Causes of the unequal sex ratio in populations of adult Richardson's ground squirrels

Abstract

Four hypotheses, postulated to explain the unequal sex ratio characteristic of populations of adult Richardson's ground squirrels (Spermophilus richardsonii), were examined in a population of this species near Hanna, Alberta.One hypothesis, postulating a disproportionate loss of adult males in early spring, was supported by our observations. Males emerged earlier and were lost from study plots in greater numbers than females; they moved more widely, which probably increased their vulnerability to predation. A second hypothesis, postulating a disproportionate loss of juvenile males during their first summer, was also supported by our data. Loss of males exceeded that of females. Juvenile males moved farther and in greater numbers than females, which seemed to make them more vulnerable to predation; significantly more juvenile males than females were among the prey of buteos feeding young. That mortality through predation falls disproportionately on males, a third hypothesis, was therefore also supported. However, our data on loss from midsummer to the following spring do not support the hypothesis that differential mortality occurs over winter.