Concentration of Free Glucocorticoids in Plasma and Mortality in the Australian Bush Rat (Rattus fuscipes Waterhouse)

We investigated the adrenocortical response to the stress of capture and anesthesia, and its association with seasonal mortality patterns in the Australian bush rat ( Rattus fuscipes ). In females, there were two marked seasonal peaks in the concentration of total corticosterone B of stressed plasma, one in the summer breeding period and the other in winter, a time of high mortality in females. The increase in summer was matched by a greater increase in maximum corticosterone-binding capacity of plasma so that free, biologically active, corticosterone B in plasma remained low. At the time of winter mortality, total corticosterone B exceeded maximum corticosterone-binding capacity and free corticosterone B in plasma was much higher than at other times. In males, total corticosterone B in plasma was much lower than in females during the summer period when mortality of males was high but maximum corticosterone-binding capacity was even lower, so that free corticosterone B in plasma was much higher than at other times. A similar increase in some males occurred at the time of winter mortality. The association of higher free corticosterone B in plasma and high mortality suggest an excessive adrenocortical response to stress causing increased susceptibility to disease, as in some dasyurid marsupials.