Hibernating female big brown bats (Eptesicus fuscus) adjust huddling and drinking behaviour, but not arousal frequency, in response to low humidity

Many mammals hibernate during winter, reducing energy expenditure via bouts of torpor. The majority of a hibernator's energy reserves are used to fuel brief, but costly, arousals from torpor. Although arousals likely serve multiple functions, an important one is to restore water stores depleted during torpor. Many hibernating bat species require high humidity, presumably to reduce torpid water loss, but big brown bats (Eptesicus fuscus) appear tolerant of a wide humidity range. We tested the hypothesis that hibernating female E. fuscus use behavioural flexibility during torpor and arousals to maintain water balance and reduce energy expenditure. We predicted: (1) E. fuscus hibernating in dry conditions would exhibit more compact huddles during torpor and drink more frequently than bats in high humidity conditions; and (2) frequency and duration of torpor bouts and arousals, and thus, total loss of body mass would not differ between bats in both environments. We housed hibernating E. fuscus in temperature- and humidity-controlled incubators at 50% or 98% relative humidity (8°C, 110 days). Bats in the dry environment maintained a more compact huddle during torpor and drank more frequently during arousals. Bats in both environments had a similar number of arousals, but arousal duration was shorter in the dry environment. However, total loss of body mass over hibernation did not differ between treatments indicating that both groups used similar amounts of energy. Our results suggest that behavioural flexibility allows hibernating E. fuscus to maintain water balance and reduce energy costs across a wide range of hibernation humidities.

多数哺乳动物会在冬季进入冬眠，通过周期性蛰伏（torpor）降低能量消耗。冬眠动物的大部分能量储备，均用于支撑从蛰伏中苏醒（arousal）这一短暂但高能耗的过程。尽管苏醒过程可能兼具多重功能，其中一项重要作用是恢复蛰伏期间消耗的体内水分储备。许多冬眠蝙蝠物种需要高湿度环境，推测此举旨在降低蛰伏过程中的水分流失，但大棕蝠（Eptesicus fuscus）似乎能耐受宽泛的湿度范围。本研究针对冬眠期的雌性大棕蝠展开实验，验证其可通过蛰伏与苏醒阶段的行为灵活性维持体内水分平衡并降低能量消耗这一假说。我们提出两项预测：（1）在低湿度环境中冬眠的大棕蝠，蛰伏期间会形成更紧凑的集群，且苏醒时饮水频率高于高湿度环境中的个体；（2）蛰伏周期与苏醒过程的频率、时长，以及最终的总体重损失量，在两种环境下的个体间不会存在差异。我们将冬眠中的大棕蝠饲养于温湿度可控的恒温培养箱中，设置50%与98%两种相对湿度（温度维持8℃，饲养时长110天）。实验结果显示，低湿度环境中的蝙蝠在蛰伏期间保持了更紧凑的集群，且苏醒时饮水频率更高。两种环境中的蝙蝠苏醒次数相近，但低湿度环境下的苏醒时长更短。不过冬眠期内的总体重损失量在两组间并无显著差异，表明两组个体的能量消耗总量相近。本研究结果表明，行为灵活性使得大棕蝠能够在宽泛的冬眠湿度范围内维持水分平衡，并降低能量消耗。