Seasonal changes in daily torpor patterns of free-ranging female and male Daubenton's bats (Myotis daubentonii)

Daily torpor can provide significant energy and water savings in bats during cold ambient temperatures and food scarcity. However, it may reduce rates of foetal and juvenile development. Therefore, reproductive females should optimize development by minimizing times in torpor. To test this hypothesis, the use of torpor by female and male free-ranging Daubenton's bats (Myotis daubentonii) during reproduction (gestation, lactation, and post-lactation period) was investigated in 1998 and 1999. Temperature-sensitive radio transmitters were attached to the bats to measure skin temperature. Simultaneously, ambient temperature was recorded. While both sexes became torpid during daytime, male bats used daily torpor (> 6 degrees C below individual active temperature) significantly more often during reproductive period (mean: 78.4% of day time in May and 43% in June) than females. Female bats went into daily torpor, particularly in late summer when juveniles were weaned (mean: 66.6% of daytime). Lowest skin temperatures occurred in a female bat with 21.0 degrees C during post-lactation. Skin temperatures of male bats fluctuated from 16.8 degrees C in torpor to 37.2 degrees C during times of activity. There was a significant effect of reproductive period on skin temperature in females whereas mean ambient temperature had no significant effect. However, mean ambient temperature affected mean skin temperatures in males. Our findings indicate that female Daubenton's bats adopt their thermoregulatory behaviour in particular to optimize the juvenile development.