Constitution and body proportions in different strains of rats. A study of race formation in breeding isolates.

(1) There are considerable differences in body constitution in different strains of rats, ranging from very robust to very gracile forms. The differences are greater in the larger males than in the smaller females. (2)Craniofacial and postcranial proportions of the trunk in domesticated strains of rats differ either uniformly or mosaically from the wild form. (3)Nasal shape differs greatly in rats ranging from extreme leptorrhine to extreme platyrrhine forms, giving a total range greater than in human populations. (4)Fisher 344 rats have long tails and extremities and Buffalo and GRL short tails and extremities. (5)Wistar and ACI rats have longer tails than wild rats but do not differ significantly from wild rats in the relative length of their other extremities. (6)The relationship between tail and extremity length is under genetic control which is concordant in Fisher, Buffalo and GRL rats, but discordant in the Lewis, Wistar and ACI strains. (7)There is no connection between relative extremity length and total body constitution since short-limbed strains occupy both the highest (Buffalo) and lowest (GRL) levels o f robusticity, and strains that do not differ in relative extremity length from wild rats differ greatly from each other in constitutional type (Wistar and ACI). (8)Differences of relative extremity length and nasal shape in rats have their parallels in human populations. But in human populations they follow Allen's ecological "rule" and can be duplicated experimentally. In rats here used, however, they do not result from any known ecological pressures, but from the genetic factors acting in breeding isolates.