Abstract
Weanling female litter mates were equally proportioned at the start of the study between a breeding and a non-breeding group. At 10 weeks of age, each rat of the breeding group was housed individually with a male until 5 litters were produced (or exceptionally for a maximum of 42 weeks) before the male was removed. Females of the non-breeding group were kept singly in similar cages.
2 years after the start of the study all the surviving rats of both groups were killed. Comprehensive necropsies were performed on the decedents and survivors and the pituitary gland of each rat was examined histologically.
The 48% survival of the female rats which were allowed to breed was better, but not significantly so, than the 38% of those not allowed to breed. Overall fewer, 70% (35 of 50) of the mated female rats had pituitary tumours compared with 80% (40 of 50) of the non-mated group. Fewer of the decedents, 69% (18 of 26), of the mated females had pituitary tumours compared with 94% (29 of 31) in the decedents of the non-mated group. More survivors, 71% (17 of 24), of the mated females had pituitary tumours compared with 58% (11 of 19) of the surviving non-mated rats.
A tumour was first observed at day 328 in the non-mated group, whilst in the mated group the first tumour was identified at day 450. It is suggested that breeding delays the appearance of pituitary tumours in the female rat.