Temporal decrease of body temperature in middle-aged C57BL/6J mice.

Colonic temperature of C57BL/6J mice of both sexes between 12 and 19 months was measured at normal ambient temperature in four independent studies. One study was cross-sectional and the others were longitudinal. In all samples, colonic temperature at about 14 months was always lower than temperature at earlier ages. Furthermore, temperature always returned to earlier levels within 2 weeks; however, it remained unstable up to the age of 18 months after which it returned to the pre-14-month level until finally it fell late in life (after 23 months). Additionally, parametric studies showed that there were differences in the reliability of temperature probes made by different manufacturers to measure absolute temperature. These differences, however, did not affect relative differences among temperatures. Further parametric studies showed that colonic temperature in restrained mice rises 1.2 degrees C within the first 5 min after restraint indicating that duration of restraint is an important factor in studies such as these.