Low-frequency Calls Research Articles

Behavioural context of call production in humpback whale calves: identification of potential begging calls in a mysticetes species.

Baleen whale calves vocalize, but the behavioural context and role of their social calls in mother-calf interactions are yet to be documented further. We investigated the context of call production in humpback whale (Megaptera novaeangliae) calves using camera-equipped animal-borne multi-sensor tags. Behavioural states, including suckling sessions, were identified using accelerometer, depth and video data. Call types were categorized through clustering techniques. We found that call types and rates predict the occurrence of a given state. Milling, resting and travelling were associated with a median call rate of 0 calls min-1, while surface play, tagging responses and suckling were associated with higher call rates, averaging up to a median of 0.5 calls min-1 for suckling. Suckling sessions were mainly associated with two sets of low-frequency calls corresponding to previously described burping, barking and snorting sounds. Surface play sessions featured mid-frequency calls with whoop-like sounds and other call types. These results address the significance of vocal signalling in mother-calf communication and the calf's development, including the first identification of potential begging calls. Overall, this study offers new insights into baleen whale behaviour, underscores the importance of social calls in mother-calf interactions and enhances our understanding of communication systems in aquatic mammalian mother-young pairs.

Tracking Whale Calls in the Lower St. Lawrence Seaway at Land Seismometers

The Lower St. Lawrence Seaway (LSLS) is critical to Canada’s economy both as part of a major marine shipping corridor and a site of intensive fishing. Every year, fin whales and blue whales frequent the LSLS feeding ground. Understanding the mechanisms driving whale habitat usage is key for making informed decisions on shipping and fishing, reducing whale collision risks and mitigating noise pollution. We detect whales in the LSLS with land seismometers by using a method that relies on the intervals of the regularly repeating low frequency calls. The resulting catalogue contains 14,076 fin whale detections and 3,739 blue whale detections between February 2020 and January 2022. These detections follow the overall pattern of hydrophones, with most detections from fall to early winter in the Estuary and until mid-winter/spring in the Gulf. High detection rates in the Northwest Gulf throughout the winter months demonstrate that this region is potentially utilized year-round. This labelled catalogue may be suitable for developing a deep learning-based whale call detection algorithm. Making use of seismometers and deep learning can increase whale monitoring coverage within the LSLS and elsewhere.

Underwater soundscape in Seaview Bay, Antarctica, and triple ascending trill of the leopard seal (Hydrurga leptonyx) underwater vocalizations.

The underwater soundscape was recorded in Seaview Bay off Inexpressible Island, Ross Sea region Marine protected area, for 3 days in December 2021. Leopard seal Hydrurga leptonyx vocalizations were a prominent sound source that led to variations in ambient sound pressure levels in a frequency range of approximately 150-4500 Hz. Among the 14 call types previously identified, except ultrasound vocalizations, six types of broadcast calls were classified, and their acoustic characteristics were analyzed. We focused on the acoustic characteristics of four low-frequency calls, clustered in a relatively narrow bandwidth, which have been relatively less studied. We identified a new call type of a triple ascending trill consisting of three trill parts, expanding upon the findings of previous studies. The audio data extracted from leopard seal vocalization videos, recorded by a monitoring camera on sea ice, enhanced the reliability of identifications of the underwater triple ascending trill. We present the unique results of underwater passive acoustic monitoring conducted at Seaview Bay, designated as Antarctic Specially Protected Area No 178. Our results could contribute to the development of detection and localization algorithms for leopard seal vocalizations and can be used as fundamental data for studies related to the vocalization and behavior of this species.

Characterization and mitigation of artifacts derived from NGS library preparation due to structure-specific sequences in the human genome

BackgroundHybridization capture-based targeted next generation sequencing (NGS) is gaining importance in routine cancer clinical practice. DNA library preparation is a fundamental step to produce high-quality sequencing data. Numerous unexpected, low variant allele frequency calls were observed in libraries using sonication fragmentation and enzymatic fragmentation. In this study, we investigated the characteristics of the artifact reads induced by sonication and enzymatic fragmentation. We also developed a bioinformatic algorithm to filter these sequencing errors.ResultsWe used pairwise comparisons of somatic single nucleotide variants (SNVs) and insertions and deletions (indels) of the same tumor DNA samples prepared using both ultrasonic and enzymatic fragmentation protocols. Our analysis revealed that the number of artifact variants was significantly greater in the samples generated using enzymatic fragmentation than using sonication. Most of the artifacts derived from the sonication-treated libraries were chimeric artifact reads containing both cis- and trans-inverted repeat sequences of the genomic DNA. In contrast, chimeric artifact reads of endonuclease-treated libraries contained palindromic sequences with mismatched bases. Based on these distinctive features, we proposed a mechanistic hypothesis model, PDSM (pairing of partial single strands derived from a similar molecule), by which these sequencing errors derive from ultrasonication and enzymatic fragmentation library preparation. We developed a bioinformatic algorithm to generate a custom mutation “blacklist” in the BED region to reduce errors in downstream analyses.ConclusionsWe first proposed a mechanistic hypothesis model (PDSM) of sequencing errors caused by specific structures of inverted repeat sequences and palindromic sequences in the natural genome. This new hypothesis predicts the existence of chimeric reads that could not be explained by previous models, and provides a new direction for further improving NGS analysis accuracy. A bioinformatic algorithm, ArtifactsFinder, was developed and used to reduce the sequencing errors in libraries produced using sonication and enzymatic fragmentation.

Evolutionary novelties underlie sound production in baleen whales.

Baleen whales (mysticetes) use vocalizations to mediate their complex social and reproductive behaviours in vast, opaque marine environments1. Adapting to an obligate aquatic lifestyle demanded fundamental physiological changes to efficiently produce sound, including laryngeal specializations2-4. Whereas toothed whales (odontocetes) evolved a nasal vocal organ5, mysticetes have been thought to use the larynx for sound production1,6-8. However, there has been no direct demonstration that the mysticete larynx can phonate, or if it does, how it produces the great diversity of mysticete sounds9. Here we combine experiments on the excised larynx of three mysticete species with detailed anatomy and computational models to show that mysticetes evolved unique laryngeal structures for sound production. These structures allow some of the largest animals that ever lived to efficiently produce frequency-modulated, low-frequency calls. Furthermore, we show that this phonation mechanism is likely to be ancestral to all mysticetes and shares its fundamental physical basis with most terrestrial mammals, including humans10, birds11, and their closest relatives, odontocetes5. However, these laryngeal structures set insurmountable physiological limits to the frequency range and depth of their vocalizations, preventing them from escaping anthropogenic vessel noise12,13 and communicating at great depths14, thereby greatly reducing their active communication range.

BovineTalk: machine learning for vocalization analysis of dairy cattle under the negative affective state of isolation.

There is a critical need to develop and validate non-invasive animal-based indicators of affective states in livestock species, in order to integrate them into on-farm assessment protocols, potentially via the use of precision livestock farming (PLF) tools. One such promising approach is the use of vocal indicators. The acoustic structure of vocalizations and their functions were extensively studied in important livestock species, such as pigs, horses, poultry, and goats, yet cattle remain understudied in this context to date. Cows were shown to produce two types of vocalizations: low-frequency calls (LF), produced with the mouth closed, or partially closed, for close distance contacts, and open mouth emitted high-frequency calls (HF), produced for long-distance communication, with the latter considered to be largely associated with negative affective states. Moreover, cattle vocalizations were shown to contain information on individuality across a wide range of contexts, both negative and positive. Nowadays, dairy cows are facing a series of negative challenges and stressors in a typical production cycle, making vocalizations during negative affective states of special interest for research. One contribution of this study is providing the largest to date pre-processed (clean from noises) dataset of lactating adult multiparous dairy cows during negative affective states induced by visual isolation challenges. Here, we present two computational frameworks-deep learning based and explainable machine learning based, to classify high and low-frequency cattle calls and individual cow voice recognition. Our models in these two frameworks reached 87.2 and 89.4% accuracy for LF and HF classification, with 68.9 and 72.5% accuracy rates for the cow individual identification, respectively.

Tradeoffs in sound quality and cost for passive acoustic devices

ABSTRACT Ecologists often collect data with automatic sensors such as passive acoustic devices. Traditional commercial passive acoustic devices that record ultrasonic sounds are usually > 700USD; less costly options now exist but the sound quality is unknown. We compared a low-cost passive acoustic device to a traditional device for quality of bat call data recorded. We simultaneously deployed two device types at seven sites to record free-flying bats over three nights. We paired call files from both devices when they occurred ≤ 3 s apart and contained ≥ 3 pulses; for each file pair and pulse, we measured indicators of recording quality, specifically duration, maximum frequency, and bandwidth. We compared the proportion of files classified as low frequency, mid frequency, or Myotis bats. The low-cost device recorded lower quality pulses (i.e., shorter duration, lower maximum frequency, and shorter bandwidth). The more expensive device recorded a higher proportion of Myotis calls and a lower proportion of low-frequency calls, but there was no difference across devices in the proportion of calls classified as mid-frequency bats. Less expensive alternatives for passive acoustic devices may be inappropriate for studies requiring high-quality data but may be critical when research objectives call for multiple devices and are constrained by budget.

Comparison of ultrasonic isolation calls of pure-breeding and interspecies hybrid Phodopus dwarf hamster pups

In mammalian cross-species hybrids, parameters of voice calls, produced by vocal fold vibrations, are intermediate between parental species. Inheritance of ultrasonic calls, produced by whistle mechanism, is unstudied for hybrids. We examined 4000 pup ultrasonic isolation-induced calls for peak power of call fundamental frequency and for call duration in 4–8-day-old captive hamsters of four Study Groups: pure Phodopus sungorus; pure P. campbelli of two populations (Mongolian and Kosh-Agach) and hybrids between male P. sungorus and female P. campbelli (Kosh-Agach). All Study Groups produced two categories of ultrasonic calls: Low-Frequency centered around 41 kHz and High-Frequency centered around 60 kHz, but in different percentages. Between populations, only Low-Frequency calls were shorter and higher-frequency in Mongolian P. campbelli. Between species, only High-Frequency calls were shorter and higher-frequency in P. sungorus. In hybrids, Low-Frequency calls were shorter and lower-frequency than in either parental species, whereas High-Frequency calls were longer and lower-frequency in hybrids than in pure P. sungorus but similar with another parental species. We discuss that interspecific hybridization may give rise to offspring with new properties of ultrasonic calls.

Vocalisation rates of the North Atlantic right whale (Eubalaena glacialis)

Vocalisation rates were measured from North Atlantic right whales (Eubalaena glacialis) in spring 1999-2000 in the Great South Channel and off Cape Cod, USA, and in summer 1999-2000 in the Bay of Fundy, Canada. Vocalisations were classed as either ‘moans’, ‘low-frequency (LF) calls’ or ‘gunshots’. Towed hydrophone recordings (36.1 hours) were made in 21 encounters where loose aggregations of right whales were within about 1,000m. Recordings were also made using acoustic tags attached by suction cups to ten different whales (29.5 hours). Tags also recorded depth data. Moan rates (sounds per aggregation per hour) were correlated with size of whale aggregation. Individual whales produced moans at ~ 0-10 per hour (recorded from tags and the towed hydrophone). Small aggregations (2-10) gave higher moan rates (usually < ~ 60 per hr) and larger aggregations ( > 10) higher still ( ~ 70-700 per hr) (recorded from towed hydrophone). Results from the Bay of Fundy indicate high moan rates at night. Moans were usually produced in clusters. Tag data showed that moans were usually produced when whales were within about 10m of the surface. A passive acoustic system could potentially provide supplementary information on the distribution of aggregations of right whales. This could be useful for management (1) in the long term, by aiding the prediction of right whale distribution, or (2) as a real-time tool for helping to route shipping away from concentrations of right whales. The empirical evidence presented here on vocalisation rates will assist in assessing feasibility. The clustering of moans and the tendency to produce them near the surface could hamper detection and localisation efforts. Further research is underway to investigate other important practical issues such as detectability and source levels.

Mid-frequency song and low-frequency calls of sei whales in the Falkland Islands.

Although sei whales (Balaenoptera borealis) are distributed throughout the globe, their behaviour and vocal repertoire are poorly described. We used passive acoustic monitoring to describe the vocal behaviour of sei whales in the Falkland Islands, between December 2018 and April 2019. We isolated more than 2000 low-frequency calls for manual classification, of which 510 calls with high signal-to-noise ratio were quantitatively measured. Five categories of stereotyped call types in the 15-230 Hz range were described, some with multiple subcategories. These included some similar to previously described calls (e.g. downsweeps), but others that were novel in acoustic structure and frequency band. In the mid-frequency range, we documented a highly stereotyped, hierarchically structured and rhythmically repetitive song display. Songs were arranged in phrases with a structure composed of repetitive sub-phrases, and a diverse variety of sounds in the 1-5 kHz range. Singing commenced in late February, despite the presence of whales and calls since early December, and continued through April. These acoustic properties and behavioural characteristics indicate that this is likely a male breeding display similar to songs and singing of other balaenopterids. This is the first detailed description of a song display for sei whales, highlighting the importance of the Falkland Islands.

A rallid ballad: Communal signaling is correlated with year-round territoriality in the most duet-rich family of birds (Gruiformes: Rallidae)

Abstract Duetting, in which 2 or more individuals call in coordinated unison, is common in birds, yet most research investigating the function of avian duets has focused only on oscine passerines. However, we have discovered that duetting occurs in 61 species (59%) in the family Rallidae (rails), one of the highest known rates in any bird clade, and that rail duets are also significantly associated with year-round territoriality and habitat type. We applied a comparative approach to study the occurrence of duetting relative to socio-ecological traits and call properties in 103 rail species, with the prediction that duetting rails produce low-frequency calls that spread well over short distances in densely vegetated habitats. Using a model correcting for phylogenetic signal, we found that duetting is the ancestral state in Rallidae, and that duetting rails show a trending effect for being sedentary and nonmigratory. Contrary to our predictions, there was no significant correlation between rail duetting and social bond length, breeding system, breeding latitude from the equator, or sexual dimorphism, as year-round territoriality and forest or heterogeneous habitats were the strongest predictors of duets. Despite the prevalence of duetting in Rallidae, few studies have tested duet functions such as territory defense in this family, and our comparative phylogenetic study lays the groundwork for future research, as little remains known about the behavioral ecology and vocal interactions of many rails.

Comparative Analysis of the Vocal Repertoires of the Indri (Indri indri) and the Diademed Sifaka (Propithecus diadema)

Strepsirrhine vocalisations are extraordinarily diverse and cross-species comparisons are needed to explore how this variability evolved. We contributed to the investigation of primate acoustic diversity by comparing the vocal repertoire of two sympatric lemur species, Propithecus diadema and Indri indri. These diurnal species belong to the same taxonomic family and have similar activity patterns but different social structures. These features make them excellent candidates for an investigation of the phylogenetic, environmental, and social influence on primate vocal behavior. We recorded 3 P. diadema groups in 2014 and 2016. From 1,872 recordings we selected and assigned 3814 calls to 9 a priori call types, on the basis of their acoustic structure. We implemented a reproducible technique performing an acoustic feature extraction relying on frequency bins, t-SNE data reduction, and a hard-clustering analysis. We first quantified the vocal repertoire of P. diadema, finding consistent results for the 9 putatively identified call types. When comparing this repertoire with a previously published repertoire of I. indri, we found highly species-specific repertoires, with only 2% of the calls misclassified by species identity. The loud calls of the two species were very distinct, while the low-frequency calls were more similar. Our results pinpoint the role of phylogenetic history, social and environmental features on the evolution of communicative systems and contribute to a deeper understanding of the evolutionary roots of primate vocal differentiation. We conclude by arguing that standardized and reproducible techniques, like the one we employed, allow robust comparisons and should be prioritized in the future.

Endurance rivalry and female choice jointly influence male mating success in the emerald treefrog (Zhangixalus prasinatus), a lek-chorusing anuran

BackgroundEndurance rivalry and female choice are two important mechanisms of sexual selection in lek-breeding species. Endurance rivalry is when males compete for opportunities to mate by spending more time in leks than others (interaction-independent male-male competition). Because high-quality males can afford to have high lek attendance, females have a higher chance of mating with good-quality males even when they mate randomly. The good gene hypothesis proposes that females can pass good genes on to their offspring by choosing males that display elaborate morphological and/or behavioral traits that reflect the males’ genetic quality. The relative importance of lek attendance and female choice to males’ mating success in anurans is rarely evaluated. In this study, we investigated how these two mechanisms might jointly shape males’ morphological traits in the lek-chorusing emerald treefrog Zhangixalus prasinatus.ResultsOur results show that (1) male lek attendance is positively correlated with body size and condition, and males with higher lek attendance have higher mating success, (2) the dominant frequency of males’ advertisement calls are negatively correlated with body size and males producing lower frequency calls have higher mating success, (3) male body size, but not body condition, has a non-significant positive relationship with mating success and (4) females show preference for calls with lower dominant frequencies in two-choice playback.ConclusionsOverall, both endurance rivalry and female choice play an important role in the mating success of male emerald treefrogs in the field and both are influenced by male body size/condition. By mating with males that have higher lek attendance and produce lower frequency calls, selection may indirectly favor larger males.

Classification of pig calls produced from birth to slaughter according to their emotional valence and context of production

Vocal expression of emotions has been observed across species and could provide a non-invasive and reliable means to assess animal emotions. We investigated if pig vocal indicators of emotions revealed in previous studies are valid across call types and contexts, and could potentially be used to develop an automated emotion monitoring tool. We performed an analysis of an extensive and unique dataset of low (LF) and high frequency (HF) calls emitted by pigs across numerous commercial contexts from birth to slaughter (7414 calls from 411 pigs). Our results revealed that the valence attributed to the contexts of production (positive versus negative) affected all investigated parameters in both LF and HF. Similarly, the context category affected all parameters. We then tested two different automated methods for call classification; a neural network revealed much higher classification accuracy compared to a permuted discriminant function analysis (pDFA), both for the valence (neural network: 91.5%; pDFA analysis weighted average across LF and HF (cross-classified): 61.7% with a chance level at 50.5%) and context (neural network: 81.5%; pDFA analysis weighted average across LF and HF (cross-classified): 19.4% with a chance level at 14.3%). These results suggest that an automated recognition system can be developed to monitor pig welfare on-farm.

Mate Preference Plasticity in a Critically Endangered Frog: Implications for Conservation Breeding

Variation in female mate preferences for male traits remains poorly understood (both among and within females), despite having important evolutionary and conservation implications, particularly for captive breeding. Here, we investigate female mate preferences for male advertisement call frequency, and determine whether preferences vary over repeated trials, in the critically endangered southern corroboree frog,Pseudophryne corroboree. We conducted a series of phonotaxis trials in a six-speaker arena where naïve, captive-bred, virgin females were offered a choice between low, average and high frequency male advertisement calls, with a subset of females tested repeatedly. In the first trial, we found no evidence for a population-level preference for call frequency, but females spent less time in the low call zone than expected by chance. However, our results showed that female mate preferences changed over sequential trials. Females spent significantly more time in the low frequency call zone in the third trial compared to the first trial, and, in the last trial, females exhibited a significant population-level preference for low frequency calls. Subsequently, repeatability estimates of female preferences were low and did not significantly deviate from zero. Our results indicate that femaleP. corroboreemate preferences can exhibit temporal variation, and suggest that females are more attracted to low call frequencies after repeated exposure. These findings imply that femaleP. corroboreemay become choosier over time, and highlight the potential for mate preferences to exhibit phenotypic plasticity within a single reproductive cycle. Overall, these findings provide the first information on mate preferences inP. corroboree, and emphasize the importance of considering individual variation in mate choice studies. From a conservation perspective, knowledge of individual variation in female mate preferences may be used to conduct behavioral manipulations in captivity that facilitate the breeding of genetically valuable individuals, and improve the success of conservation breeding programs.

Bats (Plecotus auritus) use contact calls for communication among roost mates

Communication between group members is mediated by a diverse range of signals. Contact calls are produced by many species of birds and mammals to maintain group cohesion and associations among individuals. Contact calls in bats are typically relatively low-frequency social calls, produced only for communication. However, echolocation calls (higher in frequency and used primarily for orientation and prey detection) can also facilitate interaction among individuals and location of conspecifics in the roost. We studied calling behaviour of brown long-eared bats (Plecotus auritus) during return to maternity roosts in response to playbacks of social and echolocation calls. We hypothesised that calling by conspecifics would elicit responses in colony members. Bat responses (inspection flights and social calls production) were significantly highest during social call and echolocation call playbacks than during noise (control) playbacks. We suggest that social calling in maternity roosts of brown long-eared bat evolved to maintain associations among roostmates, rather than to find roosts or roostmates, because this species is strongly faithful to roosts and the social groups and roosts are stable over time and space. Living in a stable social group requires recognition of group members and affiliation of social bonds with group members, features that may be mediated by vocal signals.

Monitoring fin whale calls using Ocean Observatories Initiative Regional Cabled Array

The Ocean Observatory Initiative (OOI) Regional Cable Array (RCA) and the Ocean Networks Canada NEPTUNE cabled observatory provide an opportunity for long-term studies of the distribution and behavior of calling marine mammals. We will present the results of a study to build a database of “20-Hz” fin whale calls that spans from 2015 to present. Fin whale calls were collected by bottom-mounted low-frequency hydrophones and seismometers deployed on the OOI RCA and are detected automatically by spectrogram cross-correlation with a detection kernel that sweeps down linearly from ∼26 Hz to ∼19 Hz with a duration of ∼0.8 s. The kernel is tuned to detect the high amplitude portions of both the high- and low-frequency calls of the doublet calling pattern which is currently the dominant fin whale song in this area. Preliminary results show significant interannual variations at a given site in the distribution of calls during the fall and winter calling season. The onset of calling appears to be significantly earlier near the continental slope off central Oregon than at Axial Seamount, 450 km offshore. There also appears to be a higher overall rate of calling nearer the continental slope.

Ontogeny of audible squeaks in yellow steppe lemming Eolagurus luteus: trend towards shorter and low-frequency calls is reminiscent of those in ultrasonic vocalization

BackgroundRodents are thought to be produced their human-audible calls (AUDs, below 20 kHz) with phonation mechanism based on vibration of the vocal folds, whereas their ultrasonic vocalizations (USVs, over 20 kHz) are produced with aerodynamic whistle mechanism. Despite of different production mechanisms, the acoustic parameters (duration and fundamental frequency) of AUDs and USVs change in the same direction along ontogeny in collared lemming Dicrostonyx groenlandicus and fat-tailed gerbil Pachyuromys duprasi. We hypothesize that this unidirectional trend of AUDs and USVs is a common rule in rodents and test whether the AUDs of yellow steppe lemmings Eolagurus luteus would display the same ontogenetic trajectory (towards shorter and low-frequency calls) as their USVs, studied previously in the same laboratory colony.ResultsWe examined for acoustic variables 1200 audible squeaks emitted during 480-s isolation-and-handling procedure by 120 individual yellow steppe lemmings (at 12 age classes from neonates to breeding adults, 10 individuals per age class, up to 10 calls per individual, each individual tested once). We found that the ontogenetic pathway of the audible squeaks, towards shorter and lower frequency calls, was the same as the pathway of USVs revealed during 120-s isolation procedure in a previous study in the same laboratory population. Developmental milestone for the appearance of mature patterns of the squeaks (coinciding with eyes opening at 9–12 days of age), was the same as previously documented for USVs. Similar with ontogeny of USVs, the chevron-like squeaks were prevalent in neonates whereas the squeaks with upward contour were prevalent after the eyes opening.ConclusionThis study confirms a hypothesis of common ontogenetic trajectory of call duration and fundamental frequency for AUDs and USVs within species in rodents. This ontogenetic trajectory is not uniform across species.

Tenors Not Sopranos: Bio-Mechanical Constraints on Calling Song Frequencies in the Mediterranean Field-Cricket

Male crickets and their close relatives bush-crickets (Gryllidae and Tettigoniidae, respectively; Orthoptera and Ensifera) attract distant females by producing loud calling songs. In both families, sound is produced by stridulation, the rubbing together of their forewings, whereby the plectrum of one wing is rapidly passed over a serrated file on the opposite wing. The resulting oscillations are amplified by resonating wing regions. A striking difference between Gryllids and Tettigoniids lies in wing morphology and composition of song frequency: Crickets produce mostly low-frequency (2–8 kHz), pure tone signals with highly bilaterally symmetric wings, while bush-crickets use asymmetric wings for high-frequency (10–150 kHz) calls. The evolutionary reasons for this acoustic divergence are unknown. Here, we study the wings of actively stridulating male field-crickets (Gryllus bimaculatus) and present vibro-acoustic data suggesting a biophysical restriction to low-frequency song. Using laser Doppler vibrometry (LDV) and brain-injections of the neuroactivator eserine to elicit singing, we recorded the topography of wing vibrations during active sound production. In freely vibrating wings, each wing region resonated differently. When wings coupled during stridulation, these differences vanished and all wing regions resonated at an identical frequency, that of the narrow-band song (∼5 kHz). However, imperfections in wing-coupling caused phase shifts between both resonators, introducing destructive interference with increasing phase differences. The effect of destructive interference (amplitude reduction) was observed to be minimal at the typical low frequency calls of crickets, and by maintaining the vibration phase difference below 80°. We show that, with the imperfect coupling observed, cricket song production with two symmetric resonators becomes acoustically inefficient above ∼8 kHz. This evidence reveals a bio-mechanical constraint on the production of high-frequency song whilst using two coupled resonators and provides an explanation as to why crickets, unlike bush-crickets, have not evolved to exploit ultrasonic calling songs.

Selection on vocal output affects laryngeal morphology in rats.

Although laryngeal morphology often reflects adaptations for vocalization, the structural consequences of selection for particular aspects of vocal behavior remain poorly understood. In this study, we investigated the effects of increased ultrasonic calling in pups on the adult larynx morphology in selectively bred rat lines. Laryngeal morphology was assessed using multiple techniques: mineralized cartilage volumes were compared in 3D‐models derived from microCT scans, internal structure was compared using clearing and staining procedures combined with microscopy, cellular structure was compared using histology and microscopy, and element composition was assessed with scanning energy dispersive X‐ray spectroscopy. Our results show that adult rats from lines bred to produce ultrasonic calls at higher rates as pups have shorter vocal folds and a more mineralized thyroid cartilage compared to rats bred to produce ultrasonic calls at lower rates. The change in vocal fold length appears to account for differences in low‐frequency calls in these two rat lines. We suggest that the observed increases in mineralization of the thyroid cartilage in the high‐ultrasound lineage provide increased reinforcement of the laryngeal structure during ultrasonic call production. Our findings therefore demonstrate an effect of selection for vocal behavior on laryngeal morphology, with acoustic consequences.