Water Loss During Aestivation

During aestivation, species of Cyclorana risk losing a high proportion of their water mass. Unable to compensate this loss with any intake of water, this risk becomes more serious. The first defense an aestivating frog has against water loss is the cocoon it forms. Evaporative water loss (EWL) for frogs in cocoons is significantly less than that of non-cocooned frogs, and resistance to water loss is significantly greater for cocooning frogs than for non-cocooning frogs (Withers, 1998). The rate of EWL, while varying between species, is also dependent upon a few factors. First, as thickness of a cocoon increases, EWL decreases and resistance increases. This helps explain why frogs shed more and more skin for their cocoon as aestivation prolongs (1998). Second, ambient conditions, such as temperature and humidity, affect the structure of the cocoon. With little testing, an increase in relative humidity (RH) has proven to compromise the structure of a cocoon. At high RH, the water content of cocoons increases, making them more watery and less stable (1998).

Even with a cocoon, aestivating Cyclorana lose a substantial percentage of their body mass through water loss. While body mass actually increases in the first couple months of aestivation due to absorption of water in the soil, body mass over full aestivation periods decreases by between 30 and 40% (Bayomy et al., 2002). Some water is withdrawn from the bladder to possibly counter EWL, indicated by an increase in urine osmolality (Cartledge et al., 2008). Mucopolysaccharides (MPS), chains of sugar molecules, are thought to be one of the more crucial components in fighting dehydration. More specifically, hyaluronidase (HYA) is thought to bind with water to limit its loss, especially in the kidneys where it has been found to increase significantly during aestivation (Bayomy et al., 2002). The exact process by which aestivating frogs limit water loss is unknown, but it is most likely due to a combination of these and other actions.

In accord with water loss, hematocrit, the portion of blood that consists of red blood cells, remains stable during aestivation and red blood cell numbers decline (Cartledge et al., 2008). Since aestivating frogs are more dehydrated, their blood becomes more viscous, or resists flow caused by force. By reducing its number of red blood cells, aestivating frogs may optimize hematocrit and the delivery of oxygen to their tissues (2008).