The Determination of the Median Body-Weight at which Female Rats reach Maturity.



  • 1. The same statistical methods which are used in the analysis of biological assays have been applied to certain data for females of the two species, Rattus norvegicus and R. rattus, in order to determine the body-weights at which on the average the individuals become sexually mature.
  • 2. In four samples of Rattus norvegicus and in two of R. rattus it was found that the probits corresponding to the percentages of females with corpora lutea in their ovaries were linearly related to the logarithms of the body weight. This relationship means that the logarithms of the body-weight at which the individual females become mature can be regarded as being distributed normally.
  • 3. From the regression line connecting the probits with log. body-weight an estimate may be made of the weight at which 50% of the females have corpora lutea, together with the range of error attached to this estimate. Similarly, from the slope of the regression line an estimate of the standard deviation of the distribution of the individual log. body-weights at maturity may be obtained. Moreover, two such regression lines may be tested for any significant differences in either position or slope, so that the results for any two samples of females can be compared in these respects.
  • 4. In one series of data it was possible to compare the values of the parameters calculated from the regression line with similar estimates derived from a small sample of 27 females which had only ovulated once, After allowing for the fact that these 27 individuals were probably observed on the average some five days after they had first ovulated, the agreement between the two methods of estimation was satisfactory.
  • 5. One example is given of the successful application of this method to some data for male Rattus norvegicus. There are, however, certain difficulties in regard to the criterion which is adopted for defining fecundity in the male and more work needs to be done on data for this sex before it can be said that the method is generally applicable.
  • 6. We conclude from the results obtained so far that this method of analysis shows promise of being of some assistance in the study of mammalian reproduction, more particularly in regard to data for wild rodents.