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

Abstract

The object of this paper is to determine the sex ratio of the various age segments of a population of wild brown rats, Rattus norvegicus, in their natural habitat. The sex ratio at parturition was determined by the examination of 1044 embryos that were found in the reproductive tracts of 118 wild female brown rats. The age of these embryos ranged from 17 to 22 days, and since intrauterine mortality and resorption is negligible from the 17th day until parturition, the sex ratio of the embryos as determined was assumed to be comparable to the sex ratio at parturition. Of the 1044 embryos, 51.25 per cent. were male, but this figure is not significantly different from 50 per cent. (P = 0.4). The sex ratio at later ages was determined from the records of over 5000 wild rats that were trapped throughout the residential slum and poultry warehouse areas of the city of Baltimore, and from a farm adjacent to the city. Since the chronological age of the trapped rats was impossible to determine, a new age classification was used, based on physiological state as determined either by the degree of sexual activity or the weight of the animal. The 5262 rats available were grouped into 50-gram weight classes and also were divided according to the degree of sexual activity. A preponderance of females in the middle weight groups was evident (P ≤ 0.05); this preponderance of females was not due to a weight differential between the sexes at comparable ages. It is concluded that the proportions of the two sexes are about equal at birth (51.25 per cent. male); that the proportion of males is greater until the beginning of the reproductive age (56.68 per cent. male, P < 0.005); and that throughout the adult period the proportion of females is considerably greater than that of males in the population (42.75 per cent. male, P < 0.005).