Lability of tail length of the white-footed mouse, Peromyscus leucopus noveboracensis.

—White-footed mice raised at 25° C have significantly longer tails at maturity than do mice raised at 15°C. The length of distal caudal vertebrae is more affected by temperature than that of proximal caudal vertebrae, suggesting a direct effect of temperature on tail growth. The results may be invoked to explain variation observed in nature, but extrapolations to free-ranging mice involve unverified assumptions. The effect of environmental temperature on the tail length of rodents was first investigated by Sumner (1909). He demonstrated that white mice (Mus musculus) raised at warmer temperatures had longer tails than did those raised at cooler temperatures. Przibram (1925) demonstrated the same effect with Rattus rattus and albino Rattus norvegicus. A number of other workers subsequently documented this phenomenon (Ogle 1934; Ashoub 1958; Harrison et al., 1959; Knudsen 1962; Barnett, 1965) with different strains of Mus musculus. The present investigation was undertaken to determine if the white-footed mouse, Peromyscus leucopus noveboracensis, also exhibits a temperature dependent lability of tail length. This is of interest for at least three basic reasons. First, P. leucopus is a cricetid rodent, whereas all previous investigations were carried out on murid rodents. Thus the results presented here permit a somewhat broader evaluation of the significance of this phenomenon among rodents. Second, the animals used were not inbred or laboratory strains. Przibram's work with Rattus rattus is the only other investigation in which this is the case. The investigations of Harrison et al. (1959) with inbred and "hybrid" Mus musculus demonstrated the relevance of the amount of inbreeding in studies of the lability of tail length. Third, the geographic variation of tail length of the white-footed mouse is of a complex and interesting nature. Over its range, P. leucopus exhibits little subspecific variation of size or tail length, in striking contrast to P. manicutotus (Osgood, 1909). It does not exhibit the geographic variation of Allen's rule. Within any geographic area, however, P. leucopus is a highly variable species, and the differences between local populations may be as great as the described differences between subspecies. Dice (1937) investigated this variation in P. I. noveboracensis by raising animals from different populations under uniform conditions in the laboratory. By showing that interpopulation differences still exist among animals raised in the laboratory, he demonstrated that there is a genetic basis for the variation observed in the wild. He did not investigate nongenetic variation of characters, per se, but, in fact, such variation is evident in his data and was noted by him. Specifically, the tails of mice raised in the laboratory were characteristically