Seasonal Changes in Mass and Reproductive Condition of the Corn Mouse (Calomys musculinus) on the Argentine Pampa

Abstract

Gross morphological and histological examinations of gonads were used to assess seasonal and age-related reproductive condition of Calomys musculinus captured in cultivated areas of the pampa of central Argentina. All females >5 weeks of age showed evidence of reproductive activity during the spring and summer; 86% of similar-aged males had fully mature gonads during the same period. The male reproductive system appeared to be most sensitive to the unfavorable environmental conditions of autumn and winter, yet a low level of reproduction was observed throughout the winter and was attributable to animals in older age classes that did not undergo gonadal regression. Commonly used external indicators of sexual maturity were relatively poor measures of underlying physiological conditions: 80% of mature, nulliparous females captured throughout the year had closed vaginal orifices and 19% of those with past or present pregnancies were nonperforate; 10% of fully mature males (mostly captured in winter) had nonscrotal testes. Aging by eye-lens weight showed that the large number of individuals of low body mass that were captured during the winter did not represent autumn and winter recruitment, but were older animals that had grown slowly or lost mass in response to unfavorable environmental conditions.