Physiological, demographic and pathological changes in wild norway rat populations over an annual cycle

Abstract

1. 1. Randomly trapped samples of saturated (land-fill) and expanding (pen) Norway rat populations revealed variations in structure brought about by spring and summer recruitment and fall and winter depression of reproduction. 2. 2. Pituitary-adrenal hyperfunction occurred in the saturated population during the recruitment period; breeding and recruitment were curtailed in the saturated population when the population showed signs of stress. 3. 3. Recruitment in low-density, expanding pen populations did not elicit signs of stress or depress reproductive potential. 4. 4. Winter conditions elicited hyperadrenalism and reproductive depression in both saturated and expanding populations. The frequency of regressed gonads and sex accessory tissues was highest in January when no pregnancies or juvenile recruitment was apparent. 5. 5. A progression of pathological changes in the hearts, livers and kidneys of animals developed following the summer social stress which was maximal with the onset of winter. The decreasing frequency of severe metabolic disease as the winter turned to spring was attributed to probable deaths of weakened animals during the severe winter cold.