Population Ecology and Physiology of the Common Rock Rat, Zyzomys argurus (Rodentia: Muridae) in Tropical Northwestern Australia

Abstract

The common rock rat, Zyzomys argurus , undergoes a marked seasonal change in abundance in rocky habitats of tropical Australia. Four local populations had highest numbers between the late-dry (September) and mid-wet (January) periods before undergoing a three- to four-fold decrease by the end of the wet season (April). Densities differed between these populations; one population disappeared in April. Immature individuals were more abundant in September when recruitment appeared maximal; the population decline is attributable to mortality in all age categories. Both sexes were reproductively active throughout the year, but pregnant females were most abundant in April. Litter size ranged from one to six and heavier females had more embryos. Gene frequencies of Z. argurus were significantly different between the wet (January and April) and dry (July and September) seasons and the ephemeral population had different gene frequencies from the others. Differences also occurred in concentration of plasma corticosterone, maximum corticosterone-binding capacity, and corticosteroid-binding globulin-bound corticosterone. In January, individuals in all local populations had normal body weight and albumin levels, and enlarged tail bases (a fat store). They also had elevated levels of free corticosterone, neutrophils, incidence of Salmonella , and total leucocyte count; lymphocyte to neutrophil ratios differed between populations. These data indicate that populations at differing densities and in average nutritional state had greater levels of physiological and pathological stress factors in the wet season suggesting that exogenous rather than endogenous factors control stress.