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

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 proportional to Mass(0.30) and R independent of body mass at about 1-3 sec cm(-1), cocoone d frogs with EWL reduced about 50-200-fold and R about 50-200 sec cm(-1), a nd 'waterproof' frogs with EWL reduced about 5-100-fold and R about 5-100 s ec cm(-1). Cocooned frogs have an exponential reduction in EWL and fairly l inear increase in R over time, corresponding to the temporal addition of la yers to the cocoon. The biophysical properties of cocoon are generally simi lar for various species, although there is some variation in both resistanc e per thickness (5-20 x 10(4) s cm(-2)) and diffusion coefficient (0.4-2.4 x 10(-5) cm(2) s(-1)). The hygroscopic property of frog cocoon resembles th at of mammalian stratum corneum, hair and wool, and mucopolysaccharides; th ere is a slight increase in water content of cocoon over a wide range of hu midities but a very steep increase in water content and substantial hydrati on and swelling at >96% RH. This extreme hygroscopic behaviour of frog coco on at very high RH may reflect less polymer cross-linking in frog cocoon an d its high digestibility. The prevention of over-hydration of frog cocoon i n vivo may be attributed to the restriction of high water content to only v ery high RH (>96%).