Thermal Biology of Two Burrowing Arid Zone Species: The Great Desert Skink (Liopholis Kintorei) and the Brush-tailed Mulgara (Dasycerus Blythi)

Abstract

<p>More than two thirds of Australia is arid or semi-arid. These areas are predominately in the centre of the continent and despite the low rainfall and extreme thermal conditions they are inhabited by many and highly diverse animals. The groups that are among the most successful in the arid zone are the ectothermic reptilian skinks (Family Scincidae) and the endothermic carnivorous marsupial mammals (Family Dasyuridae). The aim of my study was to examine behavioural and functional traits that make life in the arid zone possible despite a substantially different thermal biology and energy expenditure. Two locally sympatric burrowing species, the vulnerable great desert skink (<i>Liopholis kintorei</i>, Scincidae) and the brush tailed mulgara (<i>Dasycercus blythi</i>, Dasyuridae) were examined using radio telemetry and behavioural observations. Daily activity patterns and body temperature fluctuations in relation to environmental temperatures were investigated to determine seasonal use of dormancy and burrow use by desert skinks over a 12-month period. Similarly, activity patterns and torpor use, which is characterised by reductions in body temperature and results in a substantial reduction in energy expenditure, and differences between torpor use and home range between summer and winter by mulgaras were investigated. </p> <p>Great desert skinks spent the majority of their time during their active months at their main burrow (65.8 ± 2.0% of days tracked), but all individuals made regular forays to other burrows (4.1 ± 2.2% of days tracked; (<i>N</i> = 16 (burrows), <i>n</i> = 3), particularly during the spring breeding season. Average <i>L. kintorei</i> T_b during summer was 32.5 ± 1.6°C, during autumn 28.4 ± 2.9°C, during winter 18.4 ± 3.2°C and during spring was 27.7 ± 3.1°C. <i>Liopholis kintorei</i> T_b was significantly different between all seasons (<i>P</i> < 0.005). They were more active than previously thought, displaying predominantly unimodal activity, particularly during autumn and the beginning of winter. In spring skinks were predominantly active after dawn, during the day and ceased activity after dusk. Movement between burrows increased 7-fold between spring and summer (<i>P</i> < 0.001) with females moving within a much smaller home range than males. During spring, skink activity was predominantly unimodal (62.5%) however, showed increased crepuscular and bimodal daily activity patterns (49.4%) during summer. They regularly switched to diurnal and highly unimodal (81.2%) activity in autumn and early winter (99.9%) when ambient temperatures were low. All skinks displayed some level of activity in the cool autumn months but there was no movement between burrows. When daily maximum temperature dropped below ~20°C, all four skinks moved to winter hibernacula 72 – 226 m from their main burrows, where they remained dormant for about three months with body temperatures falling to a minimum of 7.8°C (mean = 14.2 ± 3.5°C). Migration to separate inconspicuous hibernacula during winter may be an anti-predator strategy. Some individuals also estivated from late in summer in chambers at a depth of ~22 cm (mean body temperatures = 32.3 ± 1.8°C; mean soil temperatures at 20 cm = 32.8°C) with one female skink remaining dormant for 50 days from February to April. Because winter brumation occurred at shallow depths (~11 cm) below the surface we suggest this may provide a reproductive advantage by allowing desert skinks to be aroused as early as possible from brumation in spring by being exposed to daily fluctuating cycles of increasing ambient temperatures. </p> <p>In contrast to skinks, mulgaras were active all year round, but displayed daily torpor mainly during the cold season when the combined effects of cold winter temperatures with reduced food availability also requires regular use of torpor as a means to conserve energy. Mulgaras were largely nocturnal in summer but showed some diurnal activity in winter. Activity appeared to be highest during the first half of the night during both winter and summer. During the winter mating season, males were more active for longer than females irrespective of ambient temperature (<i>P</i> = 0.015), whilst female mulgaras appeared to increase activity with higher night-time temperatures. Duration of activity for both sexes was significantly shorter in winter compared to summer (<i>P</i> < 0.001). Home ranges were larger in winter, particularly for males which were almost four-fold greater than in summer. Ambient temperature and season were found to be significant predictors of mulgaras using daily torpor and there was a statistically significant difference in use of torpor between at summer (8 days/320; 2.5 ± 1.1%) and winter (264 days/301 days; 86.9 ± 8.1%, <i>P</i> < 0.001). In winter, body temperatures in torpid mulgaras fell as low as 11.7°C (mean = 21.8 ± 5.8°C) and the duration of torpor bouts was on average 9 hours. Infrequent, shallow use of torpor occurred during the latter part of summer but with average burrow temperatures of 31.4°C, the ability to reduce body temperatures below the torpor threshold (32°C) appears to have been restricted. Sex had no effect on torpor use during summer, but differences in torpor expression occurred between sexes during winter due to different selective pressures on reproduction (torpor frequency of males = 88.0 ± 10.6%, 150 days tracked, <i>n</i> = 3; females = 85.4 ± 5.8%, 151 days tracked, <i>n</i> = 2). Female mulgaras used deeper and more prolonged torpor than males during winter, however, they ceased using torpor from early spring, presumably once parturition and lactation began. The number of burrows used in summer for both sexes was considerably less than burrows used during winter and overall, females used more burrows than males despite smaller home ranges in winter. Burrow fidelity appeared to be higher in summer than winter for both sexes. </p> <p>Similarities between the two sympatric species included the home range of males being larger than females during their distinct breeding seasons, along with increased burrow use. Males of both species presumably traversed larger areas to increase opportunities to reproduce. Activity for both species was shorter in winter than in summer and inactivity, whether displayed as daily torpor, estivation or brumation is clearly a significant survival trait to conserve energy and water through periods of low food availability and predator avoidance. My study shows that a combination of functional and behavioural traits permit both species to persist in the arid centre of Australia</p>