Evolutionary divergence in acoustic signals: causes and consequences

Assortative Mating by an Obliquely Transmitted Local Cultural Trait Promotes Genetic Divergence: A Model

The effect of learned culture (e.g., birdsong dialects and human languages) on genetic divergence is unclear. Previous theoretical research suggests that because oblique learning allows phenotype transmission from individuals with no offspring to an unrelated individual in the next generation, the effect of sexual selection on the learned trait is masked. However, I propose that migration and spatially constrained learning can form statistical associations between cultural and genetic traits, which may allow selection on the cultural traits to indirectly affect the genetic traits. Here, I build a population genetic model that allows such statistical associations to form and find that sexual selection and divergent selection on the cultural trait can indeed help maintain genetic divergence through such statistical associations, while selection against genetic hybrids does not affect cultural trait divergence. Furthermore, I find that even when the cultural trait changes over time due to drift and mutation, it can still help maintain genetic divergence. These results suggest the role of obliquely transmitted traits in evolution may be underrated, and the lack of one-to-one associations between cultural and genetic traits may not be sufficient to disprove the role of culture in genetic divergence.

Baboon vocal repertoires and the evolution of primate vocal diversity

A remarkable and derived trait of humans is the faculty for language, and considerable research effort has been devoted to understanding the evolution of speech. In contrast to spoken language, which constitutes a (learned) symbolic communication system, the acoustic structure of nonhuman primate vocalizations is largely genetically fixed. Yet, appreciable differences between different genera and species may exist. Environmental conditions, sexual selection, and characteristics of the social system have been invoked to explain these differences. Here, we studied the acoustic variation of call types and vocal repertoires in the genus Papio. Because the genus comprises both stable groups as well as multi-level societies, and reveals striking variation in the degree of aggressiveness from south to north, it constitutes a promising model to assess the link between social system characteristics and vocal communication. We found that, the vocal repertoires of the different species were composed of the same general call types. A quantitative analysis of the acoustic features of the grunts and loud calls of chacma (Papio ursinus), olive (P. anubis), and Guinea (P. papio) baboons showed subtle acoustic differences within call types, however. Social system characteristics did not map onto acoustic variation. We found no correlation between the structure of grunts and geographic distance; the same was true for female loud calls. Only for male loud calls from three populations, call structure varied with geographic distance. Our findings corroborate the view that the structure of nonhuman primate vocalizations is highly conserved, despite the differences in social systems. Apparently, variation in rate and intensity of occurrence of signals, probably due to different behavioral dispositions in species, are sufficient to allow for plasticity at the level of the social relationships, mating patterns, and social organization.

Acoustic communication in marine shallow waters: testing the acoustic adaptive hypothesis in sand gobies

Acoustic communication is an important part of social behaviour of fish species that live or breed in shallow noisy waters. Previous studies have shown that some fish species exploit a quiet window in the background noise for communication. However, it remains to be examined whether hearing abilities and sound production of fish are adapted to marine habitats presenting high hydrodynamism. Here, we investigated whether the communication system of the painted (Pomatoschistus pictus) and the marbled (Pomatoschistus marmoratus) gobies is adapted to enhance sound transmission and reception in Atlantic shallow water environments. We recorded and measured the sound pressure levels of social vocalisations of both species, as well as snapshots of ambient noise of habitats characterised by different hydrodynamics. Hearing thresholds (in terms of both sound pressure and particle acceleration) and responses to conspecific signals were determined using the auditory evoked potential recording technique. We found that the peak frequency range (100-300 Hz) of acoustic signals matched the best hearing sensitivity in both species and appeared well adapted for short-range communication in Atlantic habitats. Sandy/rocky exposed beaches presented a quiet window, observable even during the breaking of moderate waves, coincident with the main sound frequencies and best hearing sensitivities of both species. Our data demonstrate that the hearing abilities of these gobies are well suited to detect conspecific sounds within typical interacting distances (a few body lengths) in Atlantic shallow waters. These findings lend support to the acoustic adaptive hypothesis, under the sensory drive framework, proposing that signals and perception systems coevolve to be effective within local environment constraints.

Physiological constraint on acrobatic courtship behavior underlies rapid sympatric speciation in bearded manakins

Physiology's role in speciation is poorly understood. Motor systems, for example, are widely thought to shape this process because they can potentiate or constrain the evolution of key traits that help mediate speciation. Previously, we found that Neotropical manakin birds have evolved one of the fastest limb muscles on record to support innovations in acrobatic courtship display (Fuxjager et al., 2016a). Here, we show how this modification played an instrumental role in the sympatric speciation of a manakin genus, illustrating that muscle specializations fostered divergence in courtship display speed, which may generate assortative mating. However, innovations in contraction-relaxation cycling kinetics that underlie rapid muscle performance are also punctuated by a severe speed-endurance trade-off, blocking further exaggeration of display speed. Sexual selection therefore potentiated phenotypic displacement in a trait critical to mate choice, all during an extraordinarily fast species radiation-and in doing so, pushed muscle performance to a new boundary altogether.

Multi-modal signal evolution in birds: re-examining a standard proxy for sexual selection

Sexual selection is proposed to be an important driver of speciation and phenotypic diversification in animal systems. However, previous phylogenetic tests have produced conflicting results, perhaps because they have focused on a single signalling modality (visual ornaments), whereas sexual selection may act on alternative signalling modalities (e.g. acoustic ornaments). Here, we compile phenotypic data from 259 avian sister species pairs to assess the relationship between visible plumage dichromatism-a standard index of sexual selection in birds-and macroevolutionary divergence in the other major avian signalling modality: song. We find evidence for a strong negative relationship between the degree of plumage dichromatism and divergence in song traits, which remains significant even when accounting for other key factors, including habitat type, ecological divergence and interspecific interactions. This negative relationship is opposite to the pattern expected by a straightforward interpretation of the sexual selection-diversification hypothesis, whereby higher levels of dichromatism indicating strong sexual selection should be related to greater levels of mating signal divergence regardless of signalling modality. Our findings imply a 'trade-off' between the elaboration of visual ornaments and the diversification of acoustic mating signals, and suggest that the effects of sexual selection on diversification can only be determined by considering multiple alternative signalling modalities.

Barking mad: The vocalisation of the John Dory, Zeus faber

Studies on the behavioural function of sounds are very rare within heterospecific interactions. John Dory (Zeus faber) is a solitary, predatory fish that produces sound when captured, but has not been documented to vocalize under natural conditions (i.e. in the wild). The present study provides the first in-situ recordings of John Dory vocalisations and correlates them to behavioural response of snapper (Pagrus auratus) a common species found through New Zealand. Vocalisations or ‘barks’, ranged between 200–600 Hz, with a peak frequency of 312 ± 10 Hz and averaged 139 ± 4 milliseconds in length. Baited underwater video (BUV) equipped with hydrophones determined that under natural conditions a John Dory vocalization induced an escape response in snapper present, causing them to exit the area opposite to the position of the John Dory. We speculate that the John Dory vocalisation may be used for territorial display towards both conspecifics and heterospecifics, asserting dominance in the area or heightening predatory status.

Evolutionarily conserved coding properties favour the neuronal representation of heterospecific signals of a sympatric katydid species

To function as a mechanism in premating isolation, the divergent and species-specific calling songs of acoustic insects must be reliably processed by the afferent auditory pathway of receivers. Here, we analysed the responses of interneurons in a katydid species that uses long-lasting acoustic trills and compared these with previously reported data for homologous interneurons of a sympatric species that uses short chirps as acoustic signals. Some interneurons of the trilling species respond exclusively to the heterospecific chirp due to selective, low-frequency tuning and "novelty detection". These properties have been considered as evolutionary adaptations in the sensory system of the chirper, which allow it to detect signals effectively during the simultaneous calling of the sympatric sibling species. We propose that these two mechanisms, shared by the interneurons of both species, did not evolve in the chirper to guarantee its ability to detect the chirp under masking conditions. Instead we suggest that chirpers evolved an additional, 2-kHz component in their song and exploited pre-existing neuronal properties for detecting their song under masking noise. The failure of some interneurons to respond to the conspecific song in trillers does not prevent intraspecific communication, as other interneurons respond to the trill.

High-frequency vocalizations in Andean hummingbirds

Hummingbirds possess a number of unique features. A recent report in Current Biology[1] showed that the Black Jacobin (Florisuga fusca), a Brazilian hummingbird in the Topazes clade, produces high-frequency vocalizations outside the known hearing range of birds. Here, we report that four hummingbird species in the Andean clade [2,3] also exhibit high-frequency vocalizations: the Ecuadorian Hillstar (Oreotrochilus chimborazo), with the highest fundamental frequency (mean F0 = 13.4 kHz), the Buff-tailed Coronet (Boissonneaua flavescens), Speckled Hummingbird (Adelomyia melanogenys) and Violet-tailed Sylph (Aglaiocercus coelestis). The presence of high-frequency vocalizations in hummingbirds belonging to different lineages poses the question of whether high-frequency vocalizations in this group of birds have been, so far, overlooked. These Andean species are closely related but live in two different habitats. Our characterization of ambient noise in each habitat suggests that the hummingbirds are exposed to different acoustic challenges, and that the frequency content of the vocalizations of the cloud-forest species is adapted accordingly. The function of these signals and the selection pressures driving their evolution remain unknown.

Understanding sources of variance and correlation among features of Bornean gibbon (Hylobates muelleri) female calls

Acoustic signals serve important functions in mate choice, resource defense, and species recognition. Quantifying patterns and sources of variation in acoustic signals can advance understanding of the evolutionary processes that shape behavioral diversity more broadly. Animal vocalization datasets are inherently multivariate and hierarchical, wherein multiple features are estimated from calls of many individuals across different recording locations. Patterns of variation within different hierarchical levels-notwithstanding the challenges they present for modeling and inference-can provide insight into processes shaping vocal variation. The current work presents a multivariate, variance components model to investigate three levels of variance (within-female, between-female, and between-site) in Bornean gibbon calls. For six of the eight features estimated from call spectrograms, between-female variance was the most important contributor to total variance. For one feature, trill rate, there were site-level differences, which may be related to geographic isolation of certain gibbon populations. There was also a negative relationship between trill rate and duration of the introduction, suggesting trade-offs in the production of gibbon calls. Given substantial inter-individual variation in gibbon calls, it seems likely that there has been selection to confer information regarding caller identity, but mechanisms leading to site-level variation in trill rate remain to be determined.

Evolutionary ecology of chemosensation and its role in sensory drive

All behaviors of an organism are rooted in sensory processing of signals from its environment, and natural selection shapes sensory adaptations to ensure successful detection of cues that maximize fitness. Sensory drive, or divergent selection for efficient signal transmission among heterogeneous environments, has been a useful hypothesis for describing sensory adaptations, but its current scope has primarily focused on visual and acoustic sensory modalities. Chemosensation, the most widespread sensory modality in animals that includes the senses of smell and taste, is characterized by rapid evolution and has been linked to sensory adaptations to new environments in numerous lineages. Yet, olfaction and gustation have been largely underappreciated in light of the sensory drive hypothesis. Here, we examine why chemosensory systems have been overlooked and discuss the potential of chemosensation to shed new insight on the sensory drive hypothesis and vice versa. We provide suggestions for developing a framework to better incorporate studies of chemosensory adaptation that have the potential to shape a more complete, coherent, and holistic interpretation of the sensory drive.

Electric pulse characteristics can enable species recognition in African weakly electric fish species

Communication is key to a wide variety of animal behaviours and multiple modalities are often involved in this exchange of information from sender to receiver. The communication of African weakly electric fish, however, is thought to be predominantly unimodal and is mediated by their electric sense, in which species-specific electric organ discharges (EODs) are generated in a context-dependent and thus variable sequence of pulse intervals (SPI). While the primary function of the electric sense is considered to be electrolocation, both of its components likely carry information regarding identity of the sender. However, a clear understanding of their contribution to species recognition is incomplete. We therefore analysed these two electrocommunication components (EOD waveform and SPI statistics) in two sympatric mormyrid Campylomormyrus species. In a set of five playback conditions, we further investigated which components may drive interspecific recognition and discrimination. While we found that both electrocommunication components are species-specific, the cues necessary for species recognition differ between the two species studied. While the EOD waveform and SPI were both necessary and sufficient for species recognition in C. compressirostris males, C. tamandua males apparently utilize other, non-electric modalities. Mapped onto a recent phylogeny, our results suggest that discrimination by electric cues alone may be an apomorphic trait evolved during a recent radiation in this taxon.

25 Years of sensory drive: the evidence and its watery bias

It has been 25 years since the formalization of the Sensory Drive hypothesis was published in the American Naturalist (1992). Since then, there has been an explosion of research identifying its utility in contributing to our understanding of inter- and intra-specific variation in sensory systems and signaling properties. The main tenet of Sensory Drive is that environmental characteristics will influence the evolutionary trajectory of both sensory (detecting capabilities) and signaling (detectable features and behaviors) traits in predictable directions. We review the accumulating evidence in 154 studies addressing these questions and categorized their approach in terms of testing for environmental influence on sensory tuning, signal characteristics, or both. For the subset of studies that examined sensory tuning, there was greater support for Sensory Drive processes shaping visual than auditory tuning, and it was more prevalent in aquatic than terrestrial habitats. Terrestrial habitats and visual traits were the prevalent habitat and sensory modality in the 104 studies showing support for environmental influence on signaling properties. An additional 19 studies that found no supporting evidence for environmental influence on signaling traits were all based in terrestrial ecosystems and almost exclusively involved auditory signals. Only 29 studies examined the complete coevolutionary process between sensory and signaling traits and were dominated by fish visual communication. We discuss biophysical factors that may contribute to the visual and aquatic bias for Sensory Drive evidence, as well as biotic factors that may contribute to the lack of Sensory Drive processes in terrestrial acoustic signaling systems.

Current knowledge on bioacoustics of the subfamily Lophyohylinae (Hylidae, Anura) and description of Ocellated treefrog Itapotihyla langsdorffii vocalizations

Background

Anuran vocalizations, such as advertisement and release calls, are informative for taxonomy because species recognition can be based on those signals. Thus, a proper acoustic description of the calls may support taxonomic decisions and may contribute to knowledge about amphibian phylogeny.

Methods

Here we present a perspective on advertisement call descriptions of the frog subfamily Lophyohylinae, through a literature review and a spatial analysis presenting bioacoustic coldspots (sites with high diversity of species lacking advertisement call descriptions) for this taxonomic group. Additionally, we describe the advertisement and release calls of the still poorly known treefrog, Itapotihyla langsdorffii. We analyzed recordings of six males using the software Raven Pro 1.4 and calculated the coefficient of variation for classifying static and dynamic acoustic properties.

Results and Discussion

We found that more than half of the species within the subfamily do not have their vocalizations described yet. Most of these species are distributed in the western and northern Amazon, where recording sampling effort should be strengthened in order to fill these gaps. The advertisement call of I. langsdorffii is composed of 3–18 short unpulsed notes (mean of 13 ms long), presents harmonic structure, and has a peak dominant frequency of about 1.4 kHz. This call usually presents amplitude modulation, with decreasing intensity along the sequence of notes. The release call is a simple unpulsed note with an average duration of 9 ms, and peak dominant frequency around 1.8 kHz. Temporal properties presented higher variations than spectral properties at both intra- and inter-individual levels. However, only peak dominant frequency was static at intra-individual level. High variability in temporal properties and lower variations related to spectral ones is usual for anurans; The first set of variables is determined by social environment or temperature, while the second is usually related to species-recognition process. Here we review and expand the acoustic knowledge of the subfamily Lophyohylinae, highlighting areas and species for future research.

Geographical variation in the echolocation calls of bent-winged bats, Miniopterus fuliginosus

Evolutionary biologists had a long-standing interest in the evolutionary forces underlying geographical variation in the acoustic signals of animals. However, the evolutionary forces driving acoustic variation are still unclear. In this study, we quantified the geographical variation in the peak frequencies of echolocation calls in eight Miniopterus fuliginosus bat colonies, and assessed the forces that drive acoustic divergence. Our results demonstrated that seven of the colonies had very similar peak frequencies, while only one colony was significantly higher than the others. This similarity in echolocation call frequency among the seven colonies was likely due to frequent dispersal and migration, leading to male-mediated infiltration of nuclear genes. This infiltration enhances gene flow and weakens ecological selection, and also increases interactions in the presence of conspecifics. Significant correlations were not observed between acoustic distances and morphological distances, climatic differences, geographic distances or mtDNA genetic distances. However, variation in acoustic distances was significantly positive correlated with nDNA genetic distance, even after controlling for geographic distance. Interestingly, the relationship between call divergence and genetic distance was no longer significant after excluding the colony with the highest call frequency, which may be due to the minimal genetic distance among the other seven colonies. The highest frequencies of echolocation calls observed in the one colony may be shaped by selection pressure due to loud background noise in the area. Taken together, these results suggest that geographic divergence of echolocation calls may not be subject to genetic drift, but rather, that the strong selective pressure induced by background noise may lead to acoustic and genetic differentiation between JXT and the other colonies.

Urban sparrows respond to a sexually selected trait with increased aggression in noise

Animals modify acoustic communication signals in response to noise pollution, but consequences of these modifications are unknown. Vocalizations that transmit best in noise may not be those that best signal male quality, leading to potential conflict between selection pressures. For example, slow paced, narrow bandwidth songs transmit better in noise but are less effective in mate choice and competition than fast paced, wide bandwidth songs. We test the hypothesis that noise affects response to song pace and bandwidth in the context of competition using white-crowned sparrows (Zonotrichia leucophrys). We measure male response to song variation along a gradient of ambient noise levels in San Francisco, CA. We find that males discriminate between wide and narrow bandwidth songs but not between slow and fast paced songs. These findings are biologically relevant because songs in noisy areas tend to have narrow bandwidths. Therefore, this song phenotype potentially increases transmission distance in noise, but elicits weaker responses from competitors. Further, we find that males respond more strongly to stimuli in noisier conditions, supporting the ‘urban anger’ hypothesis. We suggest that noise affects male responsiveness to song, possibly leading to more territorial conflict in urban areas.

Widespread plant specialization in the polyphagous planthopper Hyalesthes obsoletus (Cixiidae), a major vector of stolbur phytoplasma: Evidence of cryptic speciation

The stolbur phytoplasma vector Hyalesthes obsoletus is generally considered as a polyphagous species associated with numerous wild and cultivated plants. However, recent research in southeastern Europe, the distribution centre of H. obsoletus and the area of most stolbur-inflicted crop diseases, points toward specific host-plant associations of the vector, indicating specific vector-based transmission routes. Here, we study the specificity of populations associated with four host-plants using mitochondrial and nuclear genetic markers, and we evaluate the evolution of host-shifts in H. obsoletus. Host-plant use was confirmed for Convolvulus arvensis, Urtica dioica, Vitex agnus-castus and Crepis foetida. Mitochondrial genetic analysis showed sympatric occurrence of three phylogenetic lineages that were ecologically delineated by host-plant preference, but were morphologically inseparable. Nuclear data supported the existence of three genetic groups (Evanno’s ΔK(3) = 803.72) with average genetic membership probabilities > 90%. While populations associated with C. arvensis and U. dioica form a homogenous group, populations affiliated with V. agnus-castus and C. foetida constitute two independent plant-associated lineages. The geographical signal permeating the surveyed populations indicated complex diversification processes associated with host-plant selection and likely derived from post-glacial refugia in the eastern Mediterranean. This study provides evidence for cryptic species diversification within H. obsoletus sensu lato: i) consistent mitochondrial differentiation (1.1–1.5%) among host-associated populations in syntopy and in geographically distant areas, ii) nuclear genetic variance supporting mitochondrial data, and iii) average mitochondrial genetic distances among host-associated meta-populations are comparable to the most closely related, morphologically distinguishable species, i.e., Hyalesthes thracicus (2.1–3.3%).

Radiation efficiency for long-range vocal communication in mammals and birds

Long-distance vocal communication by birds and mammals, including humans, is facilitated largely by radiation efficiency from the mouth or beak. Here, this efficiency is defined and quantified. It depends on frequency content of vocalization, mouth opening, head and upper body geometry, and directionality. Each of these factors is described mathematically with a piston-in-a-sphere model. While this model is considered a classic, never before has the high frequency solution been applied in detail to vocalization. Results indicate that frequency content in the 1-50 kHz range can be radiated with nearly 100% efficiency if a reactance peak in the radiation impedance is utilized with adjustments of head size, mouth opening, and beam direction. Without these adjustments, radiation efficiency is generally below 1%, especially in human speech where a high fundamental frequency is a disadvantage for intelligibility. Thus, two distinct modes of vocal communication are identified, (1) short range with optimized information transfer and (2) long range with maximum efficiency for release of acoustic power.

Synergistic selection regimens drive the evolution of display complexity in birds of paradise

Integrated visual displays that combine gesture with colour are nearly ubiquitous in the animal world, where they are shaped by sexual selection for their role in courtship and competition. However, few studies assess how multiple selection regimens operate on different components of these complex phenotypes on a macroevolutionary scale. Here, we study this issue by assessing how both sexual and ecological selection work together to influence visual display complexity in the birds of paradise. We first find that sexual dichromatism is highest in lekking species, which undergo more intense sexual selection by female choice, than non-lekking species. At the same time, species in which males directly compete with one another at communal display courts have more carotenoid-based ornaments and fewer melanin ornaments. Meanwhile, display habitat influences gestural complexity. Species that dance in the cluttered understorey have more complex dances than canopy-displaying species. Taken together, our results illustrate how distinct selection regimens each operate on individual elements comprising a complex display. This supports a modular model of display evolution, wherein the ultimate integrated display is the product of synergy between multiple factors that select for different types of phenotypic complexity.

Global macroevolution and macroecology of passerine song

Studying the macroevolution of the songs of Passeriformes (perching birds) has proved challenging. The complexity of the task stems not just from the macroevolutionary and macroecological challenge of modeling so many species, but also from the difficulty in collecting and quantifying birdsong itself. Using machine learning techniques, we extracted songs from a large citizen science dataset, and then analyzed the evolution, and biotic and abiotic predictors of variation in birdsong across 578 passerine species. Contrary to expectations, we found few links between life-history traits (monogamy and sexual dimorphism) and the evolution of song pitch (peak frequency) or song complexity (standard deviation of frequency). However, we found significant support for morphological constraints on birdsong, as reflected in a negative correlation between bird size and song pitch. We also found that broad-scale biogeographical and climate factors such as net primary productivity, temperature, and regional species richness were significantly associated with both the evolution and present-day distribution of bird song features. Our analysis integrates comparative and spatial modeling with newly developed data cleaning and curation tools, and suggests that evolutionary history, morphology, and present-day ecological processes shape the distribution of song diversity in these charismatic and important birds.

Stereotypy and variability of social calls among clustering female big-footed myotis (Myotis macrodactylus)

Echolocating bats have developed advanced auditory perception systems, predominantly using acoustic signaling to communicate with each other. They can emit a diverse range of social calls in complex behavioral contexts. This study examined the vocal repertoire of five pregnant big-footed myotis bats (Myotis macrodactylus). In the process of clustering, the last individual to return to the colony (LI) emitted social calls that correlated with behavior, as recorded on a PC-based digital recorder. These last individuals could emit 10 simple monosyllabic and 27 complex multisyllabic types of calls, constituting four types of syllables. The social calls were composed of highly stereotyped syllables, hierarchically organized by a common set of syllables. However, intra-specific variation was also found in the number of syllables, syllable order and patterns of syllable repetition across call renditions. Data were obtained to characterize the significant individual differences that existed in the maximum frequency and duration of calls. Time taken to return to the roost was negatively associated with the diversity of social calls. Our findings indicate that variability in social calls may be an effective strategy taken by individuals during reintegration into clusters of female M. macrodactylus.

The effect of isolation, fragmentation, and population bottlenecks on song structure of a Hawaiian honeycreeper

Little is known about how important social behaviors such as song vary within and among populations for any of the endemic Hawaiian honeycreepers. Habitat loss and non-native diseases (e.g., avian malaria) have resulted in isolation and fragmentation of Hawaiian honeycreepers within primarily high elevation forests. In this study, we examined how isolation of Hawai'i 'amakihi (Chlorodrepanis virens) populations within a fragmented landscape influences acoustic variability in song. In the last decade, small, isolated populations of disease tolerant 'amakihi have been found within low elevation forests, allowing us to record 'amakihi songs across a large elevational gradient (10-1800 m) that parallels disease susceptibility on Hawai'i island. To understand underlying differences among populations, we examined the role of geographic distance, elevation, and habitat structure on acoustic characteristics of 'amakihi songs. We found that the acoustic characteristics of 'amakihi songs and song-type repertoires varied most strongly across an elevational gradient. Differences in 'amakihi song types were primarily driven by less complex songs (e.g., fewer frequency changes, shorter songs) of individuals recorded at low elevation sites compared to mid and high elevation populations. The reduced complexity of 'amakihi songs at low elevation sites is most likely shaped by the effects of habitat fragmentation and a disease-driven population bottleneck associated with avian malaria, and maintained through isolation, localized song learning and sharing, and cultural drift. These results highlight how a non-native disease through its influence on population demographics may have also indirectly played a role in shaping the acoustic characteristics of a species.

Ecological drivers of song evolution in birds: Disentangling the effects of habitat and morphology

Environmental differences influence the evolutionary divergence of mating signals through selection acting either directly on signal transmission ("sensory drive") or because morphological adaptation to different foraging niches causes divergence in "magic traits" associated with signal production, thus indirectly driving signal evolution. Sensory drive and magic traits both contribute to variation in signal structure, yet we have limited understanding of the relative role of these direct and indirect processes during signal evolution. Using phylogenetic analyses across 276 species of ovenbirds (Aves: Furnariidae), we compared the extent to which song evolution was related to the direct influence of habitat characteristics and the indirect effect of body size and beak size, two potential magic traits in birds. We find that indirect ecological selection, via diversification in putative magic traits, explains variation in temporal, spectral, and performance features of song. Body size influences song frequency, whereas beak size limits temporal and performance components of song. In comparison, direct ecological selection has weaker and more limited effects on song structure. Our results illustrate the importance of considering multiple deterministic processes in the evolution of mating signals.

Geographic vocal variation and perceptual discrimination abilities in male Australian sea lions

Vocal characteristics can vary among and within populations. In species with geographic variation in the structure of vocalizations, individuals may have the ability to discriminate between calls from local and non-local individuals. The ability to distinguish differences in acoustic signals is likely to have a significant influence on the outcome of social interactions between individuals, including potentially mate selection and breeding success. Pinnipeds (seals, fur seals, sea lions and walruses) are highly vocal yet the Australian sea lion (Neophoca cinerea) is the only eared seal known to show geographic vocal variation in male barks. Barks are produced in many social interactions and encode sufficient information for both individual and colony identity to be discriminable. Yet until now, whether males could themselves discriminate these bark differences was unclear. We performed playback experiments in four breeding colonies to investigate whether males can discriminate local from non-local barks. Overall, males responded more strongly to barks from their own colony compared to barks from other colonies regardless of whether those other colonies were close or distant. Competition for females is high in Australian sea lions, but mating periods are asynchronous across colonies. The ability to correctly assess whether a male is from the same colony, thus representing a potential competitor for mates, or merely a visitor from elsewhere, may influence how males interact with others. Given the high cost of fighting, the ability to discern competitors may influence the nature of male-male interactions and ultimately influence how they allocate reproductive effort.

Rapid song divergence leads to discordance between genetic distance and phenotypic characters important in reproductive isolation

The criteria for species delimitation in birds have long been debated, and several recent studies have proposed new methods for such delimitation. On one side, there is a large consensus of investigators who believe that the only evidence that can be used to delimit species is molecular phylogenetics, and with increasing numbers of markers to gain better support, whereas on the other, there are investigators adopting alternative approaches based largely on phenotypic differences, including in morphology and communication signals. Yet, these methods have little to say about rapid differentiation in specific traits shown to be important in reproductive isolation. Here, we examine variation in phenotypic (morphology, plumage, and song) and genotypic (mitochondrial and nuclear DNA) traits among populations of yellow-rumped tinkerbird Pogoniulus bilineatus in East Africa. Strikingly, song divergence between the P. b. fischeri subspecies from Kenya and Zanzibar and P. b. bilineatus from Tanzania is discordant with genetic distance, having occurred over a short time frame, and playback experiments show that adjacent populations of P. b. bilineatus and P. b. fischeri do not recognize one another's songs. While such rapid divergence might suggest a founder effect following invasion of Zanzibar, molecular evidence suggests otherwise, with insular P. b. fischeri nested within mainland P. b. fischeri. Populations from the Eastern Arc Mountains are genetically more distant, yet share the same song with P. b. bilineatus from Coastal Tanzania and Southern Africa, suggesting they would interbreed. We believe investigators ought to examine potentially rapid divergence in traits important in species recognition and sexual selection when delimiting species, rather than relying entirely on arbitrary quantitative characters or molecular markers.

Dispersal influences genetic and acoustic spatial structure for both males and females in a tropical songbird

Animals exhibit diverse dispersal strategies, including sex‐biased dispersal, a phenomenon common in vertebrates. Dispersal influences the genetic structure of populations as well as geographic variation in phenotypic traits. Patterns of spatial genetic structure and geographic variation may vary between the sexes whenever males and females exhibit different dispersal behaviors. Here, we examine dispersal, spatial genetic structure, and spatial acoustic structure in Rufous‐and‐white Wrens, a year‐round resident tropical bird. Both sexes sing in this species, allowing us to compare acoustic variation between males and females and examine the relationship between dispersal and song sharing for both sexes. Using a long‐term dataset collected over an 11‐year period, we used banding data and molecular genetic analyses to quantify natal and breeding dispersal distance in Rufous‐and‐white Wrens. We quantified song sharing and examined whether sharing varied with dispersal distance, for both males and females. Observational data and molecular genetic analyses indicate that dispersal is female‐biased. Females dispersed farther from natal territories than males, and more often between breeding territories than males. Furthermore, females showed no significant spatial genetic structure, consistent with expectations, whereas males showed significant spatial genetic structure. Overall, natal dispersal appears to have more influence than breeding dispersal on spatial genetic structure and spatial acoustic structure, given that the majority of breeding dispersal events resulted in individuals moving only short distances. Song sharing between pairs of same‐sex animals decreases with the distance between their territories for both males and females, although males exhibited significantly greater song sharing than females. Lastly, we measured the relationship between natal dispersal distance and song sharing. We found that sons shared fewer songs with their fathers the farther they dispersed from their natal territories, but that song sharing between daughters and mothers was not significantly correlated with natal dispersal distance. Our results reveal cultural differences between the sexes, suggesting a relationship between culture and sex‐biased dispersal.

How the environment shapes animal signals: a test of the acoustic adaptation hypothesis in frogs

Long-distance acoustic signals are widely used in animal communication systems and, in many cases, are essential for reproduction. The acoustic adaptation hypothesis (AAH) implies that acoustic signals should be selected for further transmission and better content integrity under the acoustic constraints of the habitat in which they are produced. In this study, we test predictions derived from the AAH in frogs. Specifically, we focus on the difference between torrent frogs and frogs calling in less noisy habitats. Torrents produce sounds that can mask frog vocalizations and constitute a major acoustic constraint on call evolution. We combine data collected in the field, material from scientific collections and the literature for a total of 79 primarily Asian species, of the families Ranidae, Rhacophoridae, Dicroglossidae and Microhylidae. Using phylogenetic comparative methods and including morphological and environmental potential confounding factors, we investigate putatively adaptive call features in torrent frogs. We use broad habitat categories as well as fine-scale habitat measurements and test their correlation with six call characteristics. We find mixed support for the AAH. Spectral features of torrent frog calls are different from those of frogs calling in other habitats and are related to ambient noise levels, as predicted by the AAH. However, temporal call features do not seem to be shaped by the frogs' calling habitats. Our results underline both the complexity of call evolution and the need to consider multiple factors when investigating this issue.

Laryngeal airway reconstruction indicates that rodent ultrasonic vocalizations are produced by an edge-tone mechanism

Some rodents produce ultrasonic vocalizations (USVs) for social communication using an aerodynamic whistle, a unique vocal production mechanism not found in other animals. The functional anatomy and evolution of this sound production mechanism remains unclear. Using laryngeal airway reconstruction, we identified anatomical specializations critical for USV production. A robust laryngeal cartilaginous framework supports a narrow supraglottal airway. An intralaryngeal airsac-like cavity termed the ventral pouch was present in three muroid rodents (suborder Myomorpha), but was absent in a heteromyid rodent (suborder Castorimorpha) that produces a limited vocal repertoire and no documented USVs. Small lesions to the ventral pouch in laboratory rats caused dramatic changes in USV production, supporting the hypothesis that an interaction between a glottal exit jet and the alar edge generates ultrasonic signals in rodents. The resulting undulating airflow around the alar edge interacts with the resonance of the ventral pouch, which may function as a Helmholtz resonator. The proposed edge-tone mechanism requires control of intrinsic laryngeal muscles and sets the foundation for acoustic variation and diversification among rodents. Our work highlights the importance of anatomical innovations in the evolution of animal sound production mechanisms.

Differences in mating behavior between two allopatric populations of a Neotropical scorpion

Courtship and mating behavior generally evolve rapidly in diverging populations and species. The adaptation to different environments may cause behavioral divergence in characteristics involved in mate choice. Our objective in this study was to compare the sexual behavior of two distant populations of the scorpion Bothriurus bonariensis. This species has a broad distribution in South America, inhabiting Central Argentina, Uruguay and south-eastern Brazil. It is known that in this species there is a divergence in morphological patterns (body size, coloration, allometry and fluctuating asymmetry indexes) among distant populations. Considering the differences in environmental conditions between localities, we compare the sexual behavior in intra-population and inter-population matings from Central Argentina and southern Uruguay populations. We found significant differences in mating patterns, including differences in the frequency and duration of important stimulatory courtship behaviors. In addition, most inter-population matings were unsuccessful. In this framework, the differences in reproductive behavior could indicate reproductive isolation between these populations, which coincides with their already known morphological differences. This is the first study comparing the sexual behavior of allopatric populations of scorpions; it provides new data about the degree of intraspecific geographical divergence in the sexual behavior of B. bonariensis.

Evolution and plasticity: Divergence of song discrimination is faster in birds with innate song than in song learners in Neotropical passerine birds

Plasticity is often thought to accelerate trait evolution and speciation. For example, plasticity in birdsong may partially explain why clades of song learners are more diverse than related clades with innate song. This "song learning" hypothesis predicts that (1) differences in song traits evolve faster in song learners, and (2) behavioral discrimination against allopatric song (a proxy for premating reproductive isolation) evolves faster in song learners. We tested these predictions by analyzing acoustic traits and conducting playback experiments in allopatric Central American sister pairs of song learning oscines (N = 42) and nonlearning suboscines (N = 27). We found that nonlearners evolved mean acoustic differences slightly faster than did leaners, and that the mean evolutionary rate of song discrimination was 4.3 times faster in nonlearners than in learners. These unexpected results may be a consequence of significantly greater variability in song traits in song learners (by 54-79%) that requires song-learning oscines to evolve greater absolute differences in song before achieving the same level of behavioral song discrimination as nonlearning suboscines. This points to "a downside of learning" for the evolution of species discrimination, and represents an important example of plasticity reducing the rate of evolution and diversification by increasing variability.

Environmental correlates of geographic divergence in a phenotypic trait: A case study using bat echolocation

Divergence in phenotypic traits may arise from the interaction of different evolutionary forces, including different kinds of selection (e.g., ecological), genetic drift, and phenotypic plasticity. Sensory systems play an important role in survival and reproduction, and divergent selection on such systems may result in lineage diversification. Such diversification could be largely influenced by selection in different environments as a result of isolation by environment (IbE). We investigated this process using geographic variation in the resting echolocation frequency of the horseshoe bat species, Rhinolophus damarensis, as a test case. Bats were sampled along a latitudinal gradient ranging from 16°S to 32°S in the arid western half of southern Africa. We measured body size and peak resting frequencies (RF) from handheld individual bats. Three hypotheses for the divergence in RF were tested: (1) James' Rule, (2) IbE, and (3) genetic drift through isolation by distance (IbD) to isolate the effects of body size, local climatic conditions, and geographic distance, respectively, on the resting frequency of R. damarensis. Our results did not support genetic drift because there was no correlation between RF variation and geographic distance. Our results also did not support James' Rule because there was no significant relationship between (1) geographic distances and RF, (2) body size and RF, or (3) body size and climatic variables. Instead, we found support for IbE in the form of a correlation between RF and both region and annual mean temperature, suggesting that RF variation may be the result of environmental discontinuities. The environmental discontinuities coincided with previously reported genetic divergence. Climatic gradients in conjunction with environmental discontinuities could lead to local adaptation in sensory signals and directed dispersal such that gene flow is restricted, allowing lineages to diverge. However, our study cannot exclude the role of processes like phenotypic plasticity in phenotypic variation.

Prenatal environment affects embryonic response to song

Early environmental enrichment improves postnatal cognition in animals and humans. Here, we examined the effects of the prenatal acoustic environment (parental song rate) on prenatal attention in superb fairy-wren (Malurus cyaneus) embryos, the only songbird species with evidence of prenatal discrimination of maternal calls and in ovo call learning. Because both adults also sing throughout the incubation phase, we broadcast songs to embryos and measured their heart rate response in relation to parental song rate and tutor identity (familiarity, sex). Embryos from acoustically active families (high parental song rate) had the strongest response to songs. Embryos responded (i) strongest to male songs irrespective of familiarity with the singer, and (ii) strongest if their father had a high song rate during incubation. This is the first evidence for a prenatal physiological response to particular songs (potential tutors) in the egg, in relation to the prenatal acoustic environment, and before the sensitive period for song learning.

Languages in Drier Climates Use Fewer Vowels

This study offers evidence for an environmental effect on languages while relying on continuous linguistic and continuous ecological variables. Evidence is presented for a positive association between the typical ambient humidity of a language’s native locale and that language’s degree of reliance on vowels. The vowel-usage rates of over 4000 language varieties were obtained, and several methods were employed to test whether these usage rates are associated with ambient humidity. The results of these methods are generally consistent with the notion that reduced ambient humidity eventually yields a reduced reliance of languages on vowels, when compared to consonants. The analysis controls simultaneously for linguistic phylogeny and contact between languages. The results dovetail with previous work, based on binned data, suggesting that consonantal phonemes are more common in some ecologies. In addition to being based on continuous data and a larger data sample, however, these findings are tied to experimental research suggesting that dry air affects the behavior of the larynx by yielding increased phonatory effort. The results of this study are also consistent with previous work suggesting an interaction of aridity and tonality. The data presented here suggest that languages may evolve, like the communication systems of other species, in ways that are influenced subtly by ecological factors. It is stressed that more work is required, however, to explore this association and to establish a causal relationship between ambient air characteristics and the development of languages.

Grasshopper mice employ distinct vocal production mechanisms in different social contexts

Functional changes in vocal organ morphology and motor control facilitate the evolution of acoustic signal diversity. Although many rodents produce vocalizations in a variety of social contexts, few studies have explored the underlying production mechanisms. Here, we describe mechanisms of audible and ultrasonic vocalizations (USVs) produced by grasshopper mice (genus Onychomys). Grasshopper mice are predatory rodents of the desert that produce both loud, long-distance advertisement calls and USVs in close-distance mating contexts. Using live-animal recording in normal air and heliox, laryngeal and vocal tract morphological investigations, and biomechanical modelling, we found that grasshopper mice employ two distinct vocal production mechanisms. In heliox, changes in higher-harmonic amplitudes of long-distance calls indicate an airflow-induced tissue vibration mechanism, whereas changes in fundamental frequency of USVs support a whistle mechanism. Vocal membranes and a thin lamina propria aid in the production of long-distance calls by increasing glottal efficiency and permitting high frequencies, respectively. In addition, tuning of fundamental frequency to the second resonance of a bell-shaped vocal tract increases call amplitude. Our findings indicate that grasshopper mice can dynamically adjust motor control to suit the social context and have novel morphological adaptations that facilitate long-distance communication.