The Determination of Sex and Age Ratios in the Cottontail Rabbit

Abstract

Sex and age criteria are valuable in the determination of rearing success and in the dating of reproductive events. From a study of several hundred live-trapped and dead specimens in Ohio and Pennsylvania, criteria permitting the identification of sex are given for live, dressed, and frozen specimens of Sylvilagus floridanus mearnsii. Age determination methods by means of size and weight changes and by changes in the ossification of the epiphyses of the lower foreleg are proposed for animals in their first year. Growth curves, tables and photographs illustrating these methods are given. Young of the year and adults can be separated on the basis of differences in the development of the sex organs and the proximal humeral epiphysis. Age differences of limited usefulness were found in the teeth and skull. Methods are described for handling live rabbits and for the collection of age ratio data. Sex ratios are apparently influenced by the weather at the time the specimens are colllected and possibly also by the size of the study area. Late autumn age ratios indicate that usually about 6 young per adult, or 12 young per adult female survive until the hunting season. These data indicate an avg. longevity of about 1.2 years for animals surviving until the end of their first year, and an avg. population turnover period of less than 4 years. The probable birth dates of rabbits killed in Nov. are calculated from X-rays of a collection of forepaws. A possible method for forecasting hunting success is discussed.