Ecology of Aestivation in a Cocoon-forming Frog, Cyclorana Australis (Hylidae)
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.Abstract
An excavated Cyclorana australis in its burrow chamber, showing depth in the soil.
Monthly rainfall and burrowing activity during the study period. Rainfall data for the study area were taken from a gauge at the study site, supplemented by data from the Australian Bureau of Meteorology (Darwin Airport station, approx. 6 km from the site) for months without direct data from the site. Gray horizontal bars indicate the times when frogs buried or emerged from burrows. In 2004, nine frogs were released during the normal activity season, and eight were released after all other tagged frogs had buried (dashed symbols).
Soil water potential at burrows, timing of frog burrowing, and timing of cocoon formation for 2004. Points represent means of soil taken from near the burrows, at the depth of the burrow chamber, for lowland (solid line and symbols), and upland (dashed line and open symbols) habitats. Vertical gray bars shade data points where different activities occurred; frogs entering burrows, frogs having “dry skin,” frogs with fully formed cocoons, and frogs emerging from burrows. The horizontal dotted lines indicate the threshold water potential range when there is no net exchange of water between frog and soil (see also Fig. 5); potentials above the lines indicate wetter conditions where frogs could absorb water from the soil, points below the lines indicate that frogs without cocoons would lose water to the soil.
Soil temperatures at frog burrow sites. (A) Mean monthly soil temperatures immediately adjacent to frog burrows (circles) and at 10 cm depth in full shade (squares) and full sun (triangles). Points are the mean soil temperature for that month, and error bars are the range for the whole month. (B) Representative daily variation in burrow temperature during the ten days from 1–10 July 2004. The bold line is mean temperature of soils immediately adjacent to each frog burrow, the thin line is soil temperature in full sun, and the dotted line is soil temperature in full shade. Bars across the bottom represent scotophase.
Water uptake as a function of water potential for C. australis without cocoons. Each point represents the rate of water uptake per cm2 for an individual frog. The dashed line indicates no net water flow into or out of a frog; points above the line indicate water uptake and points below indicate water loss. Lines are regressions for water uptake (y = 0.0035x + 1.204, F1,4 = 11.4, P = 0.04) and for water loss (y = 0.0009x + 0.18, F1,5 = 199.8, P < 0.0001). The threshold for water uptake lies between the x-intercepts for these lines, thus between −210 and −345 kPa.
Mechanism proposed by Tracy and Rubink (1978) for control of water exchange between a frog and its environment. Water exchange (m ˙s) is driven by the water potential difference between the skin of the ventral seat patch and the environment (ψf–ψe) and controlled, in part, by the conductance of the skin (Ks). The water potential of the seat patch is controlled by the antidiuretic hormone, arginine vasotocin (AVT), and this provides a second control of water exchange. The water potential of the blood within the core of the animal can be isolated from the water potential of the tissues in the ventral seat patch of the frog as a means to control water exchange with the environment. In the presence of AVT (produced in response to dehydration), blood flows freely between the core and seat patch, and the water potential of the core and seat patch become the same. In the absence of AVT (occurring naturally in well-hydrated frogs), blood flow to the seat patch is reduced allowing the seat patch to become dilute as water entering the seat patch is not mixed with core blood. Thus, in the absence of AVT, the apparent water potential of the frog (actually the water potential of the seat patch) is approximately −200 to −345 kPa, and in the presence of AVT, the water potential of the seat patch becomes the same as the water potential of the systemic blood (approximately −550 kPa).
Contributor Notes
Section editor: E. Schultz.