Clutter resilience via auditory stream segregation in echolocating greater mouse-eared bats.

Bats use echolocation to navigate and hunt in darkness, and must in that process segregate target echoes from unwanted clutter echoes. Bats may do this by approaching a target at steep angles relative to the plane of the background, utilizing their directional transmission and receiving systems to minimize clutter from background objects, but it remains unknown how bats negotiate clutter that cannot be spatially avoided. Here, we tested the hypothesis that when movement no longer offers spatial release, echolocating bats mitigate clutter by calling at lower source levels and longer call intervals to ease auditory streaming. We trained five greater mouse-eared bats (Myotis myotis) to land on a spherical loudspeaker with two microphones attached. We used a phantom-echo setup, where the loudspeaker/target transmitted phantom clutter echoes by playing back the bats' own calls at time delays of 1, 3 and 5 ms with a virtual target strength 7 dB higher than the physical target. We show that the bats successfully landed on the target, irrespective of the clutter echo delays. Rather than decreasing their source levels, the bats used similar source level distributions in clutter and control trials. Similarly, the bats did not increase their call intervals, but instead used the same distribution of call intervals across control and clutter trials. These observations reject our hypothesis, leading us to conclude that bats display great resilience to clutter via short auditory integration times and acute auditory stream segregation rather than via biosonar adjustments.

Superfast Lombard response in free-flying, echolocating bats.

Acoustic cues are crucial to communication, navigation, and foraging in many animals, which hence face the problem of detecting and discriminating these cues in fluctuating noise levels from natural or anthropogenic sources. Such auditory dynamics are perhaps most extreme for echolocating bats that navigate and hunt prey on the wing in darkness by listening for weak echo returns from their powerful calls in complex, self-generated umwelts.1,2 Due to high absorption of ultrasound in air and fast flight speeds, bats operate with short prey detection ranges and dynamic sensory volumes,3 leading us to hypothesize that bats employ superfast vocal-motor adjustments to rapidly changing sensory scenes. To test this hypothesis, we investigated the onset and offset times and magnitude of the Lombard response in free-flying echolocating greater mouse-eared bats exposed to onsets of intense constant or duty-cycled masking noise during a landing task. We found that the bats invoked a bandwidth-dependent Lombard response of 0.1-0.2 dB per dB increase in noise, with very short delay and relapse times of 20 ms in response to onsets and termination of duty-cycled noise. In concert with the absence call time-locking to noise-free periods, these results show that free-flying bats exhibit a superfast, but hard-wired, vocal-motor response to increased noise levels. We posit that this reflex is mediated by simple closed-loop audio-motor feedback circuits that operate independently of wingbeat and respiration cycles to allow for rapid adjustments to the highly dynamic auditory scenes encountered by these small predators.

Echolocating bats prefer a high risk-high gain foraging strategy to increase prey profitability.

Predators that target multiple prey types are predicted to switch foraging modes according to prey profitability to increase energy returns in dynamic environments. Here, we use bat-borne tags and DNA metabarcoding of feces to test the hypothesis that greater mouse-eared bats make immediate foraging decisions based on prey profitability and changes in the environment. We show that these bats use two foraging strategies with similar average nightly captures of 25 small, aerial insects and 29 large, ground-dwelling insects per bat, but with much higher capture success in the air (76%) vs ground (30%). However, owing to the 3-20 times larger ground prey, 85% of the nightly food acquisition comes from ground prey despite the 2.5 times higher failure rates. We find that most bats use the same foraging strategy on a given night suggesting that bats adapt their hunting behavior to weather and ground conditions. We conclude that these bats use high risk-high gain gleaning of ground prey as a primary foraging tactic, but switch to aerial hunting when environmental changes reduce the profitability of ground prey, showing that prey switching matched to environmental dynamics plays a key role in covering the energy intake even in specialized predators.

Bats are the only mammals capable of powered flight and therefore need a high calorie intake to survive. They hunt at night using the echoes made by their own calls to navigate and locate prey. Bats can use different tactics to hunt for food: hawking involves catching prey on the wing and requires fast aerial manoeuvring and more intense echolocation calls, while gleaning involves listening for movements of ground and water dwelling prey as the bat hovers. Some bat species specialise as hawkers or gleaners but maintain the ability to hunt with both methods. With the ever-growing impact of human activities on their habitats, it is important to understand how adaptable bats feeding habits are to changes in their environment. To find out more, Stidsholt et al. studied greater mouse-eared bats, which primarily feed by gleaning. To understand how this species chooses feeding strategies they fitted bats with tiny backpacks that could record the animal's location and foraging behaviour. They could also monitor prey sizes by recording the sounds of the bats chewing. Stidsholt et al. found that, although these bats tried to catch prey on the ground more often than in the air, they were actually more successful as airborne hunters. Despite this, gleaning was still a more profitable strategy for them, because the body mass of ground prey is higher than for airborne prey. Gleaning gave the bats a higher calorie intake, even though their capture rate was lower. Although feeding habits differed slightly between individual bats on a given night of monitoring, there were much larger changes in behaviour between different feeding nights. This shows that, although this species of bat prefers gleaning, they will switch strategies to hawking as their environment changes, for example if there is more airborne prey, or if rainfall makes it hard to hear movements on the ground. Bats tended to get enough calories for their needs but did not catch more prey than they needed to survive. Stidsholt et al. concluded that greater mouse-eared bats change their feeding strategy based on prey availability and size, as well as the bat's environment. Their study provides an important insight into how bats fit into the ecosystem and how adaptable bats might be to changes in their habitat.