Predator's paradise

Large predators typically hunt relatively large prey with high failure rates and at high costs. In contrast, small predators generally hunt relatively small, abundant prey at lower failure rates, incurring relatively lower costs. Bats are small predators with high metabolic rates during flight and most species hunt large numbers of abundant, small insects. However, nine different bat species have evolved true carnivory, targeting large, less abundant vertebrate prey. This rare hunting style among bats raises the fundamental conundrum of how these very small predators with limited energy stores and high absolute energy requirements can successfully employ a hunting strategy that usually involves prolonged searching and high-energy chasing at low success rates. Here, we address this question by quantifying the hunting behavior of 20 wild carnivorous fringe-lipped bats (Trachops cirrhosus) with sound and movement biologging tags. We equipped 20 Trachops cirrhosus bats (8 females, 12 males) with sound- and movement- tags in March 2023 and in October 2023. The tags used in this study recorded continuous data for ~5.5 hrs during one night of foraging, amounting to the first half of the bats’ nightly activity periods. Recordings generally started with the bats’ activity period, either at emergence time (via a timer) when bats had been captured and tagged in their day roosts (9 bats) or at time of release, when bats had been captured and tagged during emergence (11 bats), therefore all recording intervals covered the onset of the foraging period. Audio data was recorded with an ultrasonic microphone (FG-3329; Knowles Electronics, Itaska, IL, USA) at a sampling rate of 187.5 kHz, with 16-bit resolution, a 10 kHz 1-pole analog high-pass filter, and a clipping level of 121 dB re 20 μPa pk. Movement data was recorded with triaxial accelerometers sampling at 1000 Hz with a clipping level of 8 g. All accelerometer data was calibrated, converted into acceleration units (m/s2), and decimated to 100 Hz. The orientation of the bats was recorded with triaxial magnetometers sampling at 50 Hz. Long mastication times of up to 84 minutes after a kill show that these bats can target prey as large as their own body size. Unlike much larger predators, they do so with extreme efficiency: the bats spent only a fraction of the night in flight (median 11%, quartiles 7-14%) and complemented short search and pursuit times with high rates of hunting success (50%, 35-62%), capitalizing on a combination of eavesdropping on prey sounds and hang-and-wait ambush predation. The unusual low-risk/high-gain strategy of these predatory bats critically relies on high prey densities in pristine ecosystems. The dwindling sizes of intact ecosystems and dramatic decline in biodiversity raise great concern for the Anthropocene fate of these specialized predators.大型捕食者通常以大型猎物为捕食目标，其捕猎成功率较低且能量成本高昂。与之相对，小型捕食者多以小型且种群丰富的猎物为食，捕猎成功率更高，能量成本也相对更低。蝙蝠作为小型捕食者，飞行过程中代谢率极高，多数蝙蝠物种会捕食大量小型且种群丰富的昆虫。然而，已有9种蝙蝠演化出了真正的肉食性习性，以体型更大、种群相对稀少的脊椎动物为捕食目标。这种在蝙蝠中极为罕见的捕猎模式，引出了一个核心科学难题：这类能量储备有限、绝对能量需求极高的小型捕食者，如何能够成功采用通常需要长时间搜寻、高能耗追逐且成功率低下的捕猎策略？本研究通过搭载声音与运动生物记录标签（biologging tags），对20只野生肉食性流苏叶鼻蝠（Trachops cirrhosus）的捕猎行为进行量化分析，以此解答该问题。本研究分别于2023年3月与2023年10月，为20只流苏叶鼻蝠（8雌12雄）佩戴了声音与运动记录标签。本次研究使用的标签可在一夜的觅食过程中连续记录约5.5小时的数据，覆盖蝙蝠夜间活动时长的前半段。录音通常始于蝙蝠的活动时段：其中9只蝙蝠在日间栖息处被捕获并佩戴标签后，通过计时器在其出巢时刻启动记录；剩余11只则在出巢过程中被捕获并佩戴标签，于释放时启动记录。因此所有记录时段均覆盖了觅食活动的起始阶段。音频数据通过超声波麦克风（FG-3329; Knowles Electronics, Itaska, IL, USA）采集，采样率为187.5 kHz，采样精度为16位，搭载10 kHz单极模拟高通滤波器，削波电平为121 dB re 20 μPa峰峰值。运动数据通过三轴加速度计采集，采样率为1000 Hz，削波电平为8 g。所有加速度计数据均经过校准，转换为加速度单位（m/s²）后，被降采样至100 Hz。蝙蝠的身体方位通过三轴磁强计采集，采样率为50 Hz。 捕食后最长可达84分钟的长时间咀嚼行为表明，这类蝙蝠能够捕食与自身体型相当的猎物。与体型更大的捕食者不同，这类蝙蝠的捕猎效率极高：它们夜间飞行时长仅占总活动时长的一小部分（中位数为11%，四分位距为7%-14%），并通过监听猎物声音与静候伏击相结合的策略，辅以短暂的搜寻与追逐行为，实现了较高的捕猎成功率（中位数为50%，四分位距为35%-62%）。这类捕食性蝙蝠所采用的这种非典型的低风险高回报策略，严格依赖于原始生态系统中较高的猎物种群密度。完整生态系统的不断缩减与生物多样性的急剧下降，引发了人们对这类特化捕食者在人类世（Anthropocene）中的生存前景的深切担忧。