The Relation of Sex Ratio to Physiological Age in the Wild Brown Rat

Abstract

A comparison between the sex ratios of various segments of a population of Rattus norvegicus showed differences in the relative abundance of each sex at different stages in the individual''s life-span. Since the chronological age of any particular individual was impossible to determine, the rats were separated into 50 g. wt. classes. In addition, they were divided into 3 groupings based on their physiological state: at birth, sexually immature, and sexually mature. The sex ratio at birth was detd. by anatomical inspection of 1044 individuals of 17-22 days fetal age. Data on immature and mature individuals were obtained from the records of over 5000 rats trapped within the past 3 yrs. in the vicinity of Baltimore, Md. The proportions of the 2 sexes are about equal at birth (51.2% male), immature males are more abundant than immature females (56.7% male), but mature males are less abundant than mature females (42.8% male).