We examined burrow microclimate and cocoon formation of the burrowing frog, Cyclorana australis, from northern Australia, during the dormancy season by monitoring frogs in burrows in the field. At the beginning of the dry season, while the soils were still quite moist, frogs dug shallow burrows, with 2–8 cm of soil above the top of the burrow chamber. The frogs spent 2–3 months underground without cocoons, but they began to form cocoons once the soils dried to water potentials that would dehydrate the frogs (as determined by laboratory experiments on water exchange). Frogs remained underground for up to six months, and then emerged when soil water potentials were great enough to permit water absorption, although it is unclear whether this was the primary cue for emergence. Soil temperatures adjacent to burrows were intermediate to those in full sun and full shade at 10-cm depth and increased throughout the dormancy period. Frogs removed from burrows after 2–4 months underground had a body mass 136% of their standard mass, indicating that burrowed frogs store a considerable amount of water in the bladder while in the burrows in the early weeks of aestivation. Because C. australis absorb water during the first part of aestivation (or at least maintain water they absorbed prior to burrowing), and construct a relatively impermeable cocoon during the latter part of aestivation, when they could lose water, these frogs may never experience water stress, despite being underground in the dry season for 5–6 months.
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Vol. 2007 • No. 4