We attached temperature sensitive transmitters to 28 adult T. lineata (17 males, 13 females) that were captured and released at four grassland sites during the breeding season (austral spring-summer) over two consecutive years (Oct-Feb 2012/13 and 2013/14). Laboratory tests in calibrated incubators showed that transmitter temperature (Ttrans) was a good predictor of dragon skin (Tskin) and body temperature (Tbody) (Tskin=0.924Ttrans-0.013, R2 = 0.939, P < 0.0001; Tbody=1.063Ttrans-3.20, R2 = 0.947, P < 0.0001), with Tskin measured using copper-constantan thermocouples attached to the dorsal surface of animals, and Tbody measured using thermocouples inserted 4 mm into the cloaca.
In the field, the transmitters recorded the temperature (Ttrans) of free-ranging individuals every 10 minutes for a total of 278 lizard-days and 19735 transmitter measurements during daylight hours. At the same time, we measured microclimate temperatures throughout these grasslands. We used shaded ground temperature (Tshade) as a baseline against which to compare temperature in two other microhabitats: exposed open ground (Tsun) and the temperature in arthropod burrows (Tburrow). Ground temperatures in the open sun can be extreme in these grasslands during late spring and summer, with an average daily maximum of 59°C and temperatures sometimes exceeding 70°C (Fig. 1a). Shaded ground temperatures were cooler (average daily maximum 38°C), but even these could exceed 50°C. Burrows, in contrast, provided a more thermally buffered environment (Fig. 1a): while shaded ground temperatures ranged from − 3 to 59°C, burrow temperatures ranged from 12 to 36.5°C (mean 23.2°C). Shuttling between microhabitats allowed individuals to stay between the critical maximum (38.3 to 42.8°C, mean = 40.6 ± 0.8°C) and minimum (8.3 to 11.5°C, mean = 9.9 ± 0.8°C) skin temperatures recorded in laboratory experiments (Fig. 1b).
Plotting dragon transmitter temperature (Ttrans) as a function of shaded ground temperature (Fig. 1b) revealed how dragons altered their use of thermal microhabitats as shaded ground temperature changed. Points above the 1:1 line were transmitters that were warmer than shaded ground, indicating dragons were in more open microhabitats, while points below the line indicate that dragons were occupying cooler microhabitats such as burrows. Across the range of shaded ground temperatures (-3 to 59°C), average transmitter temperature remained within the thermal tolerance limits of the species (skin temperature 9.9 to 40.6°C; Fig. 1b) because individuals adjusted the amount of time they spent in different thermal microhabitats (Fig. 2). For example, when shaded ground temperature fell below the lower critical thermal limit for the species, average transmitter temperature remained above this limit because dragons spent more time in thermally buffered sites, such as burrows, particularly at night and during the coolest parts of the day (Fig. 2). As evidence for this, the distribution of transmitter temperatures closely matched the distribution of burrow temperatures when shaded ground temperature was less than 15°C (Fig. 3). At these temperatures, burrows were generally warmer than both shaded and open ground sites (Fig. 1a, Fig. 3).
At shaded ground temperatures between 15–25°C, the distribution of transmitter temperatures closely matched the distribution of open ground (Tsun) temperatures (Fig. 3), implying that dragons spent much of the time using open sites in this temperature range (see Fig. 2). As shaded ground temperature rose above 25°C, open ground temperatures steadily exceeded transmitter temperatures, implying that dragons increasingly avoided warmer open sunny sites at higher temperatures (Fig. 3). At shaded ground temperatures greater than 40°C, most transmitter temperatures were below this value, implying that dragons were avoiding shaded ground sites in favour of cooler microhabitats, such as burrows. The point at which shaded ground temperatures were sufficiently warm that dragons began to avoid this microhabitat is indicated by the sharp inflection in the average transmitter temperature curve when shaded ground temperature approached the upper thermal tolerance limit of the species (Fig. 1b). At shaded ground temperatures above about 35°C, average transmitter temperature remained relatively constant, and below the thermal tolerance limit, as dragons increasingly sought refuge in cooler microhabitats. These data show that dragons sought refuge in more thermally buffered environments, such as burrows, at both high and low shaded ground temperatures to maintain average temperature within their thermal limits. As such, key changes in the use of microhabitats in the field coincided with the upper and lower critical temperature thresholds of this species.
Essential activities, such as feeding and finding mates, require dragons to be above-ground49, but high and low above-ground temperatures force dragons into thermal refuges such as burrows. Seeking refuge is a well-known activity in lizards to escape high temperatures 22,26 50. However, we lack field data quantifying the extent to which high temperatures curtail above-ground activity, and hence the impact that high temperatures may have on the animal’s ability to forage above-ground to support their energetic demands. To understand the impact of a shortened activity window caused by high temperatures, we classified transmitter temperatures according to whether lizards were above or below shaded ground and burrow temperature for each transmitter temperature record. Transmitter temperatures below shaded ground temperature indicate the dragon was using a thermal refuge to escape heat, while transmitter temperatures above shaded ground temperature, but at or below burrow temperature, indicate that the dragon was using a refuge to escape cold. During daylight hours, above-ground activity was curtailed in the early morning and late evening as dragons sought refuge from cooler above-ground temperatures, particularly on days when the daily maximum air temperature was low (Fig. 4). On days when the maximum air temperature rose above 30°C, dragons increasingly sought thermal refuge from high temperatures during the middle of the day (Figs. 2 & 3). This resulted in a quadratic relationship between daily activity (the proportion of daylight hours dragons were active above-ground) and maximum daily air temperature (Fig. 5a). Dragons spent, on average, more than half the day active when daily maximum air temperatures were between about 20–32°C. As the daily maximum temperature rose or fell beyond these values, dragons spent an increasing proportion of the day in thermal refuges. On days when the maximum air temperature approached 40°C, dragons spent about 80% of daylight hours in thermal refuges to avoid the heat of more open environments and to maintain their temperature within thermal tolerance limits. This pattern is similar to that projected for the widespread military dragon (Ctenophorus isolepis)51.
To assess the implications of a shortened activity window on lizard energetics, we measured resting metabolic rates (RMR) for 12 post-absorptive individual T. lineata (6 males, 6 females) at five temperatures (20, 25, 30, 35 and 38°C) using flow-through respirometry52,53. Resting metabolic rates (RMR in ml O2 g− 1h− 1) were then used to calculate the average amount of energy expended by a dragon at rest in the field based on the recorded transmitter temperatures averaged on an hourly basis. We calculated the mean RMR of a dragon for each day that we had transmitter records by averaging the hourly RMR values and then plotted the daily mean as a function of maximum daily air temperature (Fig. 5b). The mean RMR increased as daily maximum air temperature rose, as expected for an ectotherm, but started to level out once daily maximum temperature exceeded about 32°C. This is likely to occur because dragons used thermal refuges to escape the heat on warmer days (Figs. 1b, 3, 4a) and thus limit their metabolic losses. Consequently, the mean RMR divided by the number of hours active increased sharply on days when the maximum temperature rose above 32°C (Fig. 5c). We use the term energetic cost for the ratio: mean RMR / number of hours active. This ratio measures the amount of energy dragons must obtain per active hour by feeding to balance their daytime resting metabolic losses. While metabolic losses would also occur at night, dragons usually occupied cool night-time microhabitats, even on hot days (e.g. Figure 3), meaning nocturnal metabolic losses would be low.
The resting energetic cost, and hence the energy dragons must obtain while active to offset this cost, increased about three-fold as daily maximum air temperature rose from 32 to 40°C (Fig. 5c). Hence, while activity time was reduced on days with both low and high maximum temperatures, dragons incurred a substantially higher energetic cost on hot days because of greater metabolic losses at high temperatures coupled with reduced activity time in which to recoup those losses. Confining our calculations to daylight hours only and using resting metabolic rate as our measure of energy expenditure provides a conservative measure of total daily energy expenditure. If lizards are active, rates of energy metabolism are typically 1.5-3 times resting levels54,55.
To examine changes in the energetic cost over time, we gathered data on daily maximum air temperature during breeding season (October-February) for the years 1941–2020 recorded at Canberra Airport, the nearest climate station to our study sites (all sites were within 7 km of the climate station and at the same altitude). For each day during this period, we estimated the proportion of daylight hours dragons were active and the mean energetic cost based on the maximum daily temperature and the relationships shown in Fig. 5. We then calculated the mean maximum temperature, the mean activity time, and the mean energetic cost per day during the breeding season for each year (expressed as an anomaly from the mean for the period 1960–1990). Mean daily maximum temperature fluctuated around the mean from 1940–2000 but has increased in the period 2000 to 2020 to be on average more than 2°C greater than the mean, resulting in a decline in mean daily activity during summer of up to 4% (Fig. 6). The combination of hotter days and a shorter activity window caused the mean energetic cost for individual lizards to increase by an average of more than 20% during the same period (Fig. 6). This implies that between 2000–2020, increasing maximum daytime temperatures required dragons to increase their energy intake during their active period by more than 20%, relative to the 1960–1990 mean, to offset metabolic losses.