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Individual recognition is fundamental to the social behaviour of many animals. In the context of territorial behaviour, animals in high-density populations encounter conspecific rivals and potential mates more frequently, which should enhance the individuality of territorial signals to facilitate recognition among conspecifics. We investigated vocal individuality in male territorial calls of two populations of little owls ( Athene noctua ) with different densities. Further, to explore the potential influence of local population distribution on individuality, we also examined isolated males without neighbours and clumped males with neighbours. Our findings indicate higher individuality at higher densities across both scenarios, measured using two individuality metrics: Beecher’s information statistic and Discrimination score. Clumped males exhibited significantly lower acoustic niche overlaps (i.e. higher vocal individuality) compared to isolated males. However, only a non-significant trend for lower acoustic niche overlaps (i.e. higher vocal individuality) was found for males from high density compared to low density populations. This suggests that the immediate social environment might be more influential than larger-scale population density patterns. This study suggests that vocal individuality in a territorial species is influenced by conspecific density, similar to findings in group-living and colonial species.

Humans exhibit a high level of vocal plasticity in speech production, which allows us to acquire both native and foreign languages and dialects, and adapt to local accents in social communication. In comparison, non-human primates exhibit limited vocal plasticity, especially in adulthood, which would limit their ability to adapt to different social and environmental contexts in vocal communication. Here, we quantitatively examined the ability of adult common marmosets ( Callithrix jacchus ), a highly vocal New World primate species, to modulate their vocal production in social contexts. While recent studies have demonstrated vocal learning in developing marmosets, we know much less about the extent of vocal learning and plasticity in adult marmosets. We found, in the present study, that marmosets were able to adaptively modify the spectrotemporal structure of their vocalizations when they encountered interfering sounds. Our experiments showed that marmosets shifted the spectrum of their vocalizations away from the spectrum of the interfering sounds in order to avoid the overlap. More interestingly, we found that marmosets made predictive and long-lasting spectral shifts in their vocalizations after they had experienced a particular type of interfering sound. These observations provided evidence for directional control of the vocalization spectrum and long-term vocal plasticity by adult marmosets. The findings reported here have important implications for the ability of this New World primate species in voluntarily and adaptively controlling their vocal production in social communication.

It has been observed in many songbird species that populations in noisy urban areas sing with a higher minimum frequency than do matched populations in quieter, less developed areas. However, why and how this divergence occurs is not yet understood. We experimentally tested whether chronic noise exposure during vocal learning results in songs with higher minimum frequencies in great tits (Parus major), the first species for which a correlation between anthropogenic noise and song frequency was observed. We also tested vocal plasticity of adult great tits in response to changing background noise levels by measuring song frequency and amplitude as we changed noise conditions. We show that noise exposure during ontogeny did not result in songs with higher minimum frequencies. In addition, we found that adult birds did not make any frequency or song usage adjustments when their background noise conditions were changed after song crystallization. These results challenge the common view of vocal adjustments by city birds, as they suggest that either noise itself is not the causal force driving the divergence of song frequency between urban and forest populations, or that noise induces population-wide changes over a time scale of several generations rather than causing changes in individual behaviour.

Abstract On the occasion of the centenary of the discovery of the Lombard effect, we review the literature on noise-dependent regulation of vocal amplitude in humans and other animals. The article addresses the scientific and the biological history of the Lombard effect: first, it sketches the evolution of the study of the Lombard effect, and second it reflects on the biological evolution of the effect itself. By comparing the findings from anurans, birds and mammals, we try to trace back the phylogenetic origins of this basic vocal mechanism for acoustic communication in noise. The current evidence suggests two alternative parsimonious hypotheses: either the Lombard effect is the outcome of a convergent evolution in birds and mammals or it may be a synapomorphy of all amniotes. If the latter is true, then the Lombard effect would have evolved to maintain vocal communication in the presence of noise more than 300 million years ago.

Vocal duetting occurs when two individuals produce repeated, stereotyped vocalizations, often with alternating contributions. It evolved independently in many pair living taxa. Among hominoids, only hylobatids duet, but little is known about how mated pairs coordinate singing and if individuals adjust their song to spectral and temporal aspects of another's song. If they do, this would demonstrate vocal flexibility not yet well documented in apes. To test this hypothesis, we analyzed duets of wild white-handed gibbons (Hylobates lar), quantifying female notes and the timing of male notes relative to them. We measured changes in female notes that preceded her great call phrase and that she initiated only after her mate had stopped singing. We predicted males would suspend their own song during a female's great call phrase in anticipation of her climax, and thus should interrupt abnormal female calls, because these typically fail to climax. We compared (1) interrupted great call phrases females aborted, (2) interrupted phrases that were completed, and (3) uninterrupted, completed phrases. We found that abnormal great call phrases were interrupted by males, likely because the male anticipated that the phrase would be abandoned before reaching its climax. Although female call phrases varied in length, we also found that males replied in close synchrony with their ending. Subtle spectral and temporal variations in song influenced the timing of a mate's singing and thus the structure and delivery of duets. Although ape vocal behavior is thought to be largely innate, our findings show unexpected flexibility in song expression.

Background Most vocal learning species exhibit an early critical period during which their vocal control neural circuitry facilitates the acquisition of new vocalizations. Some taxa, most notably humans and parrots, retain some degree of neurobehavioral plasticity throughout adulthood, but both the extent of this plasticity and the neurogenetic mechanisms underlying it remain unclear. Differential expression of the transcription factor FoxP2 in both songbird and parrot vocal control nuclei has been identified previously as a key pattern facilitating vocal learning. We hypothesize that the resilience of vocal learning to cognitive decline in open-ended learners will be reflected in an absence of age-related changes in neural FoxP2 expression. We tested this hypothesis in the budgerigar ( Melopsittacus undulatus ), a small gregarious parrot in which adults converge on shared call types in response to shifts in group membership. We formed novel flocks of 4 previously unfamiliar males belonging to the same age class, either “young adult” (6 mo − 1 year) or “older adult” (≥ 3 year), and then collected audio-recordings over a 20-day learning period to assess vocal learning ability. Following behavioral recording, immunohistochemistry was performed on collected neural tissue to measure FoxP2 protein expression in a parrot vocal learning center, the magnocellular nucleus of the medial striatum (MMSt), and its adjacent striatum. Results Although older adults show lower vocal diversity (i.e. repertoire size) and higher absolute levels of FoxP2 in the MMSt than young adults, we find similarly persistent downregulation of FoxP2 and equivalent vocal plasticity and vocal convergence in the two age cohorts. No relationship between individual variation in vocal learning measures and FoxP2 expression was detected. Conclusions We find neural evidence to support persistent vocal learning in the budgerigar, suggesting resilience to aging in the open-ended learning program of this species. The lack of a significant relationship between FoxP2 expression and individual variability in vocal learning performance suggests that other neurogenetic mechanisms could also regulate this complex behavior.

Abstract Duets in pair-bonding primates serve as a primary mode of communication between pairs, and duets may provide cues to conspecifics regarding the calling individual or pair. Here, we test the hypothesis that pulse elements in coppery titi monkey duets vary with condition and identity of the caller. We predicted that pulse elements would vary with age, sex, or pair-bond length. We estimated pulse rate and duration for 378 pulse elements from the duets of 74 captive titi monkeys (Plecturocebus cupreus). We found inter-individual variation in both features, and evidence for vocal convergence among pair mates in pulse rate. Age was the best predictor of pulse rate, and pulse rate decreased with age. Age and pair-bond length reliably predicted pulse duration. Our results suggest that variation in titi monkey duets reflects differences in caller condition and pair identity, and contribute to growing evidence for vocal plasticity in nonhuman primates.

Molossops temminckii is the only species of the Molossidae capable of exploring background-cluttered space. We tested the hypothesis that M. temminckii uses the clutter rejection strategy in cluttered environments and determined whether the echolocation calls of this species show large-scale geographic variation. We recorded echolocation calls of 10 individuals of M. temminckii using 4 treatments with different degrees of clutter. We compared individuals recorded in Venezuela with individuals recorded in Brazil to verify geographic variation in calls. In the cluttered environments, M. temminckii emitted short downward frequency-modulated calls with short pulse intervals. In the uncluttered environments, M. temminckii emitted long upward frequency-modulated calls with longer intervals and shorter bandwidth. The decreased pulse duration and interval in cluttered environments support the hypothesis that M. temminckii uses the clutter rejection strategy. Additionally, we found that in Brazil, M. temminckii emits shorter calls with higher minimum frequency compared with bats recorded in Venezuela, indicating large-scale geographic variation, probably because of environmental factors. Molossops temminckii é a única espécie de Molossidae com capacidade de explorar áreas com obstáculos de fundo. Nesse trabalho testamos a hipótese do uso da estratégia de rejeição de obstáculo em ambientes fechados, e determinamos se há variação geográfica de larga escala nos chamados de ecolocalização de M. temminckii. Gravamos 10 indivíduos, em 4 tratamentos, com diferentes graus de obstáculos. Comparamos os chamados de ecolocalização gravados na Venezuela e Brasil para verificar a variação geográfica. Em ambientes com obstáculos, M. temminckii emite chamados curtos, de frequência modulada descendente, com intervalos curtos entre os chamados. Nos ambientes abertos, emite chamados longos, de frequência modulada ascendente, com intervalos mais longos e menor largura de banda. A menor duração e o menor intervalo em ambientes fechados suportam a hipótese que M. temminckii utiliza a estratégia de rejeição de obstáculo. No Brasil, M. temminckii emite chamados mais curtos, com frequência mínima mais alta, em comparação com os morcegos gravados na Venezuela. Esse fato indica variação geográfica de larga escala e fatores ambientais provavelmente contribuem para essa variação.

Natural background noises are common in the acoustic environments in which most organisms have evolved. Therefore, the vocalization and sound perception systems of vocal animals are inherently equipped to overcome natural background noise. Human-generated noises, however, pose new challenges that can hamper audiovocal communication. The mechanisms animals use to cope with anthropogenic noise disturbances have been extensively explored in a variety of taxa. Bats emit echolocation pulses primarily to orient, locate and navigate, while social calls are used to communicate with conspecifics. Previous studies have shown that bats alter echolocation pulse parameters in response to background noise interference. In contrast to high-frequency echolocation pulses, relatively low-frequency components within bat social calls overlap broadly with ambient noise frequencies. However, how bats structure their social calls in the presence of anthropogenic noise is not known. Here, we hypothesized that bats leverage vocal plasticity to facilitate vocal exchanges within a noisy environment. To test this hypothesis, we subjected the Asian particolored bat, Vespertilio sinensis, to prerecorded traffic noise. We observed a significant decrease in vocal complexity (i.e., an increased frequency of monosyllabic calls) in response to traffic noise. However, an increase in the duration and frequency of social calls, as have been observed in other species, was not evident. This suggests that signal simplification may increase communication efficacy in noisy environments. Moreover, V. sinensis also increased call amplitude in response to increased traffic noise, consistent with the predictions of the Lombard effect.

ContextAnthropogenic noise is relatively new to natural soundscapes and may have adverse effects on acoustically active species. In birds, adverse effects include changes in vocalization patterns. Helicopters and songbirds are ubiquitous in protected natural areas but the effect of helicopter noise on songbirds has never been assessed.ObjectivesOur objective was to determine if helicopter noise affects temporal characteristics of songbird vocalizations within protected natural areas.MethodsWe collected soundscape recordings in three protected areas with varying levels of helicopter tour activity, including one with the second highest air tour activity in the United States. We examined songbird response to helicopter noise at the species level by measuring changes in vocalization time, and at the community level by employing the Bioacoustic Index, an acoustic measure of biological sound.ResultsWe found a strong association between increasing helicopter noise and songbird vocalizations, indicating that some bird species use temporal shifts in vocalizations to mitigate masking effects from helicopter noise. The strength and direction of the response was species-specific, suggesting differences in resilience to helicopter noise between species. Furthermore, our results suggest that bird response to helicopter noise is strongest in areas with very loud and frequent helicopter traffic.ConclusionsOur study is the first to examine and demonstrate impacts of helicopter noise on songbird vocalizations. Our results may serve as the foundation of an air tour management plan that considers reducing the number of helicopter overflights over protected natural areas and enforcing higher flight altitudes to decrease noise power levels.