Reproduction in an island population of Norway rats, Rattus norvegicus (Berkenhout), treated with an oestrogenic steroid

A population of Norway rats, Rattus norvegicus (Berkenhout), living on a small island was sampled on four occasions in 1974. Breeding began in the spring and ceased in late autumn. Sub-adult rats (< 150 g) were absent in March but they comprised 45.5% of the sample taken in June; they were even more numerous in September (65.9%) but their numbers were declining by December (36.9%). Pregnant and lactating females were trapped in June and September only.

The population was treated with a synthetic oestrogen reproduction inhibitor for 1 week in March 1975. The compound, the 3-cyclopentyl ether of 17α-hexa-1′3′-diynyloestra-1,3,5(10)-trien-17β-ol (BDH 10131), was included at 0.05% in oatmeal bait.

Further samples of rats were taken after the treatment, in April, July and September 1975 and in January 1976. Fertility was found to have been impaired; no sub-adult animals were caught and no actively breeding females were found. The capture of fewer individuals at post-treatment (29) than at pre-treatment (143) was a further indication that rat numbers remained low following the treatment.

In May 1976, 14 months after the BDH 10131 treatment, both sub-adult and pregnant animals were again present, providing evidence of renewed breeding and of population growth.