The Placental Scar as a Measure of Fertility in Rats

Abstract

To compare the number of young produced by female rats with the number of placental scars found at autopsy in the reproductive tract, virgin tame hooded and a few brown rats (Rattus norvegicus) were repeatedly bred and the number of young counted at each parturition. After producing several litters, the females were killed and the placental scars counted. The results showed only a general relationship between production of young and number of scars. There were less scars than young in 9 rats and more scars than young in 4 rats out of 21 specimens. In an analysis of 430 wild female brown rats, a correlation coeff. of 60% was found between the number of scars and the length of the rat, suggesting that number of scars is a crude measure of productivity. By determining the length at which 50% of the rats have scars, it is possible to compare the inception of pregnancy in different rat populations. The 50% point for roof rats in San Antonio, Texas, was 163 mm., for brown rats in San Antonio, 178 mm., and for brown rats in Baltimore, 224 mm. The ratio of 50% point to mean wt. for brown rats in San Antonio was 0.86 and in Baltimore was 0.93, a significant difference, which indicates that rats in San Antonio became sexually mature at a smaller size than in Baltimore.