Spatial ecology of bluetongue lizards (Tiliqua spp.) in the Australian wet–dry tropics

Abstract

New technologies for quantifying animal locations enable us to document habitat-selection patterns of cryptic taxa in extraordinary detail. Northern bluetongues (Tiliqua scincoides intermedia) and centralian bluetongues (Tiliqua multifasciata) are large heavy-bodied scincid lizards that are broadly sympatric in the wet-dry tropics of north-western Australia.We used data from GPS-based radiotelemetry (n = 49 lizards, tracked for 2-121 days, total n = 61 640 locations) to examine the size, internal structure and overlap of lizard home ranges. Despite substantial habitat differences at our two study sites (semi-arid and relatively pristine habitat at Keep River National Park, Northern Territory, vs. highly disturbed and fragmented flood plain habitat in an agricultural area near Kununurra, Western Australia), home ranges were similar between the two areas, and between the two species. Our radiotracked lizards continued to disperse into previously unused areas throughout the duration of the study, so that the total areas used by lizards continued to increase. Based on the minimum convex polygon method, total home ranges averaged 4 ha (range 2-12 ha), but only about two-thirds of each home range was used intensively. Each home range had multiple core areas, and overlap of core as well as peripheral areas (especially with same-sex conspecifics) was high at the disturbed (Western Australia) site where lizard densities were high. The concentration of lizard activity within small core areas, often used by multiple individuals, suggests that these heavily used sites are critical to lizard conservation. However, the lizards' infrequent long-distance displacements also make them vulnerable to changes in the wider landscape mosaic. Because GPS-based radiotelemetry can quantify habitat use at finer spatial and temporal scales than earlier technologies, it can provide a robust base for management of at-risk fauna.

Evaluating GPS biologging technology for studying spatial ecology of large constricting snakes

“…To study reptiles, GPS biotelemetry (as opposed to satellite tracking, e.g., the ARGOS system, for definitions see [3]) has been used only sparingly, primarily on marine turtles [e.g., 12] and crocodilians [e.g., [13][14][15]. Among squamates, only large lizards such as blue-tongue skinks and monitor lizards have been tracked using GPS tags [e.g., 16,17]. Hart et al [18] reported on the first preliminary application of GPS technology in a large constrictor, the Burmese python (Python bivitattus).…”

Habitat selection by bluetongue lizards (Tiliqua, Scincidae) in tropical Australia: a study using GPS telemetry

“…Although bluetongue lizards are active by day [42,48,49], our radiotracking studies have shown that the lizards escape lethally high temperatures in exposed sites by spending much of their time inactive within shaded retreat sites. Those sites are non-randomly distributed across the landscape, with the lizards spending on average 95% of their time in small core areas that constitute <40% of the home range [32]. Analyses in this paper show that those core areas possess a distinctive suite of attributes; basically, they are oases of cool moist sheltered conditions within the otherwise inhospitable landscape.…”

“…For example, a single small (5 ha, or 5 × 10 4 m 2 ) patch of dense riparian habitat at the WA study site was used by at least 33 adult northern bluetongues over the course of our 9-month study at that site [32]. Such cohabitation, whether simultaneous or sequential, may substantially increase rates of parasite transmission among individuals [57][58][59][60], especially given that the cool, moist conditions in such sites would enhance survival of ectoparasites such as ticks, as well as the larvae of endoparasitic organisms [61].…”

“…We also recorded morphology (mass, head width, snout to vent length, and length of hind leg, front leg, tail, and head). For details of capture methodology and transmitter attachment, see [32,37].…”

“…In the tropics, where ambient temperatures are high all year round [27], reptiles are able to maintain high, stable temperatures without the need for overt thermoregulation: that is, their selection of habitats is more likely to be driven by the need to avoid overheating than to find basking sites [28,29]. Thus, tropical reptiles may select thermally distinctive microhabitats not to heat up, but to avoid overheating and water loss [30][31][32]. In the tropics, this 'heat avoidance' behavior probably predominates most of the year, whereas many temperate reptiles face similar thermal challenges for shorter periods (for example, during hot, dry summers [33]).…”