Frequency-modulated Calls Research Articles

Studies of the acoustic activity of white whales (in captive conditions and the possibility of their signals using to control behavior of fish in fishing process

The purpose of the work: identification of acoustic activity of belugas in conditions of aviary keeping and justification of the possibility of using their signals in fishing.The material of research was digital audio recordings of hydroacoustic signals of belugas collected from 2016 to 2018.Methods used: analysis of hydroacoustic data, visualization of the daily acoustic activity of belugas in different seasons, identification of stereotypes of acoustic behavior and characteristic signals in captive conditions and during fish hunting. Novelty: for the first time a substantiation for the possibility of using signals of one of the species of toothed whales — belugas registered in the captive conditions, to influence on the behavior of hydrobionts and solve practical problems of fishing and fish protection is given.Results: the highest acoustic activity of white whales is observed during the day and noticeably decreases at night with increase and decrease in the morning and evening hours. Maximum acoustic activity precedes the feeding of dolphins.The most common tones are those containing the fundamental frequency and harmonics. Characteristic features of the sounds are high variability of duration from 0.25 to 2.5 s, pronounced frequency components in the 500–2500 Hz spectrum zones, amplitude and frequency modulation, sound pressure levels of the signal up to 500 Pa /1 m.Acoustic activity of belugas during fish hunting and in the period preceding feeding in captive conditions, stereotypical. Typical are low-frequency frequency-modulated calls and whistles in the hearing range of fish, as well as a curtain of air bubbles and body and tail slaps. Possible ways of using beluga whale signals to increase the efficiency of fishing are proposed.Practical significance: the results of the study open up the possibility of using the signals of belugas to remotely control the movement of fish, create artificial concentrations and deterrent of hydrobionts in the intended areas.

Cortical theta oscillations and 50-kHz ultrasonic vocalizations in response to tactile reward indicate positive emotion in rats

Rats emit 50-kHz ultrasonic vocalizations (USVs), which reflect positive affective states. Rhythmic stroking increases 50-kHz USVs via the mesolimbic dopaminergic system. However, little is known about the effect of tactile reward on rat brain activity. This study aimed to investigate the brain activity associated with positive emotions induced by tactile stimulation using a frontoparietal electroencephalogram (EEG) as well as through the analysis of 50-kHz USVs and behavioral activity in awake rats. During rhythmic stroking, the power of the middle theta band and harmonics calls significantly increased compared with baseline. After rhythmic stroking, fast theta oscillations significantly increased but slow theta significantly decreased, with abundant frequency-modulated (FM) calls. Light touch stimulation increased the fast theta power but decreased FM calls. However, there was no significant difference in behavior after stimulation with rhythmic stroking or light touch. These results suggest that the characteristic brain theta oscillations and 50-kHz USV profiles induced by tactile reward can detect positive affective states in rats.

Automated extraction of baleen whale calls based on the pseudo-Wigner–Ville distribution

Baleen whales produce a wide variety of frequency-modulated calls. Extraction of the time-frequency (TF) structures of these calls forms the basis for many applications, including abundance estimation and species recognition. Typical methods to extract the contours of whale calls from a spectrogram are based on the short-time Fourier transform and are, thus, restricted by a fixed TF resolution. Considering the low-frequency nature of baleen whale calls, this work represents the contours using a pseudo-Wigner-Ville distribution for a higher TF resolution at the cost of introducing cross terms. An adaptive threshold is proposed followed by a modified Gaussian mixture probability hypothesis density filter to extract the contours. Finally, the artificial contours, which are caused by the cross terms, can be removed in post-processing. Simulations were conducted to explore how the signal-to-noise ratio influences the performance of the proposed method. Then, in experiments based on real data, the contours of the calls of three kinds of baleen whales were extracted in a highly accurate manner (with mean deviations of 5.4 and 0.051 Hz from the ground-truth contours at sampling rates of 4000 and 100 Hz, respectively) with a recall of 75% and a precision of 78.5%.

Sex differences in serotonergic control of rat social behaviour

RationaleThere is increasing evidence that enhancement of the salience of social stimuli can have a beneficial effect in managing many psychiatric conditions. There are, however, clear sex-related differences in social behaviour, including the neural mechanisms responsible for different aspects of social functions. ObjectivesWe explored the role of the serotonergic system on rat social behaviour under baseline and under stressful conditions in female and male rats. MethodsRats were treated with the selective serotonin transporter (SERT) inhibitor escitalopram postnatally; a procedure known to cause a long-lasting reduction of serotonergic activity. In adulthood, social behaviour was tested in a social interaction test and in ultrasonic vocalisation (USVs) recording sessions before and after yohimbine-induced stress-like state. ResultsOur data demonstrated that both female and, to a lesser extent, male escitalopram treated rats, exposed to a novel social situation, had fewer social exploration events and emitted fewer frequency-modulated calls with trills, trills and step calls, suggesting that an impaired function of the serotonergic system reduced the positive valence of social interaction. In a stress-like state, 50 kHz flat calls were increased only in female rats, indicating an increased seeking of social contact. However, the number of flat calls in escitalopram treated female rats was significantly lower compared with control rats. ConclusionsThese data suggest that females may respond differently to serotonergic pharmacotherapy with respect to enhancement of beneficial effects of social support, especially in stress-related situations.

Harmonics and frequency-modulated calls indicate different motivational states in rats

Rats emit 50-kHz ultrasonic vocalizations (USVs) via the accumbal dopaminergic system in response to rhythmic stroking (RS). However, it is unknown whether RS can lead to approach behavior, which is a reliable marker of positive affective states. To assess the effects of RS on the behavioral response and its correlation with call subtypes, we measured the total number of 50-kHz USVs (overall and within call subtypes) and approach behavior in response to RS. Rats were randomly divided into two equal groups. One group received RS and the control group received light touch (LT). RS stimulation was administered for 30 s once a day for 7 consecutive days, and USVs were recorded immediately before, during, and after stimulation. RS induced 50-kHz USVs from day 1 and led to quick approach on day 2 and after. However, those responses were absent in the LT group. Specific frequency-modulated (FM) calls observed after RS were significantly correlated with approach latency. These results suggest that RS has rewarding effects, and the specific FM calls observed immediately after RS can be used as indices of innate positive reinforcement.

The influence of sea ice on the detection of bowhead whale calls

Bowhead whales (Balaena mysticetus) face threats from diminishing sea ice and increasing anthropogenic activities in the Arctic. Passive acoustic monitoring is the most effective means for monitoring their distribution and population trends, based on the detection of their calls. Passive acoustic monitoring, however, is influenced by the sound propagation environment and ambient noise levels, which impact call detection probability. Modeling and simulations were used to estimate detection probability for bowhead whale frequency-modulated calls in the 80–180 Hz frequency band with and without sea ice cover and under various noise conditions. Sound transmission loss for bowhead calls is substantially greater during ice-covered conditions than during open-water conditions, making call detection ~ 3 times more likely in open-water. Estimates of daily acoustic detection probability were used to compensate acoustic detections for sound propagation and noise effects in two recording datasets in the northeast Chukchi Sea, on the outer shelf and continental slope, collected between 2012 and 2013. The compensated acoustic density suggests a decrease in whale presence with the retreat of sea ice at these recording sites. These results highlight the importance of accounting for effects of the environment on ambient noise and acoustic propagation when interpreting results of passive acoustic monitoring.

Source Level of Antarctic Blue and Fin Whale Sounds Recorded on Sonobuoys Deployed in the Deep-Ocean Off Antarctica

The source levels, SL, of Antarctic blue and fin whale calls were estimated using acoustic recordings collected from directional sonobuoys deployed during an Antarctic voyage in 2019. Antarctic blue whale call types included stereotyped song and downswept frequency-modulated calls, often, respectively, referred to as Z-calls (comprising song units-A, B, and C) and D-calls. Fin whale calls included 20 Hz pulses and 40 Hz downswept calls. Source levels were obtained by measuring received levels (RL) and modelling transmission losses (TL) for each detection. Estimates of SL were sensitive to the parameters used in TL models, particularly the seafloor geoacoustic properties and depth of the calling whale. For our best estimate of TL and whale-depth, mean SL in dB re 1 μPa ± 1 standard deviation ranged between 188–191 ± 6–8 dB for blue whale call types and 189–192 ± 6 dB for fin whale call types. These estimates of SL are the first from the Southern Hemisphere for D-calls and 40 Hz downsweeps, and the largest sample size to-date for Antarctic blue whale song. Knowledge of source levels is essential for estimating the detection range and communication space of these calls and will enable more accurate comparisons of detections of these sounds from sonobuoy surveys and across international long-term monitoring networks.

Distinct Profiles of 50 kHz Vocalizations Differentiate Between Social Versus Non-social Reward Approach and Consumption.

Social animals tend to possess an elaborate vocal communication repertoire, and rats are no exception. Rats utilize ultrasonic vocalizations (USVs) to communicate information about a wide range of socially relevant cues, as well as information regarding the valence of the behavior and/or surrounding environment. Both quantitative and qualitative acoustic properties of these USVs are thought to communicate context-specific information to conspecifics. Rat USVs have been broadly categorized into 22 and 50 kHz call categories, which can be further classified into subtypes based on their sonographic features. Recent research indicates that the 50 kHz calls and their various subtype profiles may be related to the processing of social and non-social rewards. However, only a handful of studies have investigated USV elicitation in the context of both social and non-social rewards. Here, we employ a novel behavioral paradigm, the social-sucrose preference test, that allowed us to measure rats’ vocal responses to both non-social (i.e., 2, 5, and 10% sucrose) and social reward (interact with a Juvenile rat), presented concurrently. We analyzed adult male Long-Evans rats’ vocal responses toward social and non-social rewards, with a specific focus on 50 kHz calls and their 14 subtypes. We demonstrate that rats’ preference and their vocal responses toward a social reward were both influenced by the concentration of the non-social reward in the maze. In other words, rats showed a trade-off between time spent with non-social or social stimuli along with increasing concentrations of sucrose, and also, we found a clear difference in the emission of flat and frequency-modulated calls in the social and non-social reward zones. Furthermore, we report that the proportion of individual subtypes of 50 kHz calls, as well as the total USV counts, showed variation across different types of rewards as well. Our findings provide a thorough overview of rat vocal responses toward non-social and social rewards and are a clear depiction of the variability in the rat vocalization repertoire, establishing the role of call subtypes as key players driving context-specific vocal responses of rats.

Simultaneous antagonism of dopamine D1/D2/D3 receptor in the NAc reduces 50-kHz ultrasonic calls in response to rhythmic tactile stroking

Tactile stimulation such as rhythmic stroking elicits 50-kHz ultrasonic vocalizations (USVs) in rats that are thought to reflect positive affective states. Dopaminergic neurotransmission in the nucleus accumbens (NAc) is required for tactile reward-induced 50-kHz USVs; however, it is still unknown whether the accumbal dopaminergic system differentially modulates 50-kHz USV call subtypes induced by rhythmic stroking. We therefore examined both total and categorized 50-kHz USV rate, peak frequency, and duration under dopamine (DA) receptor antagonism in the NAc shell. Bilateral injection of the D1 receptor antagonist SCH 23390 (500 ng/side) plus the D2/D3 receptor antagonist raclopride (25 μg/side) significantly reduced the number of predominantly flat calls with harmonics during stimulation and the number of frequency-modulated (FM) calls after stimulation. In contrast, there were no substantial changes in total and categorized 50-kHz USVs mean peak frequencies and call durations. Therefore, emission of different subtypes of 50-kHz USVs may be differently regulated by dopaminergic transmission. The 50-kHz harmonics and FM USVs induced by rhythmic stroking may be useful behavioral markers for tactile reward in rats.

Social dysfunction in the neurodevelopmental model of schizophrenia in male and female rats: Behavioural and biochemical studies

Social dysfunction is among the core symptoms of schizophrenia. The neuropeptides oxytocin (OXT) and vasopressin (VP) are involved in the regulation of social behaviour and social cognition. There are indications that both of these neurotransmitter systems are altered in schizophrenia. Prenatal (embryonic day 17) exposure to the neurotoxin methylazoxymethanol acetate (MAM; 22 mg/kg) leads to a schizophrenia-like phenotype in rats and has been used as a model of schizophrenia symptoms. Here, we examined the social phenotype of MAM-exposed female and male rats and measured concentrations of OXT, VP and their specific receptors in various brain areas involved in the control of social behaviour.We report decreases in social behaviour and ultrasonic vocalisations (USVs) in the MAM rats during social encounters. Specifically, the duration of social interactions and number of corresponding USVs were reduced in this group. In the MAM rats, “positive” 50-kHz USVs were flatter, i.e., displayed lower bandwidth, a greater percentage of “short” calls and lower percentage of frequency-modulated calls. The MAM animals exhibited diminished interest towards social stimuli in olfactory preference tests. In the resident-intruder test, MAM exposure reduced dominance behaviour only in the males. We also report cognitive impairments, including reduced novel object recognition and cognitive inflexibility in the attentional set shifting test, and decreased OXT and OXT receptor concentrations in the prefrontal cortex and hypothalamus and VP and VP receptors in the hypothalamus in the MAM rats.Deficits in central OXT and VP systems may underlie abnormalities present in the MAM model of schizophrenia.

Echolocation calls of some bats of Gujarat, India

Abstract Bats play an important role by providing ecosystem services including pollination, seed dispersal, forest regeneration and insect pest control and also serve as bio-indicators. In the present study, we present an acoustic guide to the calls of nine species of bats from Gujarat belonging to families Rhinopomatidae (Rhinopoma hardwickii, Rhinopoma microphyllum), Emballonuridae (Taphozous melanopogon, Taphozous longimanus and Taphozous nudiventris), Rhinolophidaea (Rhinolophus lepidus), Hipposideridae (Hipposideros galeritus) and Vespertilionidae (Scotophilus heathii, Pipistrellus ceylonicus). Discriminant function analysis was used to classify the bat calls to the species level using leave-one-out cross validation. Analysis was carried out separately for constant frequency (CF) calls and frequency-modulated (FM) calls. Bats echolocating with CF calls were classified with 100% success, while in the case of FM calls, the calls were classified with 66.7% accuracy. In species-rich communities, care should be taken while using echolocation calls to identify bats producing FM calls. More such call libraries of bats from other parts of India are needed for non-invasive documentation of chiropteran fauna in different biogeographic zones.

Echolocation and Stratum Preference: Key Trait Correlates of Vulnerability of Insectivorous Bats to Tropical Forest Fragmentation

Habitat loss and fragmentation rank high amongst the most pressing threats to biodiversity. Understanding how variation in functional traits is associated with species vulnerability in fragmented landscapes is central to the design of effective conservation strategies. Here, we used a whole-ecosystem ecological experiment in the Central Amazon to investigate which functional traits of aerial-hawking insectivorous bats best predict their sensitivity to forest fragmentation. During 2014, bats were surveyed using passive bat recorders in six continuous forest sites, eight forest fragments, eight fragment edges, and eight forest clearings. The interaction between functional traits, environmental characteristics, and species distribution was investigated using a combination of RLQ and fourth-corner analyses. Our results showed that echolocation call structure, vertical stratification, and wing aspect ratio were the strongest predictors of sensitivity to forest fragmentation. Frequency of maximum energy, body mass, and relative wing loading did not show any correlation with the environmental variables. Bat species with constant-frequency calls were associated with high vegetation density, being more susceptible to forest fragmentation than species with frequency-modulated calls. Vertical stratum preference was also correlated with vegetation structure, indicating that understory species were more sensitive to forest loss than canopy species. Finally, species with high aspect ratio wings were linked to forest edges and clearings. Our findings suggest that species functional traits determine the vulnerability of aerial-hawking insectivorous bats toward fragmentation and, similarly, environmental conditions determine if a species is likely to become locally extinct due to fragmentation. Preserving structurally complex forests will be crucial to ensure the long-term persistence of the most sensitive and vulnerable species of this bat ensemble in fragmented landscapes across the Neotropics.

Echolocation in a Nocturnal Primate?

Recently it has been noted that Gursky’s spectral tarsier emits ultrasonic calls. Because high-frequency sounds are capable of travelling only short distances, their function is limited by the distance the sound can be discerned. One possible function for these short-distance, high-frequency sounds is that they might be used by the tarsiers to navigate throughout their environment. I conducted this study at Tangkoko Nature Reserve in Sulawesi, Indonesia, from May to August 2017. Using the focal follows, I recorded the start of each locomotor activity to the nearest second. I also simultaneously recorded all ultrasonic vocalisations that the focal individual emitted using a Wildlife Acoustics Ultrasonic Song Meter BAT3. Approximately 42% of the ultrasonic calls that I recorded during this study were given within 1 s prior to engaging in locomotor activity. The tarsiers gave statistically more ultrasonic calls during locomotion than during other behaviours (rest, forage and social). Four types of ultrasonic calls were given while locomoting: whistles, doubles, trills and frequency-modulated calls. Of these, the trill and the frequency-modulated call were only given during locomotion, and trills were only given prior to leaping. These results preliminarily suggest that the tarsiers do use ultrasonic calls for navigation. Studying echolocation in organisms closely related to bats is critical for understanding the evolution of echolocation.

Bats are unusually insensitive to brief low-frequency tones.

Bats use brief calls for echolocation, suggesting that they might be more sensitive to brief sounds than non-echolocating mammals. To investigate this possibility, absolute thresholds for brief tones were determined for four species of bats: The Commonvampire bat (Desmodus rotundus) and the Greater spear-nosed bat (Phyllostomus hastatus), both of which use frequency-modulated calls, the Egyptian fruit bat (Rousettus aegyptiacus), an echolocator that uses tongue-clicks rather than laryngeal calls, and the Dog-faced fruit bat (Cynopterus brachyotis), a non-echolocating species. Norway rats and a human were tested for comparison using the same acoustic stimuli. Contrary to expectations, the echolocating bats were not superior to non-echolocating mammals in detecting brief tones in the frequency range of their echolocation calls. Instead, all four species of bats were remarkably less sensitive than non-bats to brief sounds of 10kHz and below. This implies that temporal summation in the mammalian auditory system can show large species differences, and that the detection of brief sound is likely influenced by the selective pressures on each species as well as by the physical integration of energy in the auditory system. Such species differences in function are expected to be reflected in the physiology of their auditory systems.

Vocal repertoire of captive northern and southern flying squirrels (Glaucomys sabrinus and G. volans)

Studies determining and categorizing ultrasonic calls in mammals have generally been limited to laboratory experiments with mice and rats or field research on bats. However, recent studies have discovered high-frequency sounds in several mammalian taxa. We describe the vocal repertoire of high-frequency calls in two species of North American flying squirrels (Glaucomys sabrinus and G. volans). We collected passive recordings from captive colonies of G. sabrinus and G. volans to generate a library of calls for each species. Using visual and auditory assessments, as well as quantitative measurements (i.e., spectral and temporal characteristics), we identified 10 specific call-types for G. sabrinus and 27 specific call-types for G. volans. The most common call-types emitted by G. sabrinus included arc chirps, tonal chirps, two-toned chirps, trills, and upsweeps; and for G. volans included arc chirps, barks, descending crows, tonal chirps, and trills. Additionally, we recorded two call-types made by G. volans juveniles: trills and tonal chirps. Most call-types emitted by G. sabrinus were also emitted by G. volans, showing overlap in the repertoire of both species, although variation between species is evident in certain call-types. We also found both species displayed evidence of frequency alteration (i.e., modification of the frequency of one squirrel's call when simultaneously calling with another squirrel using similar call-types). Our call library demonstrates a complex vocal range of tonal and frequency-modulated calls emitted by both species. Acoustic detection of North American flying squirrels is a new monitoring technique that can be used for conservation and management purposes. Differences in the trills, tonal chirps, and arc chirps provide a means to distinguish calls between these species. Increasing understanding of behavior associated with vocalizations may provide a more complete understanding of flying squirrel ecology, including intra- and interspecific interactions.

The Behaviour and Vocalisations of Captive Geoffroy's Horseshoe Bats, Rhinolophus clivosus (Chiroptera: Rhinolophidae)

Acoustic signals are important to the biology of animals, mediating crucial activities such as social interactions (communication) as well as orientation and foraging (echolocation). Many signals used in communication are vocal, which are especially important in nocturnal animals such as bats where social interactions occur in darkness. Despite this, little is known about the social calls and behaviours of echolocating bats. To better understand their social and acoustic behaviour, we compiled an ethogram and list of vocalisations for Geoffroy's horseshoe bat (Rhinolophus clivosus). We kept three non-contemporaneous and short-term captive groups to capture interactions and social calls using simultaneous video and audio recordings. The resultant ethogram was comprised of 40 unique types of behaviour, both social and non-social. Social calls (n = 255) were assigned to different call types by their frequency-time contour and categorised by their behaviours, situational categories and functional contexts (affiliative or agonistic). From the calls observed, four acoustically distinct call types were identified: (i) cascading/rising frequency-modulated (FM) calls (n = 26), (ii) oscillatory FM calls (n = 140), (iii) noisy screech calls (n = 68), and (iv) whistle calls (n = 21) (GLM: F30, 711 = 24.28, P < 0.001). Call types showed only weak associations with certain behaviours, situational categories or functional contexts. However, calls with specific acoustic attributes accompanied the behaviour of wing swat [discriminant function analysis (DFA) classification success: 76%; GLM: F20, 108 = 4.12, P < 0.001] and situational category of flight (DFA classification success: 82%; GLM: F20, 136 = 2.97, P < 0.001). An analysis of acoustic attributes across all call types showed weak associations with functional context (Affiliative DFA classification success: 6%). Only one acoustic parameter (peak frequency) had a slight significant difference between calls emitted during agonistic and affiliative interactions (GLM: F10, 73 = 2.30, P < 0.05; post-hoc unequal n: P = 0.044). In addition to the description of distinct call types, we provide evidence of transitional calls in this bat species in which a social call transitions seamlessly into an echolocation pulse. This study gives a first glimpse into the behaviours and social vocalisations produced by R. clivosus. Basic behavioural data such as these may facilitate the design of experiments that allow greater insight into the social organisation of bats.

Welcome home: Brazilian free-tailed bats adjust their echolocation calls to detect cave opening

While both bats and dolphins use sonar to navigate their environment, the sensory challenges between the two are not equal since dolphins typically navigate in the open ocean and bats often navigate in cluttered environments. Brazilian free-tailed bats (Tadarida brasiliensis), for example, adjust their frequency modulated (FM) call structure based upon their environment, producing lower-bandwidth calls in open environments, and higher-bandwidth calls in cluttered environments. In this study, we examined how Brazilian free-tailed bats change the bandwidth of calls when locating the cave opening in a flat, non-cluttered environment. We extracted individual echolocation calls from two locations around the cave, one away from the cave opening and one next to edge of the cave opening, with the only distinctive difference in the environment of the locations being the presence of the cave edge. We found higher starting and lower stopping frequencies for the FM calls at the cave edge, resulting in an increased bandwidth. This higher bandwidth suggests the bats may rely on edge detection to locate the cave opening and change the bandwidth of their signals to improve target resolution when returning to the roost.

Altai pika (Ochotona alpina) alarm calls: individual acoustic variation and the phenomenon of call-synchronous ear folding behavior.

Non-hibernating pikas collect winter food reserves and store them in hay piles. Individualization of alarm calls might allow discrimination between colony members and conspecifics trying to steal food items from a colony pile. We investigated vocal posture, vocal tract length, and individual acoustic variation of alarm calls, emitted by wild-living Altai pikas Ochotona alpina toward a researcher. Recording started when a pika started calling and lasted as long as possible. The alarm call series of 442 individual callers from different colonies consisted of discrete short (0.073-0.157s), high-frequency (7.31-15.46kHz), and frequency-modulated calls separated by irregular intervals. Analysis of 442 discrete calls, the second of each series, revealed that 44.34% calls lacked nonlinear phenomena, in 7.02% nonlinear phenomena covered less than half of call duration, and in 48.64% nonlinear phenomena covered more than half of call duration. Peak frequencies varied among individuals but always fitted one of three maxima corresponding to the vocal tract resonance frequencies (formants) calculated for an estimated 45-mm oral vocal tract. Discriminant analysis using variables of 8 calls per series of 36 different callers, each from a different colony, correctly assigned over 90% of the calls to individuals. Consequently, Altai pika alarm calls are individualistic and nonlinear phenomena might further increase this acoustic individualization. Additionally, video analysis revealed a call-synchronous, very fast (0.13-0.23s) folding, depression, and subsequent re-expansion of the pinna confirming an earlier report of this behavior that apparently contributes to protecting the hearing apparatus from damage by the self-generated high-intensity alarm calls.

Variation in echolocation call frequencies in two species of free-tailed bats according to temperature and humidity.

Bats can actively adjust their echolocation signals to specific habitats and tasks, yet it is not known if bats also modify their calls to decrease atmospheric attenuation. Here the authors test the hypothesis that individuals emit echolocation calls ideally suited to current conditions of temperature and humidity. The authors recorded two species, Molossus molossus and Molossops temminckii, in the field under different conditions of humidity and temperature. For each species, two calls were analyzed: the shorter frequency modulated (FM) signals that bats emitted as they approached the recording microphone, and the longer constant frequency (CF) calls emitted thereafter. For each signal, the authors extracted peak frequency and duration, and compared these parameters among species, call type, and environmental conditions. The authors' results show significant differences in peak frequency and duration among environmental conditions for both call types. Bats decreased the frequency and increased duration of CF calls as atmospheric attenuation increased; using a lower-frequency call may increase the range of detection by a few meters as atmospheric attenuation increases. The same trend was not observed for FM calls, which may be explained by the primary role of these signals in short-range target localization.

Variation in frequency-modulated call type for different flight densities in Mexican Free-tailed bats

Mexican free-tailed bats (Tadarida brasiliensis) adapt their call type depending on multiple factors. These bats have two general types of echolocation sounds, frequency-modulated (FM) and constant frequency (CF). CF calls are generally produced during the search phase of foraging, and FM calls are used during approach, prey capture, and roost emergence. In this study, we examined how Mexican free-tailed bats adapt their calls during emergence to avoid echo interference. We extracted individual calls from within a 500-ms sample that corresponded to five different emergence density categories. We found that these bats produce eight different FM call types during emergence. For each call within the sample, we characterized the call shape and calculated different parameters (duration, start frequency, peak frequency, end frequency). We compared the acoustic parameters across density categories and call shape, and compared the distribution of call shape across density categories. The results indicate that the call parameters and call shape vary from bat to bat, but there is no clear trend that this variance is due to the bat density. Therefore, these bats are likely changing their call shape and frequency characteristics to distinguish their own echoes from those of other bats.