Ontogenetic influences on foraging and mass accumulation by big brown bats(Eptesicus fuscus)

1. Juvenile flying animals, including bats, often cease increasing, or even decrease in body mass after the onset of flight. The low body mass of juve nile bats may be a result of stresses associated with the initiation of fli ght, or may be an adaptation to reduce flight costs. 2. To test these hypotheses, the body masses of adults and unknown aged juv enile big brown bats (Eptesicus fuscus) were separately compared in August 1990 through to 1995. It was hypothesized that there should be little varia tion in the body mass of juveniles from year to year if low body mass is an adaptation to reduce flight costs. 3. The foraging behaviour and body masses of bats of known or estimated age were also compared in 1994 and 1995. Ambient temperature at sunset was gre ater in 1994 than in 1995. It was assumed that prey density and predictabil ity increased with temperature, and hypothesized that if low body mass resu lts from energetic stress, then this should be more apparent in 1995. Using radiotelemetry, the foraging behaviour of juvenile, yearling and adult E. fuscus was compared in 1994 and 1995 to determine whether young bats attemp t to compensate for energetic shortfalls by foraging for longer, or emergin g earlier, than adults. 4. Unknown-aged juveniles did not differ significantly in body mass in Augu st from 1990 through to 1995. Adults were significantly heavier in August i n 1990 and 1994 than in 1991, 1993 and 1995. Known-aged juveniles had lower body mass than adults in 1994, and lower wing loading than adults in 1995. Known-aged juveniles did not differ in mass between 1994 and 1995, while a dults were heavier in 1994. 5. The foraging times for juvenile and adult bats were both significantly r elated to ambient temperature. However, the slope of this relationship was steeper for weaned juveniles than for adults. The foraging times of yearlin g and adults bats were not significantly different. 6. The emergence times of juvenile and adult bats did not differ, nor did t he emergence times of yearlings and adults. Emergence time did not differ s ignificantly between 1994 and 1995. 7. The results of the study suggest that juvenile bats maintain a low body mass even under conditions that permit adults to accumulate greater fat dep osits. Juveniles do not forage earlier or for longer to compensate for poor foraging ability and increased energy expenditures resulting from the onse t of flight. This suggests that, by maintaining a low body mass, juveniles reduce flight costs and the risk of predation at a time when flight and for aging are still developing.