Predator's paradise

Description

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.

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