In BriefUsing miniature tags, Egert-Berg et al. record bats' movement and social interactions. Whereas species foraging on ephemeral resources search in groups, switching foraging sites, species foraging on predictable resources search alone, returning to the same sites. The results suggest a connection between resource predictability and group foraging. SUMMARYObservations of animals feeding in aggregations are often interpreted as events of social foraging, but it can be difficult to determine whether the animals arrived at the foraging sites after collective search [1][2][3][4] or whether they found the sites by following a leader [5, 6] or even independently, aggregating as an artifact of food availability [7, 8]. Distinguishing between these explanations is important, because functionally, they might have very different consequences. In the first case, the animals could benefit from the presence of conspecifics, whereas in the second and third, they often suffer from increased competition [3,[9][10][11][12][13]. Using novel miniature sensors, we recorded GPS tracks and audio of five species of bats, monitoring their movement and interactions with conspecifics, which could be inferred from the audio recordings. We examined the hypothesis that food distribution plays a key role in determining social foraging patterns [14][15][16]. Specifically, this hypothesis predicts that searching for an ephemeral resource (whose distribution in time or space is hard to predict) is more likely to favor social foraging [10,[13][14][15] than searching for a predictable resource. The movement and social interactions differed between bats foraging on ephemeral versus predictable resources. Ephemeral species changed foraging sites and showed large temporal variation nightly. They aggregated with conspecifics as was supported by playback experiments and computer simulations. In contrast, predictable species were never observed near conspecifics and showed high spatial fidelity to the same foraging sites over multiple nights. Our results suggest that resource (un)predictability influences the costs and benefits of social foraging. RESULTS AND DISCUSSIONWe compared the movement and social foraging behavior of five bat species (representing four families), which cover a wide range of foraging styles and exploit different resources (see Table 1). Two species rely on ephemeral resources (henceforth the ''ephemeral foragers''): (1) the greater mouse-tailed bat (Rhinopoma microphyllum, Rhinopomatidae), an open-space insectivorous bat that preys on ephemeral insect swarms [17], and (2) the Mexican fish-eating bat (Myotis vivesi, Vespertilionidae), which forages primarily over marine waters [18, 19], where it feeds on local upwellings of fish and crustaceans [18, 19] whose exact location is difficult to predict on any given night. Indeed, our analysis of the spatial distribution of marine chlorophyll (a proxy of marine food availability [20, 21]) indicates low predictability of food spatial distribution over consecutive nights ( Figure...

Active sensing, where sensory acquisition is actively modulated, is an inherent component of almost all sensory systems. Echolocating bats are a prime example of active sensing. They can rapidly adjust many of their biosonar parameters to optimize sensory acquisition. They dynamically adjust pulse design, pulse duration, and pulse rate within dozens of milliseconds according to the sensory information that is required for the task that they are performing. The least studied and least understood degree of freedom in echolocation is emission beamforming-the ability to change the shape of the sonar sound beam in a functional way. Such an ability could have a great impact on the bat's control over its sensory perception. On the one hand, the bat could direct more energy into a narrow sector to zoom its biosonar field of view, and on the other hand, it could widen the beam to increase the space that it senses. We show that freely behaving bats constantly control their biosonar field of view in natural situations by rapidly adjusting their emitter aperture-the mouth gape. The bats dramatically narrowed the beam when entering a confined space, and they dramatically widened it within dozens of milliseconds when flying toward open space. Hence, mouth-emitting bats dynamically adjust their mouth gape to optimize the area that they sense with their echolocation system. bats | beamforming | echolocation | active sensing | sensory perception T he ability to actively adjust sensory acquisition is a key feature of almost all sensory systems. A capability to selectively control the sensory "field of view" could have a major impact on sensory perception. It would allow an animal to adjust the amount of acquired information in a task-dependent manner, zooming in on an object of interest and zooming out when a wider sector should be sensed. Many animals can shift their sensory attention (e.g., by changing gaze) or their focal plane (e.g., human vision), but there are no animals that are known to constantly adjust their sensory field of view under natural conditions. Echolocating bats perceive their environment acoustically by emitting ultrasonic pulses and analyzing the received echoes (1). The volume of space that is covered by the sound pulse and therefore, sensed by the bat depends on the emitted beamform-the spatial shape of the emission (2-9). Bats could potentially benefit greatly if they could change the form of their emitted beam in a functional manner, a property usually referred to in engineering as beamforming (10).Jakobsen and coworkers (11) recently summarized some of the reasons why a bat might narrow its biosonar beam. These reasons include (i) focusing sound into a narrower sector to improve the localization of objects, (ii) eliminating undesired echoes from the back or the sides of the bat, and (iii) increasing the sensing range by directing more energy forward. All of these come with a cost of reducing the volume of space that is scanned by the bat. It is, therefore, reasonable to expect that a bat would widen it...

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Summary Every evening, from late spring to mid-summer, tens of thousands of hungry lactating female lesser long-nosed bats ( Leptonycteris yerbabuenae ) emerge from their roost and navigate over the Sonoran Desert, seeking for nectar and pollen [ 1 , 2 ]. The bats roost in a huge maternal colony that is far from the foraging grounds but allows their pups to thermoregulate [ 3 ] while the mothers are foraging. Thus, the mothers have to fly tens of kilometers to the foraging sites—fields with thousands of Saguaro cacti [ 4 , 5 ]. Once at the field, they must compete with many other bats over the same flowering cacti. Several solutions have been suggested for this classical foraging task of exploiting a resource composed of many renewable food sources whose locations are fixed. Some animals randomly visit the food sources [ 6 ], and some actively defend a restricted foraging territory [ 7 , 8 , 9 , 10 , 11 ] or use simple forms of learning, such as “win-stay lose-switch” strategy [ 12 ]. Many species have been suggested to follow a trapline, that is, to revisit the food sources in a repeating ordered manner [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. We thus hypothesized that lesser long-nosed bats would visit cacti in a sequenced manner. Using miniature GPS devices, aerial imaging, and video recordings, we tracked the full movement of the bats and all of their visits to their natural food sources. Based on real data and evolutionary simulations, we argue that the bats use a reinforcement learning strategy that requires minimal memory to create small, non-overlapping cacti-cores and exploit nectar efficiently, without social communication.