Cooling down is as important as warming up for a large-bodied tropical reptile

Abstract

An ectotherm's performance and physiological function are strongly tied to environmental temperature, and many ectotherms thermoregulate behaviourally to reach optimum body temperatures. Tropical ectotherms are already living in environments matching their thermal tolerance range and may be expected to conform to environmental temperatures. We tracked the body temperatures (T_b) of 163 estuarine crocodiles across 13 years and compared T_b of 39 crocodiles to water temperature gathered using fish-borne sensors (T_w) across 3 years (2015-2018). While T_b largely conformed closely to T_w, we found inter- and intra-individual differences in relative body temperature (T_b-T_w) that depended on sex and body size as well as the time of day and year. Deviations from T_w, especially during the warm parts of the year, suggest that thermoregulatory behaviour was taking place: we found patterns of warming and cooling events that seemed to mediate this variation in T_b. Thermoregulatory behaviour was observed most frequently in larger individuals, with warming events common during winter and cooling events common during summer. By observing free-ranging animals across multiple years, we found that estuarine crocodiles show yearly patterns of active cooling and warming behaviours that modify their body temperature, highlighting their resilience in the face of recent climate warming. Our work also provides the first evidence for thermal type in large-bodied reptiles.

Keywords: acoustic telemetry; basking; intra-individual variation; thermal type; thermoregulation.

Figure 1.

(a) The acoustic array on the Wenlock and Ducie Rivers, Cape York, Queensland, Australia, showing the locations of the acoustic receivers as yellow circles, crocodile capture sites as blue squares and fish capture sights as red triangles. Conductivity, temperature and depth loggers (CTDs) are represented by black crosses. (b) Maximum temperatures reached across the global distribution of estuarine crocodiles C. porosus. Temperature data come from WorldClim [28] and represent the maximum yearly temperature reached from 1970 to 2000, at a resolution of one pixel per 170 km².

Figure 2.

Temperature recordings from two estuarine crocodiles C. porosus showing (a) a warming event (tag ID 3076) and (b) a cooling event (tag ID 3093). The space between detections (yellow and blue rectangles) likely represented a period where the crocodiles were out of the water basking or actively cooling, as they were unable to be detected during this time.

Figure 3.

(a) Scatter plot showing the relationship between the body temperature of 39 estuarine crocodiles C. porosus and their immediate water temperature as determined using fish-borne temperature sensors. Crocodile and fish temperatures are joined by a shared location within a 3 h window. (b) The relationship between tag temperatures for five fish species and the nearest instream temperature logger. The dashed lines show a 1 : 1 relationship between temperature values.

Figure 4.

Distributions of inter- and intra-individual variation in body temperature relative to water temperature of 39 estuarine crocodiles C. porosus. (a) Behavioural type of estuarine crocodiles, such that individuals below the dotted line are on average cooler than the water, while those above are warmer. (b) Residual intra-individual variation of estuarine crocodiles, such that individuals closer to 0 are more consistent. Individuals are coloured by sex and sorted according to sex and body size where M134 is a male 1340 mm SVL.

Figure 5.

Body temperature (T_b) of estuarine crocodiles C. porosus (n = 39) relative to the water temperature (T_w) throughout the year. Crocodiles are grouped by body size, with females, large males (greater than 1700 mm SVL), medium males (1470–1700 mm SVL) and small males (less than 1470 mm SVL). Coloured lines show times of day when the slope of the relationship between temperature and time of year is nonlinear or different from 0. The black dotted lines show the relationship between temperature and time of year for other times of day, while the grey dashed line shows when T_b = T_w. Grey dots are jittered raw data.

Figure 6.

Contour plots showing the frequency of cooling and warming events of (a) 55 female and (b) 108 male estuarine crocodiles C. porosus through the year. Colours represent the modelled mean number of cooling or warming events per individual per month, with grey rectangles representing a lack of data.