Observations on the sex ratio and the position of the lactating mammary gland in the brush-tailed possum, Trichosurus vulpecula (Kerr) (Marsupialia)

Abstract

Data on age, sex, and location of the lactating mammary gland in T. vulpecula have been obtained from animals captured in 14 different localities in southern Australia. There is no evidence of differences between localities in the sex ratio of pouch young and the overall totals of 113 males and 94 females do not differ significantly from equality. When these data are combined with data on the sex ratio of T. vulpecula pouch young obtained by other workers, there is a consistent and significant excess of males. On partitioning the data obtained in this study into four age groups of 50 days, the 0 to 50-day group contains a significant excess of males, whereas the older groups show a sex ratio close to 1. Also, there is a significant regression of sex ratio on age, the proportion of males decreasing with increasing age. These observations may be accounted for by proposing that the sex ratio at birth favours an excess of males, and that differential mortality between the sexes occurs during pouch life. A sample of adults and independently living juveniles from Rocky River, Kangaroo I., S.A., contains a highly significant excess of males (202 males, 92 females). In all other population samples the sex ratio is close to 1. In samples from two of the localities, namely Rocky River and Adelaide, there is a significant departure from equality with respect to the number of pouch young suckling from the left and right teat. At Rocky River the right teat appears to be favoured, while in Adelaide the situation is reversed.