Some ohservations on reproduction in Rattus rattus ( L . ) in Rangoon , Burma ^

Ohservations on the reproduction of Rattus rattus khyensis, z white-hellied commensal roof rat common in Rangoon, are described. Breeding activity is seen in all months of the year with no marked seasonal fluctuations. Female R. r. khyensis become sexually active at approximately 130 mm head and body length and at 60 g body weight. Prevalence of pregnancy in adult females averaged 33.3 "/o for the entire 2-years sampling period. Breeding activity as judged by both pregnancy and lactation averaged 52.8 Vo of all adult females. Litter size was found to be 5.03 + .21 with very little evidence of intrauterine embryo loss. The mammary pattern was determined to be 3 + 3 = 12, caused by a twinning of the postaxial mammae. The 50 °/o point for scrotal testes in males occured at a head and body length of 124 mm and an approximate body weight of 50 g, with sexual activity evident year round. The high rate of breeding activity is indicative of a population subject to heavy population pressure, predation and stress and a consequent high mortality rate. Similar ohservations have been noted for Rattus exulans, Bandicota hengalensis and Suncus murinus in Rangoon. Recruitment of young is almost continuous and populations are maintained only by a high degree of breeding activity with little seasonal fluctuation.