It's not black or white-on the range of vision and echolocation in echolocating bats

Daubenton's bats maintain stereotypical echolocation behaviour and a lombard response during target interception in light

Most bats hunt insects on the wing at night using echolocation as their primary sensory modality, but nevertheless maintain complex eye anatomy and functional vision. This raises the question of how and when insectivorous bats use vision during their largely nocturnal lifestyle. Here, we test the hypothesis that the small insectivorous bat, Myotis daubentonii, relies less on echolocation, or dispenses with it entirely, as visual cues become available during challenging acoustic noise conditions. We trained five wild-caught bats to land on a spherical target in both silence and when exposed to broad-band noise to decrease echo detectability, while light conditions were manipulated in both spectrum and intensity. We show that during noise exposure, the bats were almost three times more likely to use multiple attempts to solve the task compared to in silent controls. Furthermore, the bats exhibited a Lombard response of 0.18 dB/dBnoise and decreased call intervals earlier in their flight during masking noise exposures compared to in silent controls. Importantly, however, these adjustments in movement and echolocation behaviour did not differ between light and dark control treatments showing that small insectivorous bats maintain the same echolocation behaviour when provided with visual cues under challenging conditions for echolocation. We therefore conclude that bat echolocation is a hard-wired sensory system with stereotyped compensation strategies to both target range and masking noise (i.e. Lombard response) irrespective of light conditions. In contrast, the adjustments of call intervals and movement strategies during noise exposure varied substantially between individuals indicating a degree of flexibility that likely requires higher order processing and perhaps vocal learning.

A perspective on neuroethology: what the past teaches us about the future of neuroethology

For 100 years, the Journal of Comparative Physiology-A has significantly supported research in the field of neuroethology. The celebration of the journal's centennial is a great time point to appreciate the recent progress in neuroethology and to discuss possible avenues of the field. Animal behavior is the main source of inspiration for neuroethologists. This is illustrated by the huge diversity of investigated behaviors and species. To explain behavior at a mechanistic level, neuroethologists combine neuroscientific approaches with sophisticated behavioral analysis. The rapid technological progress in neuroscience makes neuroethology a highly dynamic and exciting field of research. To summarize the recent scientific progress in neuroethology, I went through all abstracts of the last six International Congresses for Neuroethology (ICNs 2010-2022) and categorized them based on the sensory modalities, experimental model species, and research topics. This highlights the diversity of neuroethology and gives us a perspective on the field's scientific future. At the end, I highlight three research topics that may, among others, influence the future of neuroethology. I hope that sharing my roots may inspire other scientists to follow neuroethological approaches.

What determines the information update rate in echolocating bats

The rate of sensory update is one of the most important parameters of any sensory system. The acquisition rate of most sensory systems is fixed and has been optimized by evolution to the needs of the animal. Echolocating bats have the ability to adjust their sensory update rate which is determined by the intervals between emissions - the inter-pulse intervals (IPI). The IPI is routinely adjusted, but the exact factors driving its regulation are unknown. We use on-board audio recordings to determine how four species of echolocating bats with different foraging strategies regulate their sensory update rate during commute flights. We reveal strong correlations between the IPI and various echolocation and movement parameters. Specifically, the update rate increases when the signals' peak-energy frequency and intensity increases while the update rate decreases when flight speed and altitude increases. We suggest that bats control their information update rate according to the behavioral mode they are engaged in, while always maintaining sensory continuity. Specifically, we suggest that bats apply two modes of attention during commute flights. Our data moreover suggests that bats emit echolocation signals at accurate intervals without the need for external feedback.

Calibrated microphone array recordings reveal that a gleaning bat emits low-intensity echolocation calls even in open-space habitat

Echolocating bats use ultrasound for orientation and prey capture in darkness. Ultrasound is strongly attenuated in air. Consequently, aerial-hawking bats generally emit very intense echolocation calls to maximize detection range. However, call levels vary more than tenfold (>20 dB) between species and are tightly linked to the foraging strategy. The brown long-eared bat (Plecotus auritus) is a primarily gleaning, low-amplitude species that may occasionally hawk airborne prey. We used state-of-the-art calibrated acoustic 3D-localization and automated call analysis to measure P. auritus' source levels. Plecotus auritus emits echolocation calls of low amplitude (92 dB rmsSPL re. 20 µPa at 10 cm) even while flying in open-space. While P. auritus thus probably benefits from delayed evasive manoeuvres of eared insects, we propose that low-amplitude echolocation did not evolve as an adaptive countermeasure, but is limited by morphological constraints.

Divergence of cochlear transcriptomics between reference‑based and reference‑free transcriptome analyses among Rhinolophus ferrumequinum populations

Differences in gene expression within tissues can lead to differences in tissue function. Understanding the transcriptome of a species helps elucidate the molecular mechanisms underlying phenotypic divergence. According to the presence or absence of a reference genome of for a studied species, transcriptome analyses can be divided into reference‑based and reference‑free methods, respectively. Presently, comparisons of complete transcriptome analysis results between those two methods are still rare. In this study, we compared the cochlear transcriptome analysis results of greater horseshoe bats (Rhinolophus ferrumequinum) from three lineages in China with different acoustic phenotypes using reference‑based and reference‑free methods to explore their differences in subsequent analysis. The results gained by reference-based results had lower false-positive rates and were more accurate because differentially expressed genes among the three populations obtained by this method had greater reliability and a higher annotation rate. Some phenotype-related enrichment terms, including those related to inorganic molecules and proton transmembrane channels, were also obtained only by the reference-based method. However, the reference‑based method might have the limitation of incomplete information acquisition. Thus, we believe that a combination of reference‑free and reference‑based methods is ideal for transcriptome analyses. The results of our study provided a reference for the selection of transcriptome analysis methods in the future.

Insectivorous bats form mobile sensory networks to optimize prey localization: The case of the common noctule bat

Animals that depend on ephemeral, patchily distributed prey often use public information to locate resource patches. The use of public information can lead to the aggregation of foragers at prey patches, a mechanism known as local enhancement. However, when ephemeral resources are distributed over large areas, foragers may also need to increase search efficiency, and thus apply social strategies when sampling the landscape. While sensory networks of visually oriented animals have already been confirmed, we lack an understanding of how acoustic eavesdropping adds to the formation of sensory networks. Here we radio-tracked a total of 81 aerial-hawking bats at very high spatiotemporal resolution during five sessions over 3 y, recording up to 19 individuals simultaneously. Analyses of interactive flight behavior provide conclusive evidence that bats form temporary mobile sensory networks by adjusting their movements to neighboring conspecifics while probing the airspace for prey. Complementary agent-based simulations confirmed that the observed movement patterns can lead to the formation of mobile sensory networks, and that bats located prey faster when networking than when relying only on local enhancement or searching solitarily. However, the benefit of networking diminished with decreasing group size. The combination of empirical analyses and simulations elucidates how animal groups use acoustic information to efficiently locate unpredictable and ephemeral food patches. Our results highlight that declining local populations of social foragers may thus suffer from Allee effects that increase the risk of collapses under global change scenarios, like insect decline and habitat degradation.

TrackUSF, a novel tool for automated ultrasonic vocalization analysis, reveals modified calls in a rat model of autism

Background

Various mammalian species emit ultrasonic vocalizations (USVs), which reflect their emotional state and mediate social interactions. USVs are usually analyzed by manual or semi-automated methodologies that categorize discrete USVs according to their structure in the frequency-time domains. This laborious analysis hinders the effective use of USVs as a readout for high-throughput analysis of behavioral changes in animals.

Results

Here we present a novel automated open-source tool that utilizes a different approach towards USV analysis, termed TrackUSF. To validate TrackUSF, we analyzed calls from different animal species, namely mice, rats, and bats, recorded in various settings and compared the results with a manual analysis by a trained observer. We found that TrackUSF detected the majority of USVs, with less than 1% of false-positive detections. We then employed TrackUSF to analyze social vocalizations in Shank3-deficient rats, a rat model of autism, and revealed that these vocalizations exhibit a spectrum of deviations from appetitive calls towards aversive calls.

Conclusions

TrackUSF is a simple and easy-to-use system that may be used for a high-throughput comparison of ultrasonic vocalizations between groups of animals of any kind in any setting, with no prior assumptions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01299-y.

Neural Processing of Naturalistic Echolocation Signals in Bats

Echolocation behavior, a navigation strategy based on acoustic signals, allows scientists to explore neural processing of behaviorally relevant stimuli. For the purpose of orientation, bats broadcast echolocation calls and extract spatial information from the echoes. Because bats control call emission and thus the availability of spatial information, the behavioral relevance of these signals is undiscussable. While most neurophysiological studies, conducted in the past, used synthesized acoustic stimuli that mimic portions of the echolocation signals, recent progress has been made to understand how naturalistic echolocation signals are encoded in the bat brain. Here, we review how does stimulus history affect neural processing, how spatial information from multiple objects and how echolocation signals embedded in a naturalistic, noisy environment are processed in the bat brain. We end our review by discussing the huge potential that state-of-the-art recording techniques provide to gain a more complete picture on the neuroethology of echolocation behavior.

Bats: Vision or echolocation, why not both?

Echolocation allows bats to navigate in complete darkness. Yet, some bats also have keen eyesight. A new study shows that bats integrate these sensory modalities, even when light abundance would make it possible for them to rely solely on vision.

Functional daylight echolocation in highly visual bats

Bats rely on echolocation for operating in dim light or dark conditions. Accordingly, most research on echolocation is performed under dark conditions with a few exceptions. Bat species that emerge to forage before sunset have been shown to use echolocation even in relatively high light levels^1-3. It has been argued that for insectivorous bats, as light levels decrease, echolocation rapidly becomes advantageous over vision for detecting tiny insects during dusk or dawn² and that information from the two sensory modalities is integrated^4,5. Functional use of echolocation in broad daylight in insectivorous bats has been scarcely reported^6,7. Here, we report functional use of echolocation in broad daylight in highly visual fruit bats.

Influencing Activity of Bats by Dimly Lighting Wind Turbine Surfaces with Ultraviolet Light

Simple Summary

Bats often fly near wind turbines. The fatalities associated with this behavior continue to be an issue for wind energy development and wildlife conservation. We tested an experimental method intended to reduce bat fatalities at the wind turbines. We assumed that bats navigate over long distances at night by dim-light vision and might be dissuaded from approaching artificially lit structures. For over a year, we experimentally lit wind turbines at night with dim, flickering ultraviolet (UV) light while measuring the presence and activity of bats, birds, and insects with thermal-imaging cameras. We detected no statistical differences in the activity of the bats, insects, or birds at a test turbine when lit with UV light compared with that of unlit nights. Additional experiments to test this or other possible bat-deterrence methods may benefit from considering subtle measures of animal response that can provide useful information on the possible behavioral effects of fatality-reduction experiments.

Abstract

Wind energy producers need deployable devices for wind turbines that prevent bat fatalities. Based on the speculation that bats approach turbines after visually mistaking them for trees, we tested a potential light-based deterrence method. It is likely that the affected bats see ultraviolet (UV) light at low intensities. Here, we present the results of a multi-month experiment to cast dim, flickering UV light across wind turbine surfaces at night. Our objectives were to refine and test a practical system for dimly UV-illuminating turbines while testing whether the experimental UV treatment influenced the activity of bats, birds, and insects. We mounted upward-facing UV light arrays on turbines and used thermal-imaging cameras to quantify the presence and activity of night-flying animals. The results demonstrated that the turbines can be lit to the highest reaches of the blades with “invisible” UV light, and the animal responses to such experimental treatment can be concurrently monitored. The UV treatment did not significantly change nighttime bat, insect, or bird activity at the wind turbine. Our findings show how observing flying animals with thermal cameras at night can help test emerging technologies intended to variably affect their behaviors around wind turbines.

Silence and reduced echolocation during flight are associated with social behaviors in male hoary bats (Lasiurus cinereus)

Bats are renowned for their sophisticated echolocation. However, recent research has indicated that bats may be less reliant on echolocation than has long been assumed. To test the hypothesis that bats reduce their use of echolocation to avoid eavesdropping by conspecifics, we deployed miniature tags that recorded ultrasound and accelerations on 10 wild hoary bats (Lasiurus cinereus) for one or two nights. This resulted in 997 10-s recordings. Bats switched between periods predominated by their typical high-intensity echolocation, or periods predominated by micro calls (unusually short, quiet calls), or no detectable calls (“silence”). Periods of high-intensity echolocation included high rates of feeding buzzes, whereas periods of micro calls and silence included high rates of social interactions with other bats. Bats switched back to high-intensity echolocation during actual social interactions. These data support the hypothesis that bats use reduced forms of echolocation and fly in silence to avoid eavesdropping from conspecifics, perhaps in the context of mating-related behavior. They also provide the strongest demonstration to date that bats fly for extended periods of time without the use of echolocation.

Multimodal cues facilitate ripe-fruit localization and extraction in free-ranging pteropodid bats

Sensory cues play an important role in any plant-animal interaction. Yet, we know very little about the cues used by wild mammals during fruit selection. Existing evidence mainly comes from captive studies and suggests that the pteropodid bats rely on olfaction to find fruits. In this study, we avoided captivity-generated stressors and provide insights from natural selective forces by performing manipulative experiments on free-ranging fruit bats (Cynopterus sphinx) in a wild setting, in a tree species that exhibits a bat-fruit syndrome (Madhuca longifolia var. latifolia). We find that visual cues are necessary and sufficient to locate ripe fruits. Fruit experiments exhibiting visual cues alone received more bat visits than those exhibiting other combinations of visual and olfactory cues. Ripe fruit extractions were higher by bats that evaluated fruits by perching than hovering, indicating an additional cue, i.e., haptic cue. Visual cues appear to be informative over short distances, whereas olfactory and haptic cues facilitate the fruit evaluation for those bats that used hovering and perching strategies, respectively. This study also shows that adult bats were more skillful in extracting ripe fruits than the young bats, and there was a positive correlation between the weight of selected fruits and bat weight. This study suggests that the integration of multimodal cues (visual, olfactory and haptic) facilitate ripe-fruit localization and extraction in free-ranging pteropodid bats.

Floral UV Features of Plant Species From a Neotropical Savanna

Despite the wide interest in flower colours, only after the end of the nineteenth-century studies started to comprise floral UV reflection, which is invisible to humans but visible to the major groups of pollinators. Many flowers and inflorescences display colour patterns, an important signal for pollinators, promoted by the presence of at least two different colours within flowers or inflorescences, including colours in the UV waveband. For Neotropical savanna plant species, we characterised floral UV features using UV-photography and reflectance measurements. We tested (i) whether floral UV features were constrained by their shared ancestry, (ii) whether floral UV features were associated with pollinators, and (iii) whether floral UV features were associated with floral traits mediating these interactions, including floral resource, type of attraction unit and presence/absence of non-UV colour patterns. Of 80 plant species, ca. 70% were UV-patternless, most of them UV-absorbing. Approximately 30% presented one of three types of UV-patterns: bullseye, contrasting corolla markings oriented toward floral resources or contrasting reproductive structures, which were all considered as floral guides. Floral UV features were phylogenetically constrained and were associated with pollinators, floral resources and attraction unit, but not with non-UV colour patterns. UV-patternless flowers were associated with most of the pollination systems, while UV-patterned flowers were mainly associated with bee-pollination. UV-absorbing flowers comprised the only category with hawkmoth- and butterfly-pollinated flowers, and a high percentage of hummingbird-pollinated species. Nocturnal pollinated species were also commonly UV-absorbing, except for one UV-reflecting bat-pollinated species and one beetle-pollinated species with UV-reflecting stigmas. All types of floral UV features were associated with nectar; however, flowers with contrasting reproductive structures were mainly associated with pollen. There was an association between UV-absorbing species and the presence of inflorescences and intermediate attraction units. Our results evince that phylogenetic relatedness can constraint floral UV features' diversification, but combinations of evolutionary and ecological processes may be expected in this scenario.

Visual cues enhance obstacle avoidance in echolocating bats

Studies have shown that bats are capable of using visual information for a variety of purposes, including navigation and foraging, but the relative contributions of visual and auditory modalities in obstacle avoidance has yet to be fully investigated, particularly in laryngeal echolocating bats. A first step requires the characterization of behavioral responses to different combinations of sensory cues. Here, we quantified the behavioral responses of the insectivorous big brown bat, Eptesicus fuscus, in an obstacle avoidance task offering different combinations of auditory and visual cues. To do so, we utilized a new method that eliminates the confounds typically associated with testing bat vision and precludes auditory cues. We found that the presence of visual and auditory cues together enhances bats' avoidance response to obstacles compared with cues requiring either vision or audition alone. Analyses of flight and echolocation behaviors, such as speed and call rate, did not vary significantly under different obstacle conditions, and thus are not informative indicators of a bat's response to obstacle stimulus type. These findings advance the understanding of the relative importance of visual and auditory sensory modalities in guiding obstacle avoidance behaviors.

Fireflies produce ultrasonic clicks during flight as a potential aposematic anti-bat signal

Fireflies are known for emitting light signals for intraspecific communication. However, in doing so, they reveal themselves to many potential nocturnal predators from a large distance. Therefore, many fireflies evolved unpalatable compounds and probably use their light signals as anti-predator aposematic signals. Fireflies are occasionally attacked by predators despite their warning flashes. Bats are among the most substantial potential firefly predators. Using their echolocation, bats might detect a firefly from a short distance and attack it in between two flashes. We thus aimed to examine whether fireflies use additional measures of warning, specifically focusing on sound signals. We recorded four species from different genera of fireflies in Vietnam and Israel and found that all of them generated ultrasonic clicks centered around bats' hearing range. Clicks were synchronized with the wingbeat and are probably produced by the wings. We hypothesize that ultrasonic clicks can serve as part of a multimodal aposematic display.

The masked seducers: Lek courtship behavior in the wrinkle-faced bat Centurio senex (Phyllostomidae)

Centurio senex is an iconic bat characterized by a facial morphology deviating far from all other New World Leaf Nosed Bats (Phyllostomidae). The species has a bizarrely wrinkled face and lacks the characteristic nose leaf. Throughout its distribution from Mexico to Northern South America the species is most of the time rarely captured and only scarce information on its behavior and natural history is available. Centurio senex is frugivorous and one of the few bats documented to consume also hard seeds. Interestingly, the species shows a distinct sexual dimorphism: Adult males have more pronounced facial wrinkles than females and a fold of skin under the chin that can be raised in style of a face mask. We report the first observations on echolocation and mating behavior of Centurio senex, including synchronized audio and video recordings from an aggregation of males in Costa Rica. Over a period of 6 weeks we located a total of 53 perches, where during the first half of the night males were hanging with raised facial masks at a mean height of 2.35 m. Most of the time, the males moved just their wing tips, and spontaneously vocalized in the ultrasound range. Approaches of other individuals resulted in the perching male beating its wings and emitting a very loud, low frequency whistling call. Following such an encounter we recorded a copulation event. The observed aggregation of adult C. senex males is consistent with lek courtship, a behavior described from only few other bat species.

Echolocating bats detect but misperceive a multidimensional incongruent acoustic stimulus

Coherent perception relies on integrating multiple dimensions of a sensory modality, for example, color and shape in vision. We reveal how different acoustic dimensions, specifically echo intensity and sonar aperture (or width), are important for correct perception by echolocating bats. We flew bats down a corridor blocked by objects with different intensity-aperture combinations. To our surprise, bats crashed straight into large (aperture) walls with weak echo intensity as if they did not exist. The echolocation behavior of the bats indicated that they did detect the wall, suggesting that crashing was not a result of limited sensory sensitivity, but of a perceptual deficit. We systematically manipulated intensity and aperture by changing the materials and width of different reflectors, and we conclude that a coherent echo-based percept is created only when these two acoustic dimensions have certain relations which are typical for objects in nature (e.g., large and intense or small and weak reflectors). Nevertheless, we show that these preferred relations are not innate. We show that young pups are not constrained to these relations and that new intensity-aperture associations can also be learned by adult bats.

Reinforcement Learning Enables Resource Partitioning in Foraging Bats

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 [7-11] 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 [13-22]. 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.

Retinal characterization in the eyes of two bats endemic in the Egyptian fauna, the Egyptian fruit bat (Rousettus aegyptiacus) and insectivorous bat (Pipistrellus kuhlii), using the light microscope and transmission electron microscope

Bats are the only mammals that can fly in the dark without eye usage. This study was conducted to describe the structural and functional adaptations of the retina of two bats very common in the Egyptian fauna having a different lifestyle: the Egyptian fruit bat (Rousettus aegyptiacus) and insectivorous bat (Pipistrellus kuhlii). Seven eyes were collected from adult individuals of each species. Examination of the retina using a light microscope and a transmission electron microscope was carried out. The retina of P. kuhlii was thicker than that of R. aegyptiacus, which had many projections extended from the choroid layer into retina forming papillae. Despite rods being dominant in retinae of both species, cone photoreceptors were encountered in both retinae. The outer plexiform layer of R. aegyptiacus was arranged into islets between the outer nuclear layer produced differences in its thickness. However, the retina of P. kuhlii showed a normal arrangement of retinal structure. The retinal pigment epithelium of both bat species consists of a single layer of the cuboidal cells with a round to oval vesicular nuclei, which showed a lack of pigmentation in R. aegyptiacus and poor pigmentation in the P. kuhlii. In conclusion, our investigation detected many structural and ultrastructural differences between the two bat species. The presence of many projections protruded from the choroid layer of R. aegyptiacus retina is considered the most characteristic difference between the retinae of R. aegyptiacus and P. kuhlii.

A sensorimotor model shows why a spectral jamming avoidance response does not help bats deal with jamming

For decades, researchers have speculated how echolocating bats deal with masking by conspecific calls when flying in aggregations. To date, only a few attempts have been made to mathematically quantify the probability of jamming, or its effects. We developed a comprehensive sensorimotor predator-prey simulation, modeling numerous bats foraging in proximity. We used this model to examine the effectiveness of a spectral Jamming Avoidance Response (JAR) as a solution for the masking problem. We found that foraging performance deteriorates when bats forage near conspecifics, however, applying a JAR does not improve insect sensing or capture. Because bats constantly adjust their echolocation to the performed task (even when flying alone), further shifting the signals' frequencies does not mitigate jamming. Our simulations explain how bats can hunt successfully in a group despite competition and despite potential masking. This research demonstrates the advantages of a modeling approach when examining a complex biological system.

The ontogeny of a mammalian cognitive map in the real world

How animals navigate over large-scale environments remains a riddle. Specifically, it is debated whether animals have cognitive maps. The hallmark of map-based navigation is the ability to perform shortcuts, i.e., to move in direct but novel routes. When tracking an animal in the wild, it is extremely difficult to determine whether a movement is truly novel because the animal's past movement is unknown. We overcame this difficulty by continuously tracking wild fruit bat pups from their very first flight outdoors and over the first months of their lives. Bats performed truly original shortcuts, supporting the hypothesis that they can perform large-scale map-based navigation. We documented how young pups developed their visual-based map, exemplifying the importance of exploration and demonstrating interindividual differences.

Hematologic Values of Jamaican Fruit Bats (Artibeus jamaicensis) and the Effects of Isoflurane Anesthesia

Jamaican fruit bats (Artibeus jamaicensis) are used as an animal model for several viruses, including Middle East respiratory syndrome virus, dengue virus, Zika virus, and Tacaribe virus. However, despite ongoing studies regarding these pathogens, little is known regarding the bats' normal physiology. In this study, phlebotomy of the propetagial (cephalic) vein was performed to establish baseline hematologic parameters in an apparently healthy, captive population of Jamaican fruit bats. Furthermore, we compared results from physically restrained and isoflurane-anesthetized bats. Our findings indicate significant increases in WBC count, lymphocytes, and monocytes in the anesthetized bats. However, RBC and platelet parameters were not different between the 2 groups. This information on the normal hematologic parameters of Jamaican fruit bats, adds to our overall understanding of the normal physiology of this species, and expands our knowledge on bat species in general.

Decision making in foraging bats

Foraging is a complex and cognitively demanding behavior. Although it is often regarded as a mundane task, foraging requires the continuous weighting and integration of many sources of information with varying levels of credence. Bats are extremely diverse in their ecology and behavior, and thus demonstrate a wide variety of foraging strategies. In this review, we examine the different factors influencing the decision process of bats during foraging. Technological developments of recent years will soon enable real-time tracking of environmental conditions, of the position and quality of food items, the location of conspecifics, and the bat's movement history. Monitoring these variables alongside the continuous movement of the bat will facilitate the testing of different decision-making theories such as the use of reinforcement learning in wild free ranging bats and other animals.

The benefits of insect-swarm hunting to echolocating bats, and its influence on the evolution of bat echolocation signals

Predation on swarms of prey, especially using visual information, has drawn much interest in studies of collective movement. Surprisingly, in the field of biosonar this aspect of prey detection, which is probably very common, has received little to no attention. Here, we combine computer simulations and actual echo measurements to accurately estimate the echo sound pressure of insect swarms of different size and density. We show that swarm echo sound pressure increases with 3dB for every doubling of insect number, irrespective of swarm density. Thus swarms will be much easier to detect than single insects. Many of the insects bats eat are so small that they are only detectable by echolocation at very short distances. By focusing on detection of swarms of insects, a bat may increase its operating range and diversify its diet. Interestingly, interference between the sound waves reflected from a swarm of insects can sometimes result in echoes that are much weaker than echoes from single insects. We show that bats can reduce this problem by increasing the bandwidth of their echolocation calls. Specifically, a bandwidth of 3–8 kHz would guarantee receiving loud echoes from any angle relative to the swarm. Indeed, many bat species, and specifically bats hunting in open spaces, where swarms are abundant, use echolocation signals with a bandwidth of several kHz. Our results might also explain how the first echolocating bats that probably had limited echolocation abilities, could detect insects through swarm hunting.

When bats hunt, they often encounter insects that fly in swarms. Echolocating bats emit sonar signals to search for prey and it is currently unknown what such swarms look like to a bat. Unlike vision, sonar senses the delay or distance to objects directly. We show that when bats hunt for insects in the sky, the echoes from the insects in a swarm will most of the time sum up and therefore become much louder than the echo of a single insect. Every time an insect swarm would double in number, a bat would hear an echo that is 3dB stronger. This could enable a bat to detect prey from longer distances and some bats might thus profit from swarm hunting. However, the echoes reflected from the many insects in the swarm also create acoustic interference so that sometimes the summed echo is actually weak at a certain frequency. We show how bats could deal with this drawback. It is known that most bats do not use sonar signals with a single tone but that they modulate their tones. Our analysis shows that this modulation can solve the problem of spectral interference ensuring that the swarm-echo is always loud.

M, Mpodozis J. Anatomical Specializations Related to Foraging in the Visual System of a Nocturnal Insectivorous Bird, the Band-Winged Nightjar (Aves: Caprimulgiformes)

Nocturnal animals that rely on their visual system for foraging, mating, and navigation usually exhibit specific traits associated with living in scotopic conditions. Most nocturnal birds have several visual specializations, such as enlarged eyes and an increased orbital convergence. However, the actual role of binocular vision in nocturnal foraging is still debated. Nightjars (Aves: Caprimulgidae) are predators that actively pursue and capture flying insects in crepuscular and nocturnal environments, mainly using a conspicuous “sit-and-wait” tactic on which pursuit begins with an insect flying over the bird that sits on the ground. In this study, we describe the visual system of the band-winged nightjar (Systellura longirostris), with emphasis on anatomical features previously described as relevant for nocturnal birds. Orbit convergence, determined by 3D scanning of the skull, was 73.28°. The visual field, determined by ophthalmoscopic reflex, exhibits an area of maximum binocular overlap of 42°, and it is dorsally oriented. The eyes showed a nocturnal-like normalized corneal aperture/axial length index. Retinal ganglion cells (RGCs) were relatively scant, and distributed in an unusual oblique-band pattern, with higher concentrations in the ventrotemporal quadrant. Together, these results indicate that the band-winged nightjar exhibits a retinal specialization associated with the binocular area of their dorsal visual field, a relevant area for pursuit triggering and prey attacks. The RGC distribution observed is unusual among birds, but similar to that of some visually dependent insectivorous bats, suggesting that those features might be convergent in relation to feeding strategies.

The Neural Basis of Dim-Light Vision in Echolocating Bats

Echolocating bats evolved a sophisticated biosonar imaging system that allows for a life in dim-light habitats. However, especially for far-range operations such as homing, bats can support biosonar by vision. Large eyes and a retina that mainly consists of rods are assumed to be the optical adjustments that enable bats to use visual information at low light levels. In addition to optical mechanisms, many nocturnal animals evolved neural adaptations such as elongated integration times or enlarged spatial sampling areas to further increase the sensitivity of their visual system by temporal or spatial summation of visual information. The neural mechanisms that underlie the visual capabilities of echolocating bats have, however, so far not been investigated. To shed light on spatial and temporal response characteristics of visual neurons in an echolocating bat, Phyllostomus discolor, we recorded extracellular multiunit activity in the retino-recipient superficial layers of the superior colliculus (SC). We discovered that response latencies of these neurons were generally in the mammalian range, whereas neural spatial sampling areas were unusually large compared to those measured in the SC of other mammals. From this we suggest that echolocating bats likely use spatial but not temporal summation of visual input to improve visual performance under dim-light conditions. Furthermore, we hypothesize that bats compensate for the loss of visual spatial precision, which is a byproduct of spatial summation, by integration of spatial information provided by both the visual and the biosonar systems. Given that knowledge about neural adaptations to dim-light vision is mainly based on studies done in non-mammalian species, our novel data provide a valuable contribution to the field and demonstrate the suitability of echolocating bats as a nocturnal animal model to study the neurophysiological aspects of dim-light vision.

Functional Shifts in Bat Dim-Light Visual Pigment Are Associated with Differing Echolocation Abilities and Reveal Molecular Adaptation to Photic-Limited Environments

Bats are excellent models for studying the molecular basis of sensory adaptation. In Chiroptera, a sensory trade-off has been proposed between the visual and auditory systems, though the extent of this association has yet to be fully examined. To investigate whether variation in visual performance is associated with echolocation, we experimentally assayed the dim-light visual pigment rhodopsin from bat species with differing echolocation abilities. While spectral tuning properties were similar among bats, we found that the rate of decay of their light-activated state was significantly slower in a nonecholocating bat relative to species that use distinct echolocation strategies, consistent with a sensory trade-off hypothesis. We also found that these rates of decay were remarkably slower compared with those of other mammals, likely indicating an adaptation to dim light. To examine whether functional changes in rhodopsin are associated with shifts in selection intensity upon bat Rh1 sequences, we implemented selection analyses using codon-based likelihood clade models. While no shifts in selection were identified in response to diverse echolocation abilities of bats, we detected a significant increase in the intensity of evolutionary constraint accompanying the diversification of Chiroptera. Taken together, this suggests that substitutions that modulate the stability of the light-activated rhodopsin state were likely maintained through intensified constraint after bats diversified, being finely tuned in response to novel sensory specializations. Our study demonstrates the power of combining experimental and computational approaches for investigating functional mechanisms underlying the evolution of complex sensory adaptations.

The role of ecological factors in shaping bat cone opsin evolution

Bats represent one of the largest and most striking nocturnal mammalian radiations, exhibiting many visual system specializations for performance in light-limited environments. Despite representing the greatest ecological diversity and species richness in Chiroptera, Neotropical lineages have been undersampled in molecular studies, limiting the potential for identifying signatures of selection on visual genes associated with differences in bat ecology. Here, we investigated how diverse ecological pressures mediate long-term shifts in selection upon long-wavelength (Lws) and short-wavelength (Sws1) opsins, photosensitive cone pigments that form the basis of colour vision in most mammals, including bats. We used codon-based likelihood clade models to test whether ecological variables associated with reliance on visual information (e.g. echolocation ability and diet) or exposure to varying light environments (e.g. roosting behaviour and foraging habitat) mediated shifts in evolutionary rates in bat cone opsin genes. Using additional cone opsin sequences from newly sequenced eye transcriptomes of six Neotropical bat species, we found significant evidence for different ecological pressures influencing the evolution of the cone opsins. While Lws is evolving under significantly lower constraint in highly specialized high-duty cycle echolocating lineages, which have enhanced sonar ability to detect and track targets, variation in Sws1 constraint was significantly associated with foraging habitat, exhibiting elevated rates of evolution in species that forage among vegetation. This suggests that increased reliance on echolocation as well as the spectral environment experienced by foraging bats may differentially influence the evolution of different cone opsins. Our study demonstrates that different ecological variables may underlie contrasting evolutionary patterns in bat visual opsins, and highlights the suitability of clade models for testing ecological hypotheses of visual evolution.

Integrating vision and echolocation for navigation and perception in bats

How animals integrate information from various senses to navigate and generate perceptions is a fundamental question. Bats are ideal animal models to study multisensory integration due to their reliance on vision and echolocation, two modalities that allow distal sensing with high spatial resolution. Using three behavioral paradigms, we studied different aspects of multisensory integration in Egyptian fruit bats. We show that bats learn the three-dimensional shape of an object using vision only, even when using both vision and echolocation. Nevertheless, we demonstrate that they can classify objects using echolocation and even translate echoic information into a visual representation. Last, we show that in navigation, bats dynamically switch between the modalities: Vision was given more weight when deciding where to fly, while echolocation was more dominant when approaching an obstacle. We conclude that sensory integration is task dependent and that bimodal information is weighed in a more complex manner than previously suggested.

Bat sonar and wing morphology predict species vertical niche

The use of echolocation allows insectivorous bats to access unique foraging niches by locating obstacles and prey with ultrasounds in complete darkness. To avoid interspecific competition, it is likely that sonar features and wing morphology co-evolved with species vertical distribution, but due to the technical difficulties of studying flight in the vertical dimension, this has never been demonstrated with empirical measurements. The authors equipped 48 wind masts with arrays of two microphones and located the vertical distribution of a community of 19 bat species and two species groups over their annual activity period (>8000 nights). The authors tested the correlation between the proportion of flights at height and the acoustic features of bat calls as well as their wing morphology. The authors found that call peak frequency and bandwidth are good predictors of bat use of the vertical space regardless of their acoustic strategies (i.e., gleaning, hawking, or detecting prey flutter). High wing aspect ratios and high wing loadings were associated with high proportions of time spent at height, confirming hypotheses from the literature.

Experienced Migratory Bats Integrate the Sun's Position at Dusk for Navigation at Night

From bats to whales, millions of mammals migrate every year. However, their navigation capacity for accomplishing long-distance movements remains remarkably understudied and lags behind by five decades compared to other animals [1, 2]-partly because, unlike for other taxa, such as birds and sea turtles, no small-scale orientation assay has so far been developed. Yet recently, bats became a model to investigate which cues mammals use for long-range navigation, and, surprisingly for nocturnal animals, sunset cues, and particularly polarized-light cues, appear to be crucial for calibration of the magnetic-compass system in non-migratory bats [3-5]. This does not appear to hold for a species of migratory bat, however [6], and thus the nature of the information used by migratory bats for navigation remains unclear. Here, we asked whether the position of the solar disk per se is relevant for compass orientation in a migratory bat, Pipistrellus pygmaeus. Using a new experimental assay that measures takeoff orientation, we tested the orientation of bats exposed to a shifted sunset azimuth using a mirror at dusk. Bats exposed to a 180°-rotated azimuth of the setting sun and released after translocation during the same night shifted their heading direction by ∼180° compared to control bats. However, first-year migrants had no clear orientation either as controls or after the same treatment. This suggests that learning the migratory direction is a key component in the navigational system of naive bats in this species. Our study provides rare evidence for the specific cues and mechanisms that migratory mammals use for navigation.

Inconspicuous echolocation in hoary bats (Lasiurus cinereus)

Echolocation allows bats to occupy diverse nocturnal niches. Bats almost always use echolocation, even when other sensory stimuli are available to guide navigation. Here, using arrays of calibrated infrared cameras and ultrasonic microphones, we demonstrate that hoary bats (Lasiurus cinereus) use previously unknown echolocation behaviours that challenge our current understanding of echolocation. We describe a novel call type ('micro' calls) that has three orders of magnitude less sound energy than other bat calls used in open habitats. We also document bats flying close to microphones (less than 3 m) without producing detectable echolocation calls. Acoustic modelling indicates that bats are not producing calls that exceed 70-75 dB at 0.1 m, a level that would have little or no known use for a bat flying in the open at speeds exceeding 7 m s^-1 This indicates that hoary bats sometimes fly without echolocation. We speculate that bats reduce echolocation output to avoid eavesdropping by conspecifics during the mating season. These findings might partly explain why tens of thousands of hoary bats are killed by wind turbines each year. They also challenge the long-standing assumption that bats-model organisms for sensory specialization-are reliant on sonar for nocturnal navigation.

The Monumental Mistake of Evicting Bats from Archaeological Sites—A Reflection from New Delhi

We highlight the importance of an integrated management policy for archaeological monuments and the insect-eating bats that roost inside them. We refer to India, but the issue is general and of worldwide significance. There is increasing evidence that the ecosystem services provided by insect-eating bats in agricultural fields are of vital economic importance, which is likely to increase as chemical pest-control methods become inefficient due to evolving multi-resistance in insects. We visited five archaeological sites in the city of New Delhi. We found bats at all five locations, and three of them harbored large colonies (many thousands) of mouse-tailed bats and tomb bats. These bats likely disperse over extensive areas to feed, including agricultural fields in the vicinity and beyond. All insect-eating bats should be protected and properly managed as a valuable resource at the archaeological sites where they occur. We firmly believe that “fear” of bats can be turned into curiosity by means of education and that their presence should instead enhance the value of the sites. We suggest some means to protect the bats roosting inside the buildings, while mitigating potential conflicts with archaeological and touristic interests.

Social communication in bats

Bats represent one of the most diverse mammalian orders, not only in terms of species numbers, but also in their ecology and life histories. Many species are known to use ephemeral and/or unpredictable resources that require substantial investment to find and defend, and also engage in social interactions, thus requiring significant levels of social coordination. To accomplish these tasks, bats must be able to communicate; there is now substantial evidence that demonstrates the complexity of bat communication and the varied ways in which bats solve some of the problems associated with their unique life histories. However, while the study of communication in bats is rapidly growing, it still lags behind other taxa. Here we provide a comprehensive overview of communication in bats, from the reasons why they communicate to the diversity and application of different signal modalities. The most widespread form of communication is the transmission of a signaller's characteristics, such as species identity, sex, individual identity, group membership, social status and body condition, and because many species of bats can rely little on vision due to their nocturnal lifestyles, it is assumed that sound and olfaction are particularly important signalling modes. For example, research suggests that secretions from specialized glands, often in combination with urine and saliva, are responsible for species recognition in several species. These olfactory signals may also convey information about sex and colony membership. Olfaction may be used in combination with sound, particularly in species that emit constant frequency (CF) echolocation calls, to recognize conspecifics from heterospecifics, yet their simple structure and high frequency do not allow much information of individual identity to be conveyed over long distances. By contrast, social calls may encode a larger number of cues of individual identity, and their lower frequencies increase their range of detection. Social calls are also known to deter predators, repel competitors from foraging patches, attract group mates to roost sites, coordinate foraging activities, and are used during courtship. In addition to sound, visual displays such as wing flapping or hovering may be used during courtship, and swarming around roost sites may serve as a visual cue of roost location. However, visual communication in bats still remains a poorly studied signal modality. Finally, the most common form of tactile communication known in bats is social grooming, which may be used to signal reproductive condition, but also to facilitate and strengthen cooperative interactions. Overall, this review demonstrates the rapid advances made in the study of bat social communication during recent years, and also identifies topics that require further study, particularly those that may allow us to understand adaptation to rapidly changing environmental conditions.

Sensing in a noisy world: lessons from auditory specialists, echolocating bats

All animals face the essential task of extracting biologically meaningful sensory information from the ‘noisy’ backdrop of their environments. Here, we examine mechanisms used by echolocating bats to localize objects, track small prey and communicate in complex and noisy acoustic environments. Bats actively control and coordinate both the emission and reception of sound stimuli through integrated sensory and motor mechanisms that have evolved together over tens of millions of years. We discuss how bats behave in different ecological scenarios, including detecting and discriminating target echoes from background objects, minimizing acoustic interference from competing conspecifics and overcoming insect noise. Bats tackle these problems by deploying a remarkable array of auditory behaviors, sometimes in combination with the use of other senses. Behavioral strategies such as ceasing sonar call production and active jamming of the signals of competitors provide further insight into the capabilities and limitations of echolocation. We relate these findings to the broader topic of how animals extract relevant sensory information in noisy environments. While bats have highly refined abilities for operating under noisy conditions, they face the same challenges encountered by many other species. We propose that the specialized sensory mechanisms identified in bats are likely to occur in analogous systems across the animal kingdom.

Perceptual Ranges, Information Gathering, and Foraging Success in Dynamic Landscapes

How organisms gather and utilize information about their landscapes is central to understanding land-use patterns and population distributions. When such information originates beyond an individual's immediate vicinity, movement decisions require integrating information out to some perceptual range. Such nonlocal information, whether obtained visually, acoustically, or via chemosensation, provides a field of stimuli that guides movement. Classically, however, models have assumed movement based on purely local information (e.g., chemotaxis, step-selection functions). Here we explore how foragers can exploit nonlocal information to improve their success in dynamic landscapes. Using a continuous time/continuous space model in which we vary both random (diffusive) movement and resource-following (advective) movement, we characterize the optimal perceptual ranges for foragers in dynamic landscapes. Nonlocal information can be highly beneficial, increasing the spatiotemporal concentration of foragers on their resources up to twofold compared with movement based on purely local information. However, nonlocal information is most useful when foragers possess both high advective movement (allowing them to react to transient resources) and low diffusive movement (preventing them from drifting away from resource peaks). Nonlocal information is particularly beneficial in landscapes with sharp (rather than gradual) patch edges and in landscapes with highly transient resources.

Weakly electric fish learn both visual and electrosensory cues in a multisensory object discrimination task

Weakly electric fish use electrosensory, visual, olfactory and lateral line information to guide foraging and navigation behaviors. In many cases they preferentially rely on electrosensory cues. Do fish also memorize non-electrosensory cues? Here, we trained individuals of gymnotiform weakly electric fish Apteronotus albifrons in an object discrimination task. Objects were combinations of differently conductive materials covered with differently colored cotton hoods. By setting visual and electrosensory cues in conflict we analyzed the sensory hierarchy among the electrosensory and the visual sense in object discrimination. Our experiments show that: (i) black ghost knifefish can be trained to solve discrimination tasks similarly to the mormyrid fish; (ii) fish preferentially rely on electrosensory cues for object discrimination; (iii) despite the dominance of the electrosense they still learn the visual cue and use it when electrosensory information is not available; (iv) fish prefer the trained combination of rewarded cues over combinations that match only in a single feature and also memorize the non-rewarded combination.

Finding flowers in the dark: nectar-feeding bats integrate olfaction and echolocation while foraging for nectar

Nectar-feeding bats depend mainly on floral nectar to fulfil their energetic requirements. Chiropterophilous flowers generally present strong floral scents and provide conspicuous acoustic echoes to attract bats. While floral scents are assumed to attract bats over long distances, acoustic properties of flower structures may provide detailed information, thus supporting the localization of a flower at close ranges. So far, to our knowledge, there is no study trying to understand the relative importance as well as the combination of these generally coupled cues for detection (presence) and localization (exact position) of open flowers in nature. For a better comprehension of the significance of olfaction and echolocation in the foraging behaviour of nectar-feeding bats, we conducted two-choice experiments with Leptonycteris yerbabuenae. We tested the bats' behaviour in three experimental scenarios with different cues: (i) olfaction versus echolocation, (ii) echolocation versus echolocation and olfaction, and (iii) olfaction versus echolocation and olfaction. We used the floral scent of the bat-pollinated cactus Pachycereus pringlei as olfactory cue and an acrylic paraboloid as acoustic cue. Additionally, we recorded the echolocation behaviour of the bats and analysed the floral scent of P. pringlei. When decoupled cues were offered, bats displayed no preference in choice for any of the two cues. However, bats reacted first to and chose more often the coupled cues. All bats echolocated continuously and broadcast a long terminal group before a successful visit. The floral scent bouquet of P. pringlei is composed of 20 compounds, some of which (e.g. methyl benzoate) were already reported from chiropterophilous plants. Our investigation demonstrates for the first time to our knowledge, that nectar-feeding bats integrate over different sensory modes for detection and precise localization of open flowers. The combined information from olfactory and acoustic cues allows bats to forage more efficiently.

Perceptual Modalities Guiding Bat Flight in a Native Habitat

Flying animals accomplish high-speed navigation through fields of obstacles using a suite of sensory modalities that blend spatial memory with input from vision, tactile sensing, and, in the case of most bats and some other animals, echolocation. Although a good deal of previous research has been focused on the role of individual modes of sensing in animal locomotion, our understanding of sensory integration and the interplay among modalities is still meager. To understand how bats integrate sensory input from echolocation, vision, and spatial memory, we conducted an experiment in which bats flying in their natural habitat were challenged over the course of several evening emergences with a novel obstacle placed in their flight path. Our analysis of reconstructed flight data suggests that vision, echolocation, and spatial memory together with the possible exercise of an ability in using predictive navigation are mutually reinforcing aspects of a composite perceptual system that guides flight. Together with the recent development in robotics, our paper points to the possible interpretation that while each stream of sensory information plays an important role in bat navigation, it is the emergent effects of combining modalities that enable bats to fly through complex spaces.

Hippocampal global remapping for different sensory modalities in flying bats

Hippocampal place cells encode the animal's spatial position. However, it is unknown how different long-range sensory systems affect spatial representations. Here we alternated usage of vision and echolocation in Egyptian fruit bats while recording from single neurons in hippocampal areas CA1 and subiculum. Bats flew back and forth along a linear flight track, employing echolocation in darkness or vision in light. Hippocampal representations remapped between vision and echolocation via two kinds of remapping: subiculum neurons turned on or off, while CA1 neurons shifted their place fields. Interneurons also exhibited strong remapping. Finally, hippocampal place fields were sharper under vision than echolocation, matching the superior sensory resolution of vision over echolocation. Simulating several theoretical models of place-cells suggested that combining sensory information and path integration best explains the experimental sharpening data. In summary, here we show sensory-based global remapping in a mammal, suggesting that the hippocampus does not contain an abstract spatial map but rather a 'cognitive atlas', with multiple maps for different sensory modalities.