The effects of changes in the environmental temperature on the growth of tail bones in the mouse.

The tails of baby mice grow rapidly and independently of environmental temperature because they are kept warm by their mothers. After weaning, at approximately 3 weeks of age, tail growth is strictly related to environmental temperatures. During the first 2 weeks after weaning growth rates of 1.2-1.4 mm/day/tail were seen at 33 degrees and a maximum of 2.43 mm/day/tail was observed in one group kept at 36 degrees. Animals kept at 8 degrees or 4 degrees showed tail growth rates of 0.4 mm/day or less. However, the tails of animals transferred from either hot to cold or cold to hot during their first 2 weeks after weaning immediately grew at the same rate as those of animals kept in these conditions continuously, thus indicating that heat was acting directly on bone growth. The tails of animals kept continuously in the hot environment at 33 degrees completed their growth early so that their growth rate fell below that of controls after about 3 weeks of treatment (when they were 6-7 weeks old) and below that of "cold" animals after about 4 weeks (7-8 weeks old). The tails of the "control" and "cold" animals grew slowly for a very long time, 150-195 days. Even so, because of the very rapid early growth of tails in the hot environment, their final length was always greater than either the "control" or "cold" tails.