Body mass changes in male Daubenton's bats Myotis daubentonii (Chiroptera, Vespertilionidae) during the seasonal activity period

Abstract

Body mass changes of adult and juvenile male Daubenton’s bats were studied in a summer habitat in Hesse (Germany) in the years 1998-2001. Animals from the resident population were regularly captured along the flight path from April to October, and body mass was determined using a digital balance. Body mass of adult individuals differed significantly among fortnightly periods (Kruskal-Wallis ANOVA by ranks: H = 141.75, p < 0.001). Body mass increased significantly from the second half of April (median: 7.0 g) to the first half of May (median: 7.8 g). Between the second half of May and the first half of August, median body mass fluctuated between 7.8 g and 8.3 g. From the second half of July (median: 7.9 g) to the second half of August (median: 8.4 g), and the second half of September (median: 9.3 g) body mass again significantly increased. The highest median body mass (9.6 g) was recorded in the second half of October. Median body mass of juvenile males ranged between 6.5 g in the first half of July and 8.5 g in the second half of October. Differences in body mass over the studied period were significant (H = 21.70, p < 0.01). During the months July, August, and September, body mass of adult males was significantly higher than that of juveniles, whereas in October the difference was no longer of statistical significance. It is suggested that the decrease in body mass observed in some adult males in September was related to their reproductive activity.