Abstract
The experiments here reported represent an attempt to determine whether a relation exists between the growth rate and first estrus in the rat.
It has been common knowledge for a decade that rats and mice may be retarded in growth rate and in size at weaning by the amount of available nourishment. Many breeders have found that optimal conditions of growth and size in an animal colony may be attained by limiting the size of the nursing litter to 5 or 6 animals.
Litters of rats were selected for this study over a period of more than a year. They were taken from every month in the year, but few were studied during July and August. Records were obtained on 286 rats which lived to first estrus. These represented 63 litters. To indicate the random selection of litters for study, the serial numbers of the animals reported range from 13,992, born Sept. 27, 1935, to 16,721, born Oct. 4, 1936. Those of Group D were studied between January and May of 1937.
The sex of the young was noted at birth, and females otlly were left in each litter. The rats were originally divided into 3 categories : those lvith 2 or 3 in a litter (Group A), those with 4 or 5 (Group B), and those Ivith 10 or 11 (Group C). The latter represent in each instance 2 litters Imrn the same night, and reared by the mother of one of the litters. Thus half of this group were foster rats. Because of the high mortality of infantile rats under such conditions of crowding, there were in Group C only 7 litters with 10 each alive at weaning and 3 litters with 11 alive at weaning. Sulxequent losses reduced this number to 91 at first estrus.
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