A tumor incidence of 57 per cent was observed in 150 female rats allowed to live out their life-span as normal, control animals on standard laboratory chow. One hundred similar rats on a special fatrich diet developed an 80 per cent tumor incidence.
Ninety-five per cent of the tumors observed involved mammary gland tissue. Twelve per cent of all tumors observed were of a malignant type. Eighty-seven per cent of all tumors appeared after the rats were 540 days of age, and the mean lifespan after a tumor was first observed was 140 days.
Related studies with a smaller number of animals indicated that male rats of the same strain were 5 times less susceptible to tumor formation than females. However, three of the four tumors that were examined histologically in the males were classified as malignant.
The mean life span of normal female Sprague-Dawley rats in this laboratory was 760 ± 21 days, with individuals ranging in life span from 193 to 1100 days.