GENOTYPE AND ENVIRONMENT IN TAIL LENGTH IN MICE

Mice ( Mus musculus L.) of (i) inbred strains A/Tb, A2G/Tb and C57BL/Tb, (ii) five F 1 types produced by crossing the inbred strains, and (iii) a mixed stock derived from the three inbred strains and GFF, have been reared in environments kept at ( a ) 21° C. and ( b ) ‐3° C. Body and tail lengths were recorded at 16 weeks and, for A/Tb and A2G/Tb, also at 3 weeks. Strain A2G/Tb had a typical mouse tail, near the length of the body, with about thirty‐four vertebrae (including sacral); C57BL/Tb had a slightly shorter tail, with about thirty‐two vertebræ, and A/Tb a much shorter tail, with a blunt tip, and about twenty‐four vertebræ. Evidence is given that the condition of the tail in strain A/Tb is recessive. The mixed stock and all F 1 groups, including those produced by crossing A/Tb with C57BL/Tb, had typical long tails with about thirty‐three vertebræ. The mixed stock had the longest tails. In all classes of mice absolute tail lengths were lower at ‐3° C. than at 21° C. This was the case despite the fact that, in some classes, body length was the same in the two temperatures. Hence the effect of cold on tail length is independent of its effect, on body length. All classes of mice, except the mixed stock, had fewer caudal vertebrae in the cold. The size of the caudal vertebrae was also reduced in the cold. The mixed stock, selected for fertility at ‐ 3° C., developed progressively longer tails over eighteen generations. There was no consistent influence of cold, or of heterozygosis, on variance in tail length. But variance was higher in the reducedtail strain A/Tb than in the other two inbred strains. These observations do not support the hypothesis that the tails of mice have a thermoregulatory function in a cold environment.