Breeding Cycle, Life History and Population Dynamics of the Dugong, Dugon dugon (Sirenia: Dugongidae

Abstract

Study of the gonads of 108 dugongs from north Queensland indicates that neither females nor males are continuously in breeding condition. The gonadal activity of males in a population is not synchronized. Ovaries tend to be active in the latter half of the year. There is no evidence for females coming into oestrus soon after giving birth but conception can occur during lactation. In the Townsville-Cairns area, dugongs calve from August-September through December. Neonates are between 1.0 and 1.3 m long and weigh 20-35 kg. Dugongs of both sexes less than 2.2 m long are likely to be immature, those over 2.5 m long are probably mature, and the status of animals between 2.2 and 2.5 m long is uncertain. The pre-reproductive period seems to be very variable but is a minimum of 9-10 years for both sexes. The gestation period is about 1 year and lactation can last at least 1.5 years. The usual litter size is one. The secondary and tertiary sex ratios are 1:1. Estimates of the calving interval based on pregnancy rates, the rate of accumulation of placental scars, and calf counts from aerial surveys and photographs, range from 3 to 7 years for various populations. A simple population model has been used to calculate the relationship between calving interval and adult mortality rate for stationary populations with different pre-reproductive periods and juvenile mortality rates. Even the most optimistic schedule of reproduction and juvenile mortality demands an adult survivorship of about 90% per year for population maintenance.