Energy balance of hibernating mouse-eared bat Myotis myotis: a study with a TOBEC instrument

We used a non-invasive TOBEC method (Total Body Electric Conductivity) to e stimate lean body mass and fat content in mouse-eared bats Myotis myolis (B orkhausen, 1797) hibernating in Poznan Forts (W Poland) and in a semi-natur al cave-mine Miedzianka (SE Poland). In December, Fat content averaged 5.5 g in females (body mass = 29.4 g) and 5.3 g in males (body mass = 28.4 g). At the end of hibernation (April), fat content averaged 2.2 g in females (b ody mass = 25.6 g) and 1.4 g in males (body mass = 23.7 g). Eat content did not differ between the localities either in December or in April, hut the pattern of changes of fat content was different. We calculated the rate of energy expenditure in hibernating bats using two methods, based on independ ent samples (fat content in first-time captured individuals) and based on p aired observations (changes of fat content in re-captured individuals), and discussed problems associated with the two approaches. Both methods show t hat the bats need about 4.9 g of fat (191 kJ) to sustain a 165-day hibernat ion. However, the rate of fat usage varied considerably between the sites a nd hibernation phase. Although the average amount of fat remaining in April would be sufficient to support at least six more weeks of hibernation, the level of reserves was close to zero in some individuals.