SEASONAL-VARIATION IN THE RESTING METABOLIC-RATE OF MALE WOOD MICE APODEMUS-SYLVATICUS FROM 2 CONTRASTING HABITATS 15 KM APART

Diurnal and nocturnal resting metabolic rates of winter- and summer-ac climatized adult male wood mice Apodemus sylvaticus from two adjacent populations, 15 km apart, were measured. One population lived in decid uous woodland, and experienced a narrower daily range of temperatures than the second population, which inhabited maritime sand-dunes. Ambie nt temperature and body mass had significant effects on the resting me tabolism of mice, excluding winter-acclimatized sand-dune animals wher e only temperature explained significant amounts of the observed varia tion. Only in this latter group could a thermoneutral zone be determin ed, with a lower critical temperature of ca. 25 degrees C and resting metabolism of 0.155 W. Nocturnal resting metabolic rates were signific antly greater than diurnal levels. Winter acclimatization was associat ed with reductions in thermal conductance and resting metabolism, thus minimizing energy expenditure at rest. Site differences in thermoregu latory strategies were only found in winter, thermal conductances rema ined similar but mice from the sand-dunes had significantly lower meta bolic rates than those from the woodland. Winter acclimatization in wo od mice was influenced by factors in addition to photoperiod. Intra-sp ecific and individual variations in resting metabolism, as shown in th is study, potentially have a pronounced effect on the daily energy exp enditure of a free-living animal.