The northwestern tropics of Australia experience variable and challenging climatic conditions. For most of the year the area is hot (daytime maximum air temperatures often >40°C), with little or no rain during the extended (April to November) dry season. Any diurnally active ectotherm thus faces a high risk of lethal overheating if it remains in sun-exposed habitats; and may also be under hydric stress during prolonged dry periods [30, 43–45]. Presumably reflecting those challenges, a high proportion of large reptiles and amphibians living in this region are nocturnally rather than diurnally active [29, 46, 47]. Although bluetongue lizards are active by day [42, 48, 49], our radio-tracking 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 .
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. At both study areas, most of those oases consisted of small concentrations of relatively dense vegetation; often, a few trees with a dense shrub or grass understory beneath them, creating a fairly thick ground cover of leaf litter in which the lizards could readily conceal themselves. At the highly disturbed WA site, some of the oases were created by artificial materials (concrete, wood, and so on) rather than by live trees. Despite this difference, both natural and artificially created core areas enabled the lizards to shelter in similar abiotic conditions (cool, moist, protected).
The causal bases for this heterogeneity in shelter opportunities are clear for the artificial shelters (which were constructed in places that people live or work), but obscure for the natural landscape. Small differences in elevation, and thus drainage patterns, as well as increased organic matter [50, 51] may have contributed to differences in soil type (and thus, moisture-holding ability), and these soil differences might have been exacerbated by the more profuse growth of vegetation in these small areas . Stochastic events of tree germination and survival were probably also important; once a tree becomes established, its shade may allow the growth of numerous shrubs, and ultimately produce a small patch of moister, cooler conditions at ground level than can be found anywhere in the surrounding landscape.
It is likely that bluetongue lizards benefit from these conditions in a number of ways. Most obviously, they can avoid otherwise-lethal ground temperatures, and maintain water balance . In addition, they are hidden from such predators as snakes, varanid lizards, raptors, and dingoes . Although we did not quantify food availability, we observed more edible fruit, berries, invertebrates and small vertebrates (such as frogs) in the moist leaf litter than would be available elsewhere in the landscape. The differences in soil-water-holding capacity might also translate into a lower probability of inundation; after heavy rain, we often saw extensive pooling of water in areas between but not within core areas. Moister conditions of vegetation and soil within the patches may also provide refuges from fire, at least during low-intensity burns (for example, the early dry and wet seasons [53, 54]), particularly given the limited ability of bluetongues for rapid movement.
One of the most striking results of our study is the similarity in the attributes of core areas used by both species of bluetongue lizard, and by lizards at both sites. The two lizard species are broadly similar in overall size and morphology, but differ strongly in their geographic distributions and thus, in the range of abiotic conditions that they encounter over most of their extensive ranges. The northern bluetongue (T. s. intermedia) is restricted to the wet-dry tropics, primarily in open woodland habitat, whereas the centralian bluetongue (T. multifasciata) ranges widely across the arid interior, especially in spinifex grassland. The wet-dry tropics inhabited by northern bluetongues experience higher and more seasonally predictable rainfall than do the desert habitats of centralian bluetongues [55, 56]. Nonetheless, the broad abiotic challenges experienced by the two species are probably fairly similar: an open landscape that is lethally hot for long periods during daylight hours, with scattered patches of denser vegetation that provide shade, food, moisture, and protection from predators. At the site where the two species were sympatric (Keep River), the centralian bluetongues used spinifex clumps (rather than trees) for shelter more often than did the northern bluetongues (Figure 2b,c). This difference is consistent with the habitat types experienced by the two lizard species over the rest of their (allopatric) ranges.
The lizards’ frequent use of small isolated habitat patches, and the relative scarcity of such patches within the broader landscape, resulted in considerable overlap between individuals in patch use. For example, a single small (5 ha, or 5 × 104 m2) 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 . Such cohabitation, whether simultaneous or sequential, may substantially increase rates of parasite transmission among individuals [57–60], 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 . These focal sites for reptile activity may also play a significant role in the social systems of the species involved, because many tiliquine species display complex sociality [62–69].
The lizards’ disproportionate use of small isolated patches of distinctive habitat also has strong implications for their conservation and management. Most obviously, these scattered patches of dense vegetation are critical for population persistence; destruction of such patches (for example, by feral pigs, or by cattle that use the shaded area as a diurnal refuge from sun exposure) would seriously reduce the carrying capacity of the broader landscape for these lizards. Future work could usefully examine issues of core-area connectedness in greater detail. Previous work has suggested that the visual perceptual range of a congeneric species (T. rugosa) is about 20 m, allowing lizards to execute direct movements towards refuge sites within this distance . Thus, the perceptual range of a species is likely to play an important role in how lizards interact and move within the surrounding environment . If habitat degradation reduces connectedness among core-area patches, such that many are isolated by distances too great for the lizards to detect from the nearest remaining patch, even otherwise-suitable remnant habitat patches may remain unused.
Another conservation implication of the lizards’ frequent usage of cool moist core areas is a negative one: the same patches will attract highly toxic cane toads (Rhinella marina), an invasive species currently spreading into this region [72–75]. Both bluetongues and toads are physiologically dependent on habitats that provide buffers against environmental extremes. Bluetongues are dependent on habitats that allow them to maintain the appropriate body temperature [23, 76, 77]. Similarly, cane toads have no morphological or physiological mechanisms to prevent evaporative water loss  and select shelter sites in relation to the protection they provide from desiccation [73–76, 79]. Toads at the invasion front are highly mobile, and spend daylight hours inactive in relatively cool moist areas . Hence, toads congregate within refuge sites that are likely to be key foraging areas . Bluetongues rely on relatively slow-moving prey, and are able to ingest very large items [80, 81]; thus, if encountered, a toad would be an easy target.