Why study plasticity in multiple traits? New hypotheses for how phenotypically plastic traits interact during development and selection

Organisms can often respond adaptively to a change in their environment through phenotypic plasticity in multiple traits, a phenomenon termed as multivariate plasticity. These different plastic responses could interact and affect each other's development as well as selection on each other, but the causes and consequences of these interactions have received relatively little attention. Here, we propose a new conceptual framework for understanding how different plastic responses can affect each other's development and why organisms should have multiple plastic responses. A plastic change in one trait could alter the phenotype of a second plastic trait by changing either the cue received by the organism (cue-mediated effect) or the response to that cue (response-mediated effect). Multivariate plasticity could benefit the organism either because the plastic responses work better when expressed together (synergy) or because each response is more effective under different environmental circumstances (complementarity). We illustrate these hypotheses with case studies, focusing on interactions between behavior and morphology, plastic traits that differ in their reversibility. Future empirical and theoretical research should investigate the consequences of these interactions for additional factors important for the evolution of plasticity, such as the limits and costs of plasticity.

Influence of Anthropogenic Sounds on Insect, Anuran and Bird Acoustic Signals: A Meta-Analysis

Acoustic communication is a way of information exchange between individuals, and it is used by several animal species. Therefore, the detection, recognition and correct understanding of acoustic signals are key factors in effective communication. The priority of acoustic communication is effectiveness rather than perfection, being effective avoids affecting the sound-based communication system of the species. One of the factors that can affect effective communication is the overlap in time and frequency during signal transmission, known as signal masking. One type of sound that can cause masking is anthropogenic noise, which is currently increasing due to urban growth and consequently motorized transportation and machinery. When exposed to anthropogenic noise, animals can use compensatory mechanisms to deal with sound masking, such as the modification of acoustic parameters of their acoustic signal. Here, we performed a meta-analysis investigating whether different taxa have a general tendency for changes in acoustic parameters due to anthropogenic noise, we used taxa and acoustic parameters available in the literature that met the minimum criteria to perform a meta-analysis. We hypothesized that animals exposed to anthropogenic noise use compensation mechanisms, such as changes in dominant, maximum or minimum frequencies, call duration, note duration and call rate to deal with masking. We performed a meta-analysis, which synthesized information from 73 studies comprising 82 species of three taxa: insects, anurans and birds. Our results showed that in the presence of anthropogenic noise, insects did not change the acoustic parameters, while anurans increased call amplitude and birds increased dominant frequency, minimum and maximum frequencies, note duration and amplitude of their songs. The different responses of the groups to anthropogenic noise may be related to their particularities in the production and reception of sound or to the differences in the acoustic parameters considered between the taxa and also the lack of studies in some taxa.

How urbanization affects sexual communication

Urbanization is rapidly altering landscapes worldwide, changing environmental conditions, and creating novel selection pressures for many organisms. Local environmental conditions affect the expression and evolution of sexual signals and mating behaviors; changes in such traits have important evolutionary consequences because of their effect on reproduction. In this review, we synthesize research investigating how sexual communication is affected by the environmental changes associated with urbanization-including pollution from noise, light, and heavy metals, habitat fragmentation, impervious surfaces, urban heat islands, and changes in resources and predation. Urbanization often has negative effects on sexual communication through signal masking, altering condition-dependent signal expression, and weakening female preferences. Though there are documented instances of seemingly adaptive shifts in trait expression, the ultimate impact on fitness is rarely tested. The field of urban evolution is still relatively young, and most work has tested whether differences occur in response to various aspects of urbanization. There is limited information available about whether these responses represent phenotypic plasticity or genetic changes, and the extent to which observed shifts in sexual communication affect reproductive fitness. Our understanding of how sexual selection operates in novel, urbanized environments would be bolstered by more studies that perform common garden studies and reciprocal transplants, and that simultaneously evaluate multiple environmental factors to tease out causal drivers of observed phenotypic shifts. Urbanization provides a unique testing ground for evolutionary biologists to study the interplay between ecology and sexual selection, and we suggest that more researchers take advantage of these natural experiments. Furthermore, understanding how sexual communication and mating systems differ between cities and rural areas can offer insights on how to mitigate negative, and accentuate positive, consequences of urban expansion on the biota, and provide new opportunities to underscore the relevance of evolutionary biology in the Anthropocene.

Phenotypic integration does not constrain phenotypic plasticity: differential plasticity of traits is associated to their integration across environments

Understanding constraints to phenotypic plasticity is key given its role on the response of organisms to environmental change. It has been suggested that phenotypic integration, the structure of trait covariation, could limit trait plasticity. However, the relationship between plasticity and integration is far from resolved. Using a database of functional plasticity to drought of a Mediterranean shrub that included 20 ecophysiological traits, we assessed environmentally-induced changes in phenotypic integration and whether integration constrained the expression of plasticity, accounting for the within-environment phenotypic variation of traits. Furthermore, we provide the first test of the association between differential trait plasticity and trait integration across an optimum and a stressful environment. Phenotypic plasticity was positively associated with phenotypic integration in both environments, but this relationship was lost when phenotypic variation was considered. The similarity in the plastic response of two traits predicted their integration across environments, with integrated traits having more similar plasticity. Such variation in the plasticity of traits partly explained the lower phenotypic integration found in the stressful environment. We found no evidence that integration may constitute an internal constraint to plasticity. Rather, we present the first empirical demonstration that differences in plastic responses may involve a major reorganization of the relationships among traits, and challenge the notion that stress generally induces a tighter phenotype.

Urban Noise Restricts Song Frequency Bandwidth and Syllable Diversity in Bananaquits: Increasing Audibility at the Expense of Signal Quality

Anthropogenic noise can interfere with animal behavior through masking of acoustic communication. In response to masking, animals may change their acoustic signals as an apparent adjustment strategy, but this may have a drawback on signal quality. Songs and calls may show noise-dependent changes in frequency and duration, which may yield some masking avoidance, but may also constrain other acoustic parameters that might carry information about the sender. In the present study, we investigated whether noise-dependent reduction in frequency bandwidth or song duration restricted syllable diversity or song elaboration in a Neotropical songbird, the bananaquit (Coereba flaveola). We show that bananaquits sing higher frequency songs, of narrower bandwidth, in noisier territories, independent of variation in territory density, without significant variation in song duration. We also show that songs with higher minimum frequencies, narrower bandwidths, and shorter durations have on average a lower number of syllable types and higher syllable rates. This finding is in line with an acoustic restriction and may reflect a functional trade-off between audibility and signal value: higher frequencies may be more audible but less elaborate songs may weaken the message of sender quality. Consequently, noise pollution may not only alter avian communities, but also shape acoustic diversity and processes of sexual selection in urban environments.

Noise-Related Song Variation Affects Communication: Bananaquits Adjust Vocally to Playback of Elaborate or Simple Songs

Birds communicate through acoustic variation in their songs for territorial defense and mate attraction. Noisy urban conditions often induce vocal changes that can alleviate masking problems, but that may also affect signal value. We investigated this potential for a functional compromise in a neotropical songbird: the bananaquit (Coereba flaveola). This species occurs in urban environments with variable traffic noise levels and was previously found to reduce song elaboration in concert with a noise-dependent reduction in song frequency bandwidth. Singing higher and in a narrower bandwidth may make their songs more audible in noisy conditions of low-frequency traffic. However, it was unknown whether the associated decrease in syllable diversity affected their communication. Here we show that bananaquits responded differently to experimental playback of elaborate vs. simple songs. The variation in syllable diversity did not affect general response strength, but the tested birds gave acoustically distinct song replies. Songs had fewer syllables and were lower in frequency and of wider bandwidth when individuals responded to elaborate songs compared to simple songs. This result suggests that noise-dependent vocal restrictions may change the signal value of songs and compromise their communicative function. It remains to be investigated whether there are consequences for individual fitness and how such effects may alter the diversity and density of the avian community in noisy cities.

Species sensitivities to a global pollutant: A meta-analysis on acoustic signals in response to anthropogenic noise

Anthropogenically driven environmental changes affect our planet at an unprecedented rate. Among these changes are those in the acoustic environment caused by anthropogenic noise, which can affect both animals and humans. In many species, acoustic communication plays a crucial role to maintain social relationships by exchanging information via acoustic signals. However, how species relying on acoustic communication differ in their adjustments to anthropogenic noise is little understood. Yet, this is crucial because protecting species effectively depends on our capability to predict how species differ in their response to human-induced environmental changes. Using a phylogenetically controlled meta-analysis, we quantified differences in adjustments of acoustic signals to anthropogenic noise among species. The effect sizes included in the analysis were obtained from noise exposure experiments, as only carefully controlled experiments allow to establish cause-and-effect relationships. We found that animals changed acoustic signals when exposed to noise, but the magnitude and the direction of adjustments differed among species. Given the importance of communication in the animal kingdom, these adjustments can affect social relationships in many species. The diversity of responses among species highlights the necessity to assess the effect of environmental stressors not only for a few species, because an effect may be positive in one species but negative in another depending on the species' biology. Thus, an effective conservation approach to protect different species is to preserve natural soundscapes of ecosystems to which species have adapted to by reducing or mitigating the emission of anthropogenic noise into the environment.

Chronic exposure to urban noise during the vocal learning period does not lead to increased song frequencies in zebra finches

Abstract

It has often been observed that birds sing at a higher pitch in cities and other areas that are polluted with intense low-frequency noise. How this pattern arises remains unclear though. One prevailing idea is that songbirds adjust song frequencies to environmental noise profiles through developmental plasticity via vocal learning. However, the conclusions of previous studies testing this hypothesis are inconsistent. Here we report the findings from two song learning experiments with zebra finches (Taenopygia guttata), in which we exposed young birds to anthropogenic noise during their sensitive vocal learning period. Unlike previous studies that addressed this issue, we did not use constant synthetic noise but natural urban noise with its typical amplitude fluctuations that was broadcast at realistic sound levels. We found that noise-exposed males in neither experiment developed higher pitched songs compared to control males. This suggests that the natural fluctuations between higher and lower noise levels in cities may allow young birds to exploit relatively quiet moments to hear their tutors and themselves, permitting them to make accurate copies of even low-frequency song elements.

Significance statement

If animals are to persist in urban habitats, they often must adjust their behavior to the altered conditions. Birds in cities are often observed to sing at a higher pitch, but we are largely ignorant of how this phenomenon arises. We investigated whether low-frequency traffic noise interferes with the song learning of birds so that they develop higher pitched songs. Accordingly, we played back natural traffic noise from urban bird habitats to young birds during their learning period and then analyzed their adult songs. We found that birds that learned their songs in noise did not sing at higher frequencies compared to control males that learned their song with no noise exposure. Our results show that typical traffic noise in cities may not be sufficient to interfere with vocal learning in a way that birds develop higher-pitched songs.

The quiet spring of 2020

Anthropogenic noise has no legacy effects on bird song

Social context and noise affect within and between male song adjustments in a common passerine

Across populations, animals that inhabit areas with high anthropogenic noise produce vocalizations that differ from those inhabiting less noisy environments. Such patterns may be due to individuals rapidly adjusting their songs in response to changing noise, but individual variation is seldom explored. Therefore, we tested the hypothesis that male house wrens (Troglodytes aedon) immediately adjust their songs according to changing noise and that social context further modifies responses. We recorded songs, quantified noise, and defined social context within pairs as female fertile status and between males as number of conspecific neighbors. We used a reaction-norm approach to compare song trait intercepts (between-male effects) and slopes (within-male effects) as a function of noise. Individuals immediately adjusted song duration in response to changing noise. How they achieved adjustments varied: some sang shorter and others longer songs with greater noise, and individuals varied in the extent to which they adjusted song duration. Variation in song duration could be affected by competition as between-male noise levels interacted with number of neighbors to affect syllable duration. Neither within- nor between-male noise effects were detected for frequency traits. Rather, males with fertile mates sang lower-frequency songs and increased peak frequency with more neighbors. Among males, social context but not noise affected song frequency, whereas temporal structure varied between and within individuals depending on noise and social factors. Not all males adjusted signals the same way in response to noise, and selection could favor different levels of variation according to noise.

Effects of ecotourism on eastern yellow robin (Eopsaltria australis) vocal behaviour

Ecotourism attempts to provide sustainable tourism with a focus on experiencing natural areas to foster environmental appreciation. However, it is often seen as a paradox, since associated infrastructure and activity can have detrimental effects on wildlife. One aspect that has been overlooked in this context is the potential impact of ecotourists on vocal behaviour, especially of birds. Many birds are susceptible to anthropogenic noise, avoiding noisy areas, or changing vocal activity within them. We used passive recording techniques to quantify vocal behaviours of a native Australian songbird, the eastern yellow robin, at an ecotourist resort to investigate how tourist presence might affect vocal behaviours. We found that during peak tourism seasonal periods, robins sang fewer songs, and these songs were shorter. However, this same pattern was observed on weekdays compared with weekends, when tourist numbers are generally lower. This seemingly contradictory pattern may be explained by the fact that maintenance of the resort grounds occurred predominantly on weekdays, with noise levels comparable to those detected during periods of high tourism. Thus, ecotourism infrastructure can have layered effects on bird vocal activity: tourist numbers as well as maintenance practices should be considered within the context of local wildlife conservation.

Signal complexity communicates aggressive intent during contests, but the process is disrupted by noise

Contestants use displays to signal their aggressive intent and settle disputes before they escalate. For birds, this is often in the form of song, which can vary in structural complexity. The role of song complexity in signalling aggressive intent has not been fully established, and its efficacy could be influenced by background noise levels. Using playback experiments, we found that in European robins, Erithacus rubecula, song complexity signalled sender aggression and affected receiver response. However, increased noise impacted the ability of contestants to adjust response based on opponent song complexity. These findings provide new evidence regarding the use of acoustic signal complexity for assessing opponent aggression and that noise can influence contest behaviour by interrupting this process, which could impose fitness consequences.

Noise-induced vocal plasticity in urban white-crowned sparrows does not involve adjustment of trill performance components

Background noise can interfere with acoustic communication and subsequently influence signaling behavior. Immediate signaling flexibility (ISF) is a context-dependent form of behavioral plasticity that allows animals to temporarily change their acoustic behavior in response to noise fluctuations and potentially improve the chances of successful communication in noisy environments. The adaptive value of ISF is ultimately contingent on the response of the intended receiver, and there are differential effects on receiver response depending on which signal component is modified. However, there is scant research on whether ISF involves modification of a signal component specifically linked to mate attraction or territory defense. Our study addresses this knowledge gap and provides important insight into whether males employ short-term signal modification in a manner that could affect mate pairing success in birds. Specifically, we explore the maladaptive potential of ISF in the San Francisco, California population of Zonotrichia leucophrys nuttalli by testing for changes in trill bandwidth and rate—the specific trill structure components known to influence the receiver’s perception of vocal performance in this species—before and during noise broadcast experiments. Although Zonotrichia leucophrys nuttalli are capable of ISF, we found no evidence that noise induces temporary adjustment of the trill structure traits used by receivers to assess vocal performance.

Adaptive changes in sexual signalling in response to urbanization

Urbanization can cause species to adjust their sexual displays, because the effectiveness of mating signals is influenced by environmental conditions. Despite many examples that show that mating signals in urban conditions differ from those in rural conditions, we do not know whether these differences provide a combined reproductive and survival benefit to the urban phenotype. Here we show that male túngara frogs have increased the conspicuousness of their calls, which is under strong sexual and natural selection by signal receivers, as an adaptive response to city life. The urban phenotype consequently attracts more females than the forest phenotype, while avoiding the costs that are imposed by eavesdropping bats and midges, which we show are rare in urban areas. Finally, we show in a translocation experiment that urban frogs can reduce risk of predation and parasitism when moved to the forest, but that forest frogs do not increase their sexual attractiveness when moved to the city. Our findings thus reveal that urbanization can rapidly drive adaptive signal change via changes in both natural and sexual selection pressures.

Males increase call frequency, not intensity, in response to noise, revealing no Lombard effect in the little torrent frog

Noise is one of the main factors that can influence the processes of sound communication across a wide range of animal groups. Although the effects of ambient noise on animal communication, including anthropogenic noise, have received increasing attention, few studies have examined changes in the fine structure of acoustic signals produced by vocalizing species in constantly noisy environments. Here, we used natural recordings to determine the associations between stream noise and call parameters in the little torrent frog (Amolops torrentis). We also used playbacks of stream noise recorded in natural habitats and playbacks of white noise to examine how male vocal signals change with increasing noise levels. The results show that noise intensity has a significant effect on male call frequency, but not on call amplitude or other call characteristics. Based on this evidence, we suggest that in streamside species stream noise drives males to alter call frequency and call as loudly as possible in order to improve discriminability. These findings provide insights into the role played by ecological selection in the evolution of noise-dependent anuran vocal plasticity.

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.

Combined effects of physiological condition and environmental attributes in determining call plasticity

Evidence is growing on the ability of anurans to make immediate call adjustments in response to the propagation properties of the environment. However, our understanding of such flexibility is typically based on dichotomous stimuli (i.e. presence/absence), while its condition-dependence has been little explored. We experimentally studied the ability of adjustment and its condition dependence of advertisement calls of Hypsiboas pulchellus in response to different levels of attenuation. Individuals modified most call parameters analyzed, although the direction of adjustments was contingent on individual identity, level of attenuation and trait category (i.e. spectral or temporal). Under low attenuation conditions males lengthened some call elements, partially supporting the hypothesis of plasticity oriented towards maintaining signal transmission, but this trend was not present in the high attenuation treatment. Also, under both attenuation treatments males gave higher frequency notes, as expected from an energy-saving response to changes in social environment or reducing attractiveness due to risk perception. Across treatments individuals in better condition tended to exhibit higher plasticity. These results indicate that call structure could depend on the integration of different cues such as habitat propagation characteristics, acoustic competition for females, and predation risk, while the ability to behave flexibly depends on current body reserves.

The role of singing style in song adjustments to fluctuating sound conditions: A comparative study on Mexican birds

Many bird species adjust their songs to noisy urban conditions by which they reduce masking and counteract the detrimental impact on signal efficiency. Different species vary in their response to level fluctuations of ambient noise, but it remains unclear why they vary. Here, we investigated whether noise-dependent flexibility may relate to singing style and signal function of the flexible acoustic trait. Species with highly variable songs may generally be more flexible and strongly repetitive singers may be more limited to stray from their stringent patterns. We exposed males of four passerine species with contrasting singing styles (repertoire size, immediate or eventual variety singing and syllable diversity) to three experimental sound conditions: 1) continuous urban noise; 2) intermittent white noise and 3) conspecific song playback. We found no spectral or temporal changes in response to experimental noise exposure in any of the four species, but significant temporal adjustment to conspecific playback in one of them. We argue that the consistency in song frequency and timing may have signal value, independent of singing style, and therefore be an explanation for the general lack of noise-dependent flexibility in the four species of the current study.

Dynamic changes in display architecture and function across environments revealed by a systems approach to animal communication

Animal communication is often structurally complex and dynamic, with signaler and receiver behavior varying in response to multiple environmental factors. To date, studies assessing signal dynamics have mostly focused on the relationships between select signaling traits and receiver responses in a single environment. We use the wolf spider Schizocosa floridana to explore the relationships between courtship display form and function across two social contexts (female presence vs absence) and two light environments (light vs dark). We use traditional analytical methods to determine predictors of copulation success (i.e., signal function) and examine these predictors in a structural context by overlaying them on signal phenotype networks (Wilkins et al. 2015). This allows us to explore system design principles (degeneracy, redundancy, pluripotentiality), providing insight into hypotheses regarding complex signal evolution. We found that both social context and light environment affect courtship structure, although the predictors of mating success remain similar across light environments, suggesting system degeneracy. Contrastingly, the same display traits may serve different functions across social environments, suggesting pluripotentiality. Ultimately, our network approach uncovers a complexity in display structure and function that is missed by functional analyses alone, highlighting the importance of systems-based methodologies for understanding the dynamic nature of complex signals.

Phenotypic integration and the evolution of signal repertoires: A case study of treefrog acoustic communication

Animal signals are inherently complex phenotypes with many interacting parts combining to elicit responses from receivers. The pattern of interrelationships between signal components reflects the extent to which each component is expressed, and responds to selection, either in concert with or independently of others. Furthermore, many species have complex repertoires consisting of multiple signal types used in different contexts, and common morphological and physiological constraints may result in interrelationships extending across the multiple signals in species' repertoires. The evolutionary significance of interrelationships between signal traits can be explored within the framework of phenotypic integration, which offers a suite of quantitative techniques to characterize complex phenotypes. In particular, these techniques allow for the assessment of modularity and integration, which describe, respectively, the extent to which sets of traits covary either independently or jointly. Although signal and repertoire complexity are thought to be major drivers of diversification and social evolution, few studies have explicitly measured the phenotypic integration of signals to investigate the evolution of diverse communication systems. We applied methods from phenotypic integration studies to quantify integration in the two primary vocalization types (advertisement and aggressive calls) in the treefrogs Hyla versicolor, Hyla cinerea, and Dendropsophus ebraccatus. We recorded male calls and calculated standardized phenotypic variance-covariance (P) matrices for characteristics within and across call types. We found significant integration across call types, but the strength of integration varied by species and corresponded with the acoustic similarity of the call types within each species. H. versicolor had the most modular advertisement and aggressive calls and the least acoustically similar call types. Additionally, P was robust to changing social competition levels in H. versicolor. Our findings suggest new directions in animal communication research in which the complex relationships among the traits of multiple signals are a key consideration for understanding signal evolution.

Learning to cope: vocal adjustment to urban noise is correlated with prior experience in black-capped chickadees

Urban noise can interfere with avian communication through masking, but birds can reduce this interference by altering their vocalizations. Although several experimental studies indicate that birds can rapidly change their vocalizations in response to sudden increases in ambient noise, none have investigated whether this is a learned response that depends on previous exposure. Black-capped chickadees (Poecile atricapillus) change the frequency of their songs in response to both fluctuating traffic noise and experimental noise. We investigated whether these responses to fluctuating noise depend on familiarity with noise. We confirmed that males in noisy areas sang higher-frequency songs than those in quiet areas, but found that only males in already-noisy territories shifted songs upwards in immediate response to experimental noise. Unexpectedly, males in more quiet territories shifted songs downwards in response to experimental noise. These results suggest that chickadees may require prior experience with fluctuating noise to adjust vocalizations in such a way as to minimize masking. Thus, learning to cope may be an important part of adjusting to acoustic life in the city.

White-crowned sparrow males show immediate flexibility in song amplitude but not in song minimum frequency in response to changes in noise levels in the field

The soundscape acts as a selective agent on organisms that use acoustic signals to communicate. A number of studies document variation in structure, amplitude, or timing of signal production in correspondence with environmental noise levels thus supporting the hypothesis that organisms are changing their signaling behaviors to avoid masking. The time scale at which organisms respond is of particular interest. Signal structure may evolve across generations through processes such as cultural or genetic transmission. Individuals may also change their behavior during development (ontogenetic change) or in real time (i.e., immediate flexibility). These are not mutually exclusive mechanisms, and all must be investigated to understand how organisms respond to selection pressures from the soundscape. Previous work on white-crowned sparrows (Zonotrichia leucophrys) found that males holding territories in louder areas tend to sing higher frequency songs and that both noise levels and song frequency have increased over time (30 years) in urban areas. These previous findings suggest that songs are changing across generations; however, it is not known if this species also exhibits immediate flexibility. Here, we conducted an exploratory, observational study to ask whether males change the minimum frequency of their song in response to immediate changes in noise levels. We also ask whether males sing louder, as increased minimum frequency may be physiologically linked to producing sound at higher amplitudes, in response to immediate changes in environmental noise. We found that territorial males adjust song amplitude but not minimum frequency in response to changes in environmental noise levels. Our results suggest that males do not show immediate flexibility in song minimum frequency, although experimental manipulations are needed to test this hypothesis further. Our work highlights the need to investigate multiple mechanisms of adaptive response to soundscapes.

Coping with a changing soundscape: avoidance, adjustments and adaptations

Since the industrial age, background anthropogenic noise has become a pervasive feature of many habitable environments. This relatively recent environmental feature can be particularly challenging for organisms that use acoustic forms of communication due to its propensity for masking and decreasing the potential acoustic space of signals. Furthermore, anthropogenic noise may affect biological processes including animal interactions, physiological and behavioural responses to stimuli and cognitive development. However, animals' cognitive abilities may enable them to cope with high levels of anthropogenic noise through learning, the employment of acoustic and behavioural flexibility as well as the use of multi-modal sensory systems. We are only just beginning to understand how neural structures, endocrine systems and behaviour are mechanistically linked in these scenarios, providing us with information we can use to mitigate deleterious effects of pervasive noise on wildlife, along with highlighting the remarkable adaptability of animals to an increasingly anthropogenic world. In this review, I will focus mainly on birds, due to the amount of literature on the topic, and survey recent advancements made in two main spheres: (1) how anthropogenic noise affects cognitive processes and (2) how cognition enables animals to cope with increasingly noisy environments. I will be highlighting current gaps in our knowledge, such as how noise might impact behavioural traits such as predation, as well as how noise causes physical damage to neurotransmitters and affects stress levels, in order to direct future studies on this topic.

Biomechanical control of vocal plasticity in an echolocating bat

Many animal species adjust the spectral composition of their acoustic signals to variable environments. However, the physiological foundation of such spectral plasticity is often unclear. The source-filter theory of sound production, initially established for human speech, applies to vocalizations in birds and mammals. According to this theory, adjusting the spectral structure of vocalizations could be achieved by modifying either the laryngeal/syringeal source signal or the vocal tract, which filters the source signal. Here, we show that in pale spear-nosed bats, spectral plasticity induced by moderate level background noise is dominated by the vocal tract rather than the laryngeal source signal. Specifically, we found that with increasing background noise levels, bats consistently decreased the spectral centroid of their echolocation calls up to 3.2 kHz, together with other spectral parameters. In contrast, noise-induced changes in fundamental frequency were small (maximally 0.1 kHz) and were inconsistent across individuals. Changes in spectral centroid did not correlate with changes in fundamental frequency, whereas they correlated negatively with changes in call amplitude. Furthermore, while bats consistently increased call amplitude with increasing noise levels (the Lombard effect), increases in call amplitude typically did not lead to increases in fundamental frequency. In summary, our results suggest that at least to a certain degree echolocating bats are capable of adjusting call amplitude, fundamental frequency and spectral parameters independently.

Anthropogenic noise affects behavior across sensory modalities

Many species are currently experiencing anthropogenically driven environmental changes. Among these changes, increasing noise levels are specifically a problem for species using acoustic signals (i.e., species relying on signals that use the same sensory modality as anthropogenic noise). Yet many species use other sensory modalities, such as visual and olfactory signals, to communicate. However, we have only little understanding of whether changes in the acoustic environment affect species that use sensory modalities other than acoustic signals. We studied the impact of anthropogenic noise on the common cuttlefish Sepia officinalis, which uses highly complex visual signals. We showed that cuttlefish adjusted their visual displays by changing their color more frequently during a playback of anthropogenic noise, compared with before and after the playback. Our results provide experimental evidence that anthropogenic noise has a marked effect on the behavior of species that are not reliant on acoustic communication. Thus, interference in one sensory channel, in this case the acoustic one, affects signaling in other sensory channels. By considering sensory channels in isolation, we risk overlooking the broader implications of environmental changes for the behavior of animals.

White-throated sparrows alter songs differentially in response to chorusing anurans and other background noise

Animals can use acoustic signals to attract mates and defend territories. As a consequence, background noise that interferes with signal transmission has the potential to reduce fitness, especially in birds that rely on song. While much research on bird song has investigated vocal flexibility in response to urban noise, weather and other birds, the possibility of inter-class acoustic competition from anurans has not been previously studied. Using sound recordings from central Ontario wetlands, we tested if white-throated sparrows (Zonotrichia albicolis) make short-term changes to their singing behaviour in response to chorusing spring peepers (Pseudacris crucifer), as well as to car noise, wind and other bird vocalizations. White-throated sparrow songs that were sung during the spring peeper chorus were shorter with higher minimum frequencies and narrower bandwidths resulting in reduced frequency overlap. Additionally, sparrows were less likely to sing when car noise and the vocalizations of other birds were present. These patterns suggest that birds use multiple adjustment strategies. This is the first report to demonstrate that birds may alter their songs differentially in response to different sources of noise. This article is part of a Special Issue entitled: insert SI title.