A and C). The temperature differential between body surface and air temperature (Ts – Ta) was negative in the early phases, then as Ts increased and air temperature decreased, the differential was close to zero and positive for head and rear flipper regions during the later stages of nesting (Fig. 4B).

In this study core temperature was estimated from the surface temperature of freshly laid eggs using infrared thermometry. Measurements were made within one or two seconds of laying and several eggs were measured for each turtle. To our knowledge no other study has used this non-invasive technique to estimate core temperature, or reported sample sizes of this magnitude. Mean core temperature fell within the range of previously published values recorded during nesting, using various techniques (Table 2). Values were well below that of the estimated critical thermal maximum temperature for leatherbacks of 40 °C (Spotila et al., 1997). The greater core temperature of leatherbacks compared with hawksbills may be related to the large difference in body size between the species, leading hawksbill body temperatures to more closely follow environmental temperatures. Mean core temperatures were within the range of temperatures reported for freely swimming individuals in tropical waters (Table 1), showing that, at least up until the point of laying, nesting leatherbacks do not show core temperatures elevated beyond the range reached while in tropical seas. However, this does not necessarily mean that leatherbacks are at no risk of overheating during nesting. Thermoregulatory mechanisms are used in tropical waters (Southwood et al., 2005; Bostrom and Jones, 2007), suggesting that leatherbacks are required to avoid significant rises in core temperature. Data on core temperature changes during nesting is currently lacking, but a previous biophysical model predicted that core temperatures will continue to rise throughout the nesting process (Dudley and Porter 2014). If this were so it may be expected that body surface temperature would also rise due to changes in peripheral blood flow. This was not observed in this study, as surface temperatures tended to level out, and even decrease in the later phases, even for areas such as the lateral region of the head, where reduced insulation (Davenport et al., 2009b) and proximity to the core may lead surface temperature to more closely track core temperature (Fig. 4). The relatively constant surface temperature during laying could be explained by a decreased metabolic rate and inactivity of the flippers reducing metabolic and muscular heat production during this phase (Paladino et al., 1996). It is also noteworthy that there were significant differences in the core temperatures of turtles between years of study, with temperatures being lower during 2014 for both species (Fig. S2). Whether this co-variation is the result of chance or has an environmental cause requires further years of observation, but one potential explanation for this is that the sea temperature regimes to which individuals were exposed varied between years. Satellitederived sea surface temperatures during our fieldwork periods were also found to be significantly lower during 2014 for both study sites (Fig. S4), seemingly consistent with this hypothesis. However, due to the limited data available for SST during our field observation periods, and broad spatial scales of satellite-derived data, there is insufficient information as yet to infer a casual relationship, though this clearly merits further investigation. 4.2. Regional variation in temperature Body region, behavioural phase of nesting, and air temperature were found to be the best predictors of surface temperature of leatherbacks. Surface temperature varied by up to 1.2 °C, with the head regions, in particular the lateral region, having the greatest surface temperature. This was followed in descending order by the dorsal region of the head, rear flippers, front flippers and carapace (Fig. 3). That the carapace had the lowest surface temperature was

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Fig. 4. Regional differences in body surface temperatures of leatherback turtles during nesting. (A) Surface temperature of leatherback turtle body regions during behavioural phases of the nesting process. (B) Temperature differential between body surfaces and air temperature during nesting. (C) Change in leatherback turtle body surface temperature from values at start of prospecting/body pitting phase. Mean timings of phase transitions are indicated by the dotted lines and are: approach (A), prospecting/ body pitting (P/B), excavation (E), laying (L), refilling (R), sand scattering (SS) and return to sea (S). All temperature values are expressed as mean 7 SE. Data for which sample size was less than four individuals for a phase have been removed. All individuals emerged from the sea between approximately 8 pm and midnight. Table 2 Results of linear mixed models that account for variation in surface temperature of nesting leatherback turtles (n¼ 17 individuals). Model including body region, behavioural phase of nesting, air temperature and individual as a random effect was found to be the best fit. df¼ degrees of freedom, AICc ¼ second order Akaike information criterion and ΔAICc ¼AICc differences. Model Body Body Body Body Body Body Body

surface temperature regionþ behavioural regionþ behavioural regionþ behavioural regionþ behavioural regionþ behavioural regionþ behavioural

phase þ air temperatureþ humidity þ sand temperatureþ wind speed þ(1|individual) phase þ air temperatureþ humidity þ sand temperatureþ (1|individual) phase þ air temperatureþ humidity þ wind speed þ (1|individual) phase þ air temperatureþ humidity þ (1|individual) phase þ air temperatureþ (1|individual) phase þ (1|individual)

not surprising given its thick, poorly vascularised, insulating blubber lining (Davenport et al., 1990) that receives little blood flow during nesting (Penick, 1996). Substantial adipose tissues are also found in the head and neck, but the area around the jaw muscles are poorly insulated, and eyes are exposed (Davenport

df

AICc

ΔAICc

Akaike weight

17 16 16 15 14 13

644.4585 640.4831 640.4861 637.653 636.7247 639.5521

7.7338 3.7584 3.7614 0.9283 0 2.8274

0.0095 0.0695 0.0694 0.286 0.455 0.1107

et al., 2009b). Measurements of the lateral view of the head included both the jaw and eye and most likely explain the small temperature difference (0.2 °C) with the dorsal head region. However, in general, the head was the warmest region of the body, reflecting a general lack of insulation compared to other body

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parts. Blood flow to the skin surface increases during nesting, with the skin appearing to flush red and the pineal spot, located on the dorsal region of the head, turns pink compared to its usual white appearance at sea (Davenport et al., 2014). Blood flow has been reported to be ten times greater when the skin appears pink compared to when pale (Spotila et al., 1997). In this study, the pineal spot was 0.3–0.4 °C higher in temperature than the surrounding skin surface of the dorsal region of the head (T. J. Burns, per obs.). This may be due to bone thickness and lack of insulation immediately dorsal to the pineal gland (Davenport et al., 2014). Furthermore, the blood supply to the head lacks the counter current heat exchangers found in the flippers (Greer et al., 1973). Previously, the head region has not been included in analyses of heat exchange (Bostrom et al., 2010) but our findings suggest that heat loss from the head of nesting leatherbacks merits consideration. The likely reason for higher surface temperatures of the flippers is increased blood flow under warm ambient conditions, as has been shown in the front flippers of green (Chelonia mydas) and loggerhead (Caretta caretta) turtles (Hochscheid et al., 2002). In juvenile leatherbacks 30% of total heat loss was from the flippers which accounted for 27% of total body surface area (Bostrom et al., 2010). Furthermore as their study measured heat flux in only the front flipper, our findings, that the rear flipper is warmer than the front, suggest this effect may be even greater. Variation in the temperature between fore and rear flippers may be caused by differing levels of muscular heat produced during nesting. Other than during egg laying, the rear flippers are actively used for all phases of the nesting process. The front flippers are relatively inactive during the excavation and refilling phases, when they instead perform a supporting role for the pivoting of the body mass from side to side. During nesting, rear flippers are active for approximately 89% of the overall nesting period, compared with 46% for the front flippers (T. J. Burns and M. W. Kennedy, unpublished data). Additionally the proportion of time which different body surfaces spend in contact with sand versus air during the nesting process will likely influence heat transfer, as heat flux between the body surface and these two mediums is likely to be considerably different. However, quantifying these proportions to estimate the influence on heat transfer would require careful calculation. A limitation of the thermal imaging methods used to measure surface temperature in this study is that areas of the body surface not exposed to the air were not measurable, including the ventral surfaces of the flippers and head, and the plastron. Furthermore, these surfaces will spend most of, or, in the case of the plastron, all of the nesting process in contact with sand rather than air, which will vary in temperature with depth. It was therefore not possible to estimate heat transfer across ventral surfaces. Heat flux rates across the plastron of a juvenile leatherback under aquatic laboratory conditions were reported to be very similar to values for the carapace across a range of temperature regimes (Bostrom et al., 2010) and Davenport et al. (1990) noted that the blubber of the plastron, which was of similar thickness to that of the carapace, was poorly vascularised. 4.3. Environmental conditions Heat exchange between any mass and the environment will occur through convection, radiation, conduction, and evaporation. No significant correlation between core temperature and any environmental variable was found, a point that merits larger sample sizes to resolve. The correlation between sand temperature and Tb was closest to significance (Spearman Rank Correlation, r2 ¼0.67, p ¼0.01), this would suggest that the most important factor influencing core temperature may have been conduction (Fig. S5).

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In contrast, surface body temperature was positively correlated with air temperature suggesting that the temperature differential with air was driving heat exchange from these upper body surfaces, rather than wind speed. As both the air and sand temperature changed during nesting, the time after sunset of emergence may be an important factor determining a leatherback’s heat exchange with the environment (Fig. S3). A basic heat transfer model of the exposed upper surfaces of a nesting leatherback was developed to estimate convective and radiative heat exchange during the laying phase of nesting (Table S3). The model used mean values of body surface temperature and beach environmental conditions, as well as estimates of the surface area of different body regions. Total surface area was estimated to be 5.34 m2 where body, front flippers, rear flippers and head represented 67.6, 19.9, 8.6 and 3.7% of total surface area, respectively. For simplicity upper surfaces of the body were estimated as 50% of total body surface. The total heat loss from the upper surface of the animal was estimated to equal 164 W was and heat loss was greatest from the carapace (62.9%), followed by front flippers (21.1%), rear flippers (11.4%) and head (4.6%). Heat loss was principally by radiation 180 W while there was an overall convective heat gain of 16 W driven by the fact that the surface temperature of carapace and front flippers were below air temperature. An additional consideration when examining heat exchange in nesting leatherbacks is the effect of the covering of sand which often gathers on the body surface, particularly the carapace, during nesting. Sand throwing has been associated with thermoregulation in pinnipeds (Lewis and Campagna, 1998), whether the sand layer on nesting leatherbacks increases evaporative cooling or provides insulation is not known. 4.4. Conclusion This study found that core temperatures of nesting leatherback turtles were similar to core temperatures measured in tropical seas, suggesting that the energy expenditure involved in nesting activity and the thermal conditions encountered are not severe enough to elevate core temperatures beyond the range of freely swimming individuals. Leatherback body surfaces initially increased in temperature from their arrival on the beach before levelling off or decreasing in later nesting phases. Regional variation in surface temperature was related to heat production from the flippers during use and the degree of insulation in carapace and head. It appears that time ashore leads to relatively small increases in surface temperature, despite substantial energy expenditure while removed from the cooling influence of water.

Acknowledgements We would like to thank the Wildlife Section of the Trinidad Government for allowing us access to Fishing Pond Beach, and Turtle Village Trust and all the members of the tagging and patrolling team at Fishing Pond Village. We are particularly grateful to Sookraj Persad for accompanying us in the field throughout and for passing on his considerable experience on leatherback turtle nesting. We would also like to thank Hannah Davidson and other members of the Glasgow University expeditions to Trinidad 2013 and 2014 for assistance in the field, and the Glasgow University expeditions to Tobago 2013 and 2014 for data on hawksbill turtles. Paul Johnson provided invaluable advice and assistance with statistical modelling. Alan Cooper provided advice on the physics of heat transfer. Thanks to Margaret Reilly (Hunterian Museum, University of Glasgow) for access to museum specimen for measurement. Furthermore we would like to thank our funders, field

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T.J. Burns et al. / Journal of Thermal Biology 51 (2015) 15–22

research was supported by the Carnegie Trust for the Universities of Scotland, T. J. B. was funded by the IBAHCM Summer Studentship Scheme and the thermal imaging camera for the project was kindly donated by Douglas Neil. Funding sources had no involvement in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jtherbio.2015.03. 001.

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