The selection of optimal hibernation temperature (T_H) was possible by bats changing both the distance at which they roosted from the mine entrance (D) (T_H versus D; r_s= 0.73, n = 615, P < 0.001), and the height of hibernation place (H) (T_H vs. H; r_s= 0.16, n = 412, P < 0.01). Bats were able to select areas of high relative humidity (RH) by roosting in low temperature (RH vs. T_H; r_s= -0.26, n = 366, P < 0.001) and/or by selecting hibernation places situated lower on the mine walls (RH vs. H; r_s= -0.61, n = 280, P < 0.001). Sub-adult bats (identified by presence of the black chin spot) were found to hibernate at significantly lower temperatures (Z = -3.1, n₁ = 164, n₂ = 41, P < 0.01) and in places situated lower on the mine walls (Z = -2.2, n₁ = 164, n₂, = 41, P < 0.05) than adult individuals. In March sub-adults hibernated closer to the entrance than adult individuals (χ² = 8.18, d.f. = 1, P < 0.01). The difference in average body condition index between sub-adult and adult bats recorded in March (one-way ANOVA, F = 6.56, error d.f. = 51, P < 0.05) made individuals in their first year of life more prone to starvation at the end of hibernation period. In this month the significantly smaller distance of hibernation place of sub-adult individuals from the mine entrance (Z = -2.7, n₁ = 58, n₂, = 19, P < 0.01), resulted in significantly lower hibernation temperature, making them more endangered by predation of mammalian and avian species than adult bats hibernating deeper in the mine. The linear (r = 0.87, d.f. = 30, P < 0.001) relationship between body mass at the beginning and end of hibernation (November and March) of uniquely marked individuals indicates these bats did not forage in winter and their energy use was exclusively dependent on fat reserves accumulated prior to hibernation. The significant relationship between body mass in November and total mass loss (r = 0.59, d.f. = 30, P < 0.001) could indicate the possibility of existence of another factor, or group of factors, that could increase the energy use in hibernating M. daubentonii. These may include mating and/or energy costly defence against predators.