Evaporative water loss and the role of cocoon formation in Australian frogs

Abstract

Measurements of evaporative water loss (EWL; mg min^-1) and resistance (R; sec cm^-1) for various Australian frogs indicate three general allometric patterns: non-cocooned and non-‘waterproof’ frogs with EWL ∝ Mass^0.30 and R independent of body mass at about 1–3 sec cm^-1, cocooned frogs with EWL reduced about 50–200-fold and R about 50–200 sec cm^-1, and ‘waterproof’ frogs with EWL reduced about 5–100- fold and R about 5–100 sec cm^-1. Cocooned frogs have an exponential reduction in EWL and fairly linear increase in R over time, corresponding to the temporal addition of layers to the cocoon. The biophysical properties of cocoon are generally similar for various species, although there is some variation in both resistance per thickness (5–20 × 10⁴ s cm^-2) and diffusion coefficient (0.4–2.4 × 10 –5 cm² s^-1). The hygroscopic property of frog cocoon resembles that of mammalian stratum corneum, hair and wool, and mucopolysaccharides; there is a slight increase in water content of cocoon over a wide range of humidities but a very steep increase in water content and substantial hydration and swelling at >96% RH. This extreme hygroscopic behaviour of frog cocoon at very high RH may reflect less polymer cross-linking in frog cocoon and its high digestibility. The prevention of over-hydration of frog cocoon in vivo may be attributed to the restriction of high water content to only very high RH (>96%).