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This study examined behavioral and physiological responses (changes in inter-animal spacing, total glucocorticoids, testosterone, and body mass) to the formation of breeding and same-sex groups in two bat species, the socially gregarious Malayan Flying Fox (Pteropus vampyrus) and the less social Little Golden-mantled Flying Fox (Pteropus pumilus). We hypothesized that social instability, especially in the breeding groups and especially in P. vampyrus, would result in elevated glucocorticoids and that social facilitation of breeding and/or male-male competition would result in persistently higher levels of testosterone in breeding males. Seasonal rhythms in all measures were also predicted, and the glucocorticoid stress response was expected to vary by sex, season, and group type. Nearly all animals responded to group formation with elevated glucocorticoids, but, for breeding animals (especially aggressive male P. vampyrus), these responses persisted over time. In both species, breeding group formation resulted in elevated testosterone in males. Glucocorticoids, testosterone, testes volume, and body mass generally peaked in the breeding season in males (late summer and early autumn), but the seasonal glucocorticoid peak in females occurred in late winter and early spring. All animals responded to restraint stress with elevations in glucocorticoids that largely did not differ by sex, time of year, reproductive condition, group type, or, in lactating females, the presence of her pup. Changes in both behavior and physiology were more evident in P. vampyrus than in P. pumilus, and we believe that their underlying social differences influenced their responses to group formation and to the changing seasonal environment.

Baseline and stress-responsive glucocorticoid (GC) levels were characterized during the active period in free-ranging male and reproductive female little brown myotis (Myotis lucifugus). Bats were trapped and blood was sampled within 3 min of capture at two maternity sites during the summer and at one swarming site prior to hibernation in New England. Both GC hormones, cortisol and corticosterone, were detected, with cortisol accounting for an average of approximately 95% of total circulating GCs. Samples collected at the dusk emergence and after the first return from feeding showed significant seasonal differences across the active period (early pregnancy, mid-to-late pregnancy, lactation [and comparable mid-summer times for males], and pre-hibernation) within and between each sex. Elevated baseline values were found in mid-to-late pregnancy females at emergence, and in both males and females at the swarming site compared to other groups. Female GC values during mid-to-late pregnancy and during the pre-hibernation period were greater than those for males. Significantly higher GC levels following 15 min of restraint were exhibited by all animals in the summer and prior to hibernation. There was little variation between groups or sexes in the total GC levels reached following restraint. Taken together, these results suggest that: (1) GCs may be involved in the increased feeding and/or fat deposition characteristic of pregnancy and the pre-hibernation period, (2) GCs may be related to mating and to the generally increased levels of activity that occur during the pre-hibernation period, and (3) regardless of sex or reproductive condition, all animals maximally respond to restraint stress.