Hamilton I M, Barclay R M
Behavioural Ecology Group, Division of Ecology, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada, T2N IN4.
J Anim Ecol. 1998 Nov;67(6):930-40. doi: 10.1046/j.1365-2656.1998.6760930.x.
Juvenile flying animals, including bats, often cease increasing, or even decrease in body mass after the onset of flight. The low body mass of juvenile bats may be a result of stresses associated with the initiation of flight, or may be an adaptation to reduce flight costs. To test these hypotheses, the body masses of adults and unknown aged juvenile big brown bats (Eptesicus fuscus) were separately compared in August 1990 through to 1995. It was hypothesized that there should be little variation in the body mass of juveniles from year to year if low body mass is an adaption to reduce flight costs. 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 greater in 1994 than in 1995. It was assumed that prey density and predictability increased with temperature, and hypothesized that if low body mass results 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 attempt to compensate for energetic shortfalls by foraging for longer, or emerging earlier, than adults. Unknown-aged juveniles did not differ significantly in body mass in August from 1990 through to 1995. Adults were significantly heavier in August in 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 adults were heavier in 1994. The foraging times for juvenile and adult bats were both significantly related to ambient temperature. However, the slope of this relationship was steeper for weaned juveniles than for adults. The foraging times of yearling and adults bats were not significantly different. The emergence times of juvenile and adult bats did not differ, nor did the emergence times of yearlings and adults. Emergence time did not differ significantly between 1994 and 1995. The results of the study suggest that juvenile bats maintain a low body mass even under conditions that permit adults to accumulate greater fat deposits. Juveniles do not forage earlier or for longer to compensate for poor foraging ability and increased energy expenditures resulting from the onset 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 foraging are still developing.
包括蝙蝠在内的幼年飞行类动物，在开始飞行后，其体重往往停止增长，甚至下降。幼年蝙蝠体重较轻，可能是与开始飞行相关的压力所致，也可能是为降低飞行成本而做出的一种适应。为验证这些假设，在1990年8月至1995年期间，分别比较了成年大棕蝠（棕蝠）和年龄未知的幼年大棕蝠的体重。假设如果低体重是为降低飞行成本而做出的适应，那么幼年蝙蝠的体重每年应变化不大。1994年和1995年，还比较了已知或估计年龄的蝙蝠的觅食行为和体重。1994年日落时的环境温度高于1995年。假定猎物密度和可预测性随温度升高而增加，并假设如果低体重是由能量压力导致的，那么这在1995年应更为明显。1994年和1995年，使用无线电遥测技术比较了幼年、一岁和成年棕蝠的觅食行为，以确定幼年蝙蝠是否试图通过比成年蝙蝠觅食时间更长或更早出飞来弥补能量不足。1990年至1995年8月期间，年龄未知的幼年蝙蝠体重无显著差异。1990年和1994年8月的成年蝙蝠比1991年、1993年和1995年的成年蝙蝠体重更重。1994年，已知年龄的幼年蝙蝠体重低于成年蝙蝠，1995年，其翼载荷低于成年蝙蝠。1994年至1995年期间，已知年龄的幼年蝙蝠体重无差异，而1994年的成年蝙蝠体重更重。幼年和成年蝙蝠的觅食时间均与环境温度显著相关。然而，断奶幼年蝙蝠的这种关系斜率比成年蝙蝠更陡。一岁和成年蝙蝠的觅食时间无显著差异。幼年和成年蝙蝠的出飞时间无差异，一岁和成年蝙蝠的出飞时间也无差异。1994年和1995年的出飞时间无显著差异。研究结果表明，即使在允许成年蝙蝠积累更多脂肪储备的条件下，幼年蝙蝠仍维持低体重。幼年蝙蝠不会为弥补因开始飞行导致的觅食能力差和能量消耗增加而更早或更长时间觅食。这表明，通过维持低体重，幼年蝙蝠在飞行和觅食仍在发展的时期降低了飞行成本和被捕食的风险。
