DESICCATION AND SHELTER-SITE USE IN A TROPICAL AMPHIBIAN - COMPARING TOADS WITH PHYSICAL MODELS

1. Shelter-site selection is a critical component of the behaviour of most mobile organisms, allowing them to avoid adverse physical and bio tic environmental conditions. This paper examines the patterns of diur nal retreat-site use of tropical cane toads (Bufo marinus), with respe ct to the ability of various shelter-site types to protect against des iccation. 2. To measure desiccation rates in different shelter-site ty pes, agar models the size and shape of an adult toad were placed in si tes used by toads and allowed to desiccate. Models were put in burrows , in tall grass either close to (<1 m, 'wet grass' sites) or far from (>10 m, 'dry grass' sites) permanent water, and in the open on sandy g round ('control' sites). These measurements were carried out during dr y, wet, and moderately wet seasonal conditions. 3. Desiccation rates o f models varied dramatically among seasons. During the wet season, onl y models located in burrows lost any mass; models in other locations g ained mass. During the dry season, all models lost mass, but rates of loss were lowest in burrows and 'wet grass' sites. During the late wet season, all models lost mass at lower rates than in the dry season; m odels in control sites lost water at the highest rates, whereas models in 'wet grass' sites, 'dry grass' sites and burrows all lost water mo re slowly. 4. During each season, we radio-tracked individual toads an d recorded the characteristics of shelter sites they used. In each sea son, the sites most often used were those which provided the most prot ection from desiccation. Toads used burrows most frequently in the dry season, 'dry grass' sites in the wet season, and showed no preference for any shelter-site type during the late wet season. This suggests t hat toads select shelter sites based on their ability to reduce rates of water loss. 5. In the dry season, toads were more likely to return to burrows than to other shelter-site types. This may occur if burrows are scarce resources that are relatively difficult to find, and/or if burrows are enhanced by individuals, making it worthwhile to return t o them. In addition, burrows may provide protection from predation as well as desiccation.