STUDIES ON INDICES AND CAUSES OF PROLIFICACY IN RATS

Abstract

Studies were made on 2 inbred strains of rats having a common ancestry. The white strain had an avg. litter size of 6.5 as compared to 8.9 for the more prolific grey strain. Age at sexual maturity was found to be correlated with prolificacy, 20 [female] [female] of the white strain matured at an avg. age of 41.7 days as compared to 36.0 days for 25 [female] [female] of the more prolific grey strain. The avg. ovarian wt. at 27 days of age of 26 grey [female] [female] was 17.9 mg. as compared to 10.4 mg. for 22 white [female] [female] . The difference between the means of age at sexual maturity, and ovarian wt., for the 2 strains, is statistically significant, suggesting their use as indices of prolificacy. Comparison of body size, ovarian wt. and thyroid wt. of 24 sexually mature 9 9 of each strain showed that the wts. were significantly less in the white strain, indicating general reduction in activity of the anterior pituitary as a cause for the difference in prolificacy. However, the adrenals of the white strain were larger than those of the grey strain, and pituitary implants in day old chicks and immature 9 9 rats did not give evidence of increased thyrotrophic or gonadotrophin content of pituitaries of the grey strain. It is concluded that the increased prolificacy of the grey strain is not clearly indicated by the hormonal content of the pituitary gland.