Effects of age and body weight on the proportion of females breeding in a population of roe deer (Capreolus capreolus)

The reproductive state of female roe deer (Capreolus capreolus) (n = 140) caught during 4 consecutive winters (January and February) was determined from progesterone concentrations in blood samples. We found that (i) females older than 20 months were pregnant almost every year, indicating that though monoestrous, this species can achieve a very high conception rate (98%); (ii) a significant proportion of females 20 months of age (corresponding to the usual age at first pregnancy) had not reached sexual maturity; and (iii) body weight measured in winter could be used to predict the probability of pregnancy for young females (20 months of age) but not for older females. The correlation observed between age, body weight, and reproductive success suggests the hypothesis that the proportion of young females that breeds is density dependent. Ecological conditions mediated by social behaviour and prevailing during the spring of birth could constitute the ultimate factor determining pregnancy rate at 20 months.