Reproductive biology of female striped dolphin (Stenella coeruleoalba) from the western Mediterranean

Abstract

Data on age and length at sexual maturity, calving interval, lactation period, and ovulation and pregnancy rates were analysed from 84 female striped dolphin collected during the Mediterranean morbillivirus epizootic. Age was determined by counting growth layer groups in tooth dentine, and reproductive status was determined by the macroscopic examination of the ovaries and the uterus. Age at sexual maturity (ASM) was estimated using two different methods; giving average estimates of 12 and 12.3 years. The average length at sexual maturity was 187 cm and the calving interval was four years. The annual pregnancy rate (APR) was calculated using two different criteria. In the first, only females that were pregnant at the time of death, as evidenced by the presence of a foetus, were incorporated in the calculations. This method gave an estimate of 8.9% as the apparent pregnancy rate of the dolphins killed by the epizootic. The second method incorporated in the calculations females that were pregnant at the time of death and females that had young corpus albicans in the ovaries and a distended uterus. This method is considered to reflect the true pregnancy rate of the population and gave an estimate of 25%. Estimates of ovulation rate (0.401) and lactation period (16 months) were considered to be potentially affected by the disease and, therefore, biased. These estimates reflect a peculiar situation of the diseased dolphins but are useful for comparative purposes. Estimates of ASM and APR, parameters which are not likely to be affected by the disease, differed markedly from those of other Stenella populations and could indicate a density‐dependent response, possibly reflecting food limitations.