The Relation of the Rate of Growth to Diet. Ii.*

In 1926 Osborne and Mendel (1) published a report of some of their experiences in securing in male rats, kept under laboratory conditions, more rapid growth than has hitherto been recorded for this species in the literature with which we are familiar. The conclusion was reached that the favorable outcome of our experiments was not due to selection or marked changes produced in our stock through breeding, but was primarily the result of a more appropriate diet than had hitherto been employed regularly in the experimental feeding of rats. This implies that, owing to the shortcomings of the rations used, the inherent capacity of the rat to grow has in the past rarely been given full play in the laboratory. Consequently the published records and compilations of “norms” fail to furnish an adequate idea of the rate of growth of which the rat is capable. In the most familiar records of growth, such as are presented in Donaldson’s “The Rat,” the average time required for the change in body weight in the male from 60 to 200 gm. is 79 days; from 60 to 275 gm., it is 244 days. The corresponding figure for the female growing from 60 to 200 gm. is 129 days. The average daily gains calculated from these figures are as follows: