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Male humpback whales switch to singing in the presence of seismic air guns

Ocean noise produced by seismic exploration has been implicated in causing changes in baleen whale hearing, physiology, feeding, breeding, and migratory behaviours. Here, we observed changes in the mating tactics of humpback whales (Megaptera novaeangliae) during a one-hour exposure to nearby seismic air guns. Males employ a conditional mating strategy where they switch between singing and non-singing tactics. Singing is presumably an advertisement signal, while non-singing behaviours include seeking out and joining with females as well as forming competitive groups. During periods of active air guns, the number of male singing whales increased, and singers were more likely to be observed joining females. Conversely, non-singing males were less likely to engage in joining interactions suggesting that active air guns caused a switch in male breeding tactics. Though we cannot translate these effects into changes in breeding success, this indicates that seismic exploration has the potential to alter breeding behaviours in baleen whales.

Color-advertising strategies of invasive plants through the bee eye.

Invasive plants represent a significant global challenge as they compete with native plants for limited resources such as space, nutrients and pollinators. Here, we focused on four invasive species that are widely spread in the French Pyrenees, Buddleja davidii, Reynoutria japonica, Spiraea japonica and Impatiens glandulifera, and analyzed their visual advertisement signals with respect to those displayed by their surrounding native species using a perceptual approach based on the neural mechanisms of bee vision given that bees are regular pollinators of these plants. We collected 543 spectral reflections from the 4 invasive species, and 66 native species and estimated achromatic and chromatic similarities to the bee eye. R. japonica, S. japonica and B. davidii were inconspicuous against the foliage background and could be hardly discriminated in terms of color from their surrounding native plants. These characteristics promote generalization, potentially attracting pollinators foraging on similar native species. Two morphs of I. glandulifera were both highly salient in chromatic and achromatic terms and different from their surrounding native species. This distinctive identity facilitates detection and learning in association with rich nectar. While visual signals are not the only sensory cue accounting for invasive-plant success, our study reveals new elements for understanding biological invasion processes from the perspective of pollinator perceptual processes.

Levels of Airborne Sound And Substrate-borne Vibration Calling Are Negatively Related Across Neotropical False-leaf Katydids.

Animals often signal in multiple sensory modalities to attract mates, but the level of signaling investment in each modality can differ dramatically between individuals and across species. When functionally overlapping signals are produced in different modalities, their relative use can be influenced by many factors, including differences in signal active space, energetic costs, and predation risk. Characterizing differences in total signal investment across time can shed light on these factors, but requires long focal recordings of signal production. Neotropical pseudophylline katydids produce mate advertisement signals as airborne sound and substrate-borne vibration. Airborne calls, produced via stridulation, are extremely short, high-frequency, and longer-range signals. Conversely, substrate-borne calls produced via abdominal tremulation are longer, low-frequency, relatively more energetically costly, and shorter-range signals. To examine patterns of stridulation and tremulation across species and test hypotheses about the drivers of signal use in each modality, we recorded multimodal signaling activity over 24 hours for males from 10 pseudophylline species from a single Panamanian community. We also collected data on demographic and morphological species characteristics, and acoustic features of airborne calls, such as bandwidth, peak frequency, and duration. Finally, we generated a molecular phylogeny for these species and used phylogenetic generalized least squares models to test for relationships between variables while controlling for evolutionary relationships. We found a negative relationship between sound and vibration calling, indicating that substrate-borne vibrational signaling may compensate for reduced airborne signaling in these species. Sound call bandwidth and the proportion of males collected at lights, a proxy for the amount of male movement, also explained a significant amount of variation in sound calling across species, indicating that the overall relationship between the two types of calling signals may be mediated by the specific characteristics of the signals as well as other species traits.

Partitioning variance in a signaling trade-off under sexual selection reveals among-individual covariance in trait allocation.

Understanding the evolution of traits subject to trade-offs is challenging because phenotypes can (co)vary at both the among- and within-individual levels. Among-individual covariation indicates consistent, possibly genetic, differences in how individuals resolve the trade-off, while within-individual covariation indicates trait plasticity. There is also the potential for consistent among-individual differences in behavioral plasticity, although this has rarely been investigated. We studied the sources of (co)variance in two characteristics of an acoustic advertisement signal that trade-off with one another and are under sexual selection in the gray treefrog, Hyla chrysoscelis: call duration and call rate. We recorded males on multiple nights calling spontaneously and in response to playbacks simulating different competition levels. Call duration, call rate, and their product, call effort, were all repeatable both within and across social contexts. Call duration and call rate covaried negatively, and the largest covariance was at the among-individual level. There was extensive plasticity in calling with changes in social competition, and we found some evidence for among-individual variance in call rate plasticity. The significant negative among-individual covariance in trait values is perpendicular to the primary direction of sexual selection in this species, indicating potential limits on the response to selection.

On the function of a female-like signal type in the vibrational repertoire of Enchenopa male treehoppers (Hemiptera: Membracidae).

Animals often mimic the behaviours or signals of conspecifics of the opposite sex while courting. We explored the potential functions of a novel female-like signal type in the courtship displays of male Enchenopa treehoppers. In these plant-feeding insects, males produce plant-borne vibrational advertisement signals, to which females respond with their own duetting signals. Males also produce a signal type that resembles the female duetting responses. We experimentally tested whether this signal modifies the behaviour of receivers. First, we tested whether the female-like signal would increase the likelihood of a female response. However, females were as likely to respond to playbacks with or without them. Second, we tested whether the female-like signal would inhibit competing males, but males were as likely to produce displays after playbacks with or without them. Hence, we found no evidence that this signal has an adaptive function, despite its presence in the courtship display, where sexual selection affects signal features. Given these findings, we also explored whether the behavioural and morphological factors of the males were associated with the production of the female-like signal. Males that produced this signal had higher signalling effort (longer and more frequent signals) than males that did not produce it, despite being in worse body condition. Lastly, most males were consistent over time in producing the female-like signal or not. These findings suggest that condition-dependent or motivational factors explain the presence of the female-like signal. Alternatively, this signal might not bear an adaptive function, and it could be a way for males to warm up or practice signalling, or even be a by-product of how signals are transmitted through the plant. We suggest further work that might explain our puzzling finding that a signal in the reproductive context might not have an adaptive function.

A female mimic signal type in the vibrational repertoire of male Enchenopa treehoppers

Abstract Animals vary in the complexity and size of the signal repertoire used in communication. Often, these behavioural repertoires include multiple signal types for the same process, for instance, courtship. In Enchenopa treehoppers (Hemiptera: Membracidae) mate-searching males produce plant-borne vibrational advertisement signals. Receptive females then respond to males with their own signals. Here we describe an additional signal type in the repertoire of these males. We collected nymphs in Wisconsin, USA, and recorded the spontaneous signalling bouts of adult males and duetting signals of females using laser vibrometry. Two-thirds of males produced the additional signal type, which differed in temporal and spectral features from the main male advertisement signals, whilst resembling female duetting signals in placement and acoustic features. Our findings suggest that this might be a female mimic signal. Overall, our findings highlight the diversity in the behavioural repertoire that animals may use for reproduction.

Efficacy constraints on female directional preference stabilize a male call component in a multimodal cricket duet

Mate advertisement signals can vary in their variability, with some signals or signal components being highly stereotyped and others highly variable. One hypothesis for differences in variability suggests that receivers provide stabilizing selection for signal components important for mate recognition, resulting in low variability, while also providing directional selection for signal components indicating mate quality, correlating with greater variability. Factors that affect signal efficacy, however, can also influence signal design – and might interact with receiver preferences to influence variability. We investigated these hypotheses in the duetting cricket species Lebinthus bitaeniatus Stål. Like many other insects, these crickets use call-and-response signalling to find mates. Males produce an acoustic signal, females reply with a vibrational signal, and males use these vibrations to find females. Male calls have two components: a series of isolated syllables (ticks) followed by a rapid series of syllables (a trill). The number of ticks per call is highly variable, whereas the number of pulses in the trill is stereotyped. Male size was correlated positively with the number of trill pulses per call, but not with the number of ticks. Female response amplitude was not related to the number of ticks in the male call (the variable component) and it increased with trill duration (the stereotyped signal component), contrary to the stabilizing directional selection hypothesis. We also found, however, that the latency of the female vibrational reply became erratic as the trill increased beyond the typical duration. Since males in duetting insect species rely on the species-specific latency of the female reply to recognize the signal, this reduces the efficacy of the female reply for communication despite the females’ greater amplitude response. Given the prevalence of call-and-response signalling in insects, this relationship between female response amplitude and signal efficacy could be a common factor influencing signal variability across many species. • Male calls in a duetting cricket species have two components: ticks and a trill. • The number of ticks per call is highly variable, whereas the trill is stereotyped. • Female vibrational replies to males were unaffected by the number of ticks per call. • Females produced stronger replies to longer trills, but timing was adversely affected. • An interaction between female reply amplitude and efficacy explains uniform trills.

Spoiled for choice: number of signalers constrains mate choice based on acoustic signals

AbstractAnimal communication mediates social interactions with important fitness consequences for individuals. Receivers use signals to detect and discriminate among potential mates. Extensive research effort has focused on how receiver behavior imposes selection on signalers and signals. However, animals communicate in socially and physically complex environments with important biotic and abiotic features that are often excluded from controlled laboratory experiments, including noise. “Noise” is any factor that prevents signal detection and discrimination. The noise caused by aggregates of acoustic signalers is a well-known impediment to receivers, but how many individual signalers are required to produce the emergent effects of chorus noise on receiver behavior? In Teleogryllus oceanicus, the Australian field cricket, we assayed female preferences for a temporal property of male advertisement signals, the number of long chirp pulses, using two-, four-, six-, and eight-choice phonotaxis experiments. We found that, as the number of individual signalers increased, receivers became less likely to respond phonotactically and less likely to express their well-documented preference for more long chirp pulses. We found that very few individual signalers can create a sufficiently noisy environment, due either to acoustic interference or choice overload, to substantially impair female preference expression. Our results suggest that receivers may not always be able to express their well-documented mating preferences in nature.

Noise elicits the Lombard effect in midshipman mating vocalizations

Anthropogenic noise pollution is an emerging global threat to fish populations. Among a suite of deleterious effects, noise can potentially impede reproductive success in some fishes by masking their mate advertisement vocalizations. Using the plainfin midshipman fish (Porichthys notatus), a marine toadfish that produces a distinctive “hum” during courtship, we investigated how noise affects male vocalizations and spawning success in the wild. We recorded nesting males for three days and measured the frequency, amplitude, and duration of their vocalizations before, during, and after exposure to artificial noise (a c. 118-Hz tone). We also counted eggs in nests exposed to 10 days of artificial noise versus control nests that were not exposed to artificial noise. Males exposed to noise reduced the number of vocalizations they produced, reduced the frequency of their vocalizations, and increased the amplitude of their mating hum (Lombard effect). However, chronic noise exposure did not clearly affect spawning success, suggesting that the Lombard effect allowed males to sustain clear advertisement signals when competing with the relatively weak artificial noise source.

Zebra finch song is a very short-range signal in the wild: evidence from an integrated approach.

Birdsong is typically seen as a long-range signal functioning in mate attraction and territory defense. Among birds, the zebra finch is the prime model organism in bioacoustics, yet almost exclusively studied in the lab. In the wild, however, zebra finch song differs strikingly from songbirds commonly studied in the wild as zebra finch males sing most after mating and in the absence of territoriality. Using data from the wild, we here provide an ecological context for a wealth of laboratory studies. By integrating calibrated sound recordings, sound transmission experiments and social ecology of zebra finches in the wild with insights from hearing physiology we show that wild zebra finch song is a very short-range signal with an audible range of about nine meters and that even the louder distance calls do not carry much farther (up to about fourteen meters). These integrated findings provide an ecological context for the interpretation of laboratory studies of this species and indicate that the vocal communication distance of the main laboratory species for avian acoustics contrasts strikingly with songbirds that use their song as a long-range advertisement signal.

Low-amplitude noise elicits the Lombard effect in plainfin midshipman mating vocalizations in the wild

Anthropogenic noise pollution is an emerging global threat to fish populations. Among a suite of deleterious effects, noise can potentially impede reproductive success in some fishes by masking their mate advertisement vocalizations. Using the plainfin midshipman fish, Porichthys notatus , a marine toadfish that produces a distinctive ‘hum’ during courtship, we investigated how noise affects male vocalizations and spawning success in the wild. We recorded nesting males for 3 days and measured the frequency (i.e. pitch), amplitude and duration of their vocalizations before, during and after exposure to artificial noise (∼118 Hz tone). We also counted eggs in nests exposed to 10 days of artificial noise versus control nests that were not exposed to artificial noise. Males exposed to noise produced fewer vocalizations, reduced the frequency of vocalizations and increased the amplitude of their mating hum (Lombard effect). However, chronic artificial noise exposure did not significantly affect spawning success, suggesting that the Lombard effect allowed males to sustain clear advertisement signals when competing with a relatively weak artificial noise source. Future studies are needed to determine whether such vocal adjustments incur costs for males, and how common anthropogenic noises, such as boat engines, affect spawning and reproductive success. • Artificial noise elicited an amplitude increase in mate advertisement vocalizations. • Artificial noise elicited peak frequency decreases in three different vocalizations. • All vocalizations occurred less often in the presence of artificial noise. • Exposure to artificial noise for 10 days did not appear to affect spawning success.

Daily Signaling Rate and the Duration of Sound per Signal are Negatively Related in Neotropical Forest Katydids.

Researchers have long examined the structure of animal advertisement signals, but comparatively little is known about how often these signals are repeated and what factors predict variation in signaling rate across species. Here, we focus on acoustic advertisement signals to test the hypothesis that calling males experience a tradeoff between investment in the duration or complexity of individual calls and investment in signaling over long time periods. This hypothesis predicts that the number of signals that a male produces per 24 h will negatively correlate with (1) the duration of sound that is produced in each call (the sum of all pulses) and (2) the number of sound pulses per call. To test this hypothesis, we measured call parameters and the number of calls produced per 24 h in 16 species of sympatric phaneropterine katydids from the Panamanian rainforest. This assemblage also provided us with the opportunity to test a second taxonomically specific hypothesis about signaling rates in taxa such as phaneropterine katydids that transition from advertisement calls to mating duets to facilitate mate localization. To establish duets, male phaneropterine katydids call and females produce a short acoustic reply. These duets facilitate searching by males, females, or both sexes, depending on the species. We test the hypothesis that males invest either in calling or in searching for females. This hypothesis predicts a negative relationship between how often males signal over 24 h and how much males move across the landscape relative to females. For the first hypothesis, there was a strong negative relationship between the number of signals and the duration of sound that is produced in each signal, but we find no relationship between the number of signals produced per 24 h and the number of pulses per signal. This result suggests the presence of cross-taxa tradeoffs that limit signal production and duration, but not the structure of individual signals. These tradeoffs could be driven by energetic limitations, predation pressure, signal efficacy, or other signaling costs. For the second hypothesis, we find a negative relationship between the number of signals produced per day and proportion of the light trap catch that is male, likely reflecting males investing either in calling or in searching. These cross-taxa relationships point to the presence of pervasive trade-offs that fundamentally shape the spatial and temporal dynamics of communication.

Vibrational playbacks and microscopy to study the signalling behaviour and female physiology of Philaenusspumarius

AbstractThe meadow spittlebug Philaenus spumarius, vector of the bacterium Xylella fastidiosa, relies on vibrational communication to accomplish mating: the female calls to establish a duet with a male. A deeper knowledge of the species’ reproductive biology and behaviour would provide useful information for developing control techniques based on principles of ‘biotremology’, which studies the vibrational behaviour of animals. Playback tests were conducted on single females and male–female pairs of P. spumarius from June to October 2018, and the features of the recorded calling signals were analysed using a wavelet decomposition. Dissections were performed on females to evaluate the relationship between calling activity and ovarioles development. From August onwards, females started to emit calling signals and to develop ovarioles. Female calling activity, duration of their chirps and their responsiveness to mating increased as the season progressed, and they were correlated with ovarioles’ development and presence of mature eggs. Hence, the ovarian maturation represents a key factor in association with the development of the sexual behaviour of P. spumarius females. Conversely, males produced advertisement signals soon after adult eclosion in May, but these signals were not involved in the pair formation process. Mating was achieved only when males produced courtship signals in response to female calling signals and established with them vibrational duets starting from August. Here, we provide new information regarding the P. spumarius’ ethology and hypothesize that potential mating disruption techniques should consider the insect physiology and be applied when both sexes are responsive to mating signals.

The relationship between a combinatorial processing rule and a continuous mate preference function in an insect.

Mate choice involves processing signals that can reach high levels of complexity and feature multiple components, even in small animals with tiny brains. This raises the question of whether and how such organisms deal with this complexity. One solution involves combinatorial processing, whereby different signal elements are processed as single units. Combinatorial processing has been described in several mammals and birds, and recently in a vibrationally signalling insect, Enchenopa treehoppers. Here, we ask about the relationship between combinatorial rules and mate preferences for continuously varying signal features. Enchenopa male advertisement signals are composed of two elements: a 'whine' followed by a set of pulses. The dominant frequency of the whine and element combination both matter to females. We presented synthetic signals varying in element order (natural [whine-pulses], reverse [pulses-whine]) and in frequency to Enchenopa females and recorded their responses. The reverse combination resulted in a decrease in attractiveness of the signals, and also slightly changed the shape of the preference for frequency. We found that females could be classified into three 'types': females with both a strong preference and a strong combinatorial rule, females with both a weak preference and weak rule, and females with a strong preference but a weak rule. Our results suggest that in Enchenopa signal processing, the mate preference for a continuous signal feature 'takes precedence' over, but also interacts with, the combinatorial rule. The relationship between the preference and the rule could evolve to take different forms according to selection on mate choice decisions. We suggest that exploring the relationship between such preferences and rules in species with more complex signals will bring insight into the evolution of the multi-component communication systems.

Species Recognition Is Constrained by Chorus Noise, but Not Inconsistency in Signal Production, in Cope’s Gray Treefrog (Hyla chrysoscelis)

Optimal mate choice based on the assessment of communication signals can be constrained by multiple sources of noise. One well-known impediment to acoustically guided mating decisions is the ambient noise created by multiple signaling individuals in large social groups, in which ambient noise can mask signals by impairing signal recognition and discrimination by receivers. Although studied far less often, another potential source of noise in communication systems stems from variability or inconsistency in how signalers produce their signals. Consistency is especially important in the context of mate choice because sexual advertisement signals are frequently produced repeatedly through time and are composed of constituent parts (e.g., notes, pulses) that are repeated within signals. Inconsistent signal production within individuals has the potential to mask between-individual differences that are often the target of receiver decision-making. In this study of Cope’s gray treefrog, Hyla chrysoscelis, we tested the hypothesis that ambient noise and inconsistent signaling, both independently and synergistically, impair discrimination of species identity. We assayed female discrimination based on pulse rate, a signal of species identity, in quiet and at three levels of ambient noise designed to simulate a breeding chorus. We used synthetic advertisement calls that were invariant or generated with one of three experimental levels of inconsistency in pulse rate, chosen based on levels of within-individual variation observed in natural calls. Pulse rate discrimination was impaired by average and above-average levels of chorus noise, but not by inconsistency in signal production. Receivers spent slightly more time making decisions at the highest level of chorus noise, but response latencies were unaffected by inconsistency. There was no evidence of synergism between ambient noise and inconsistency. Our results suggest that ambient noise, but not inconsistency in signal production, may limit sexual selection on a signal of species identity in natural settings.

Extraterritorial forays by great tits are associated with dawn song in unexpected ways.

Conspicuous male signals often play an important role in both attracting mates and deterring rivals. In territorial species with extrapair mating, female and male forays to other territories may be an important component underlying female choice and male mating success and might be influenced by male advertisement signals. Yet, whether off-territory foraying is associated with male signals is still not well understood. Here, we tested how female and male forays are associated with short-range visual and long-range acoustic signals (dawn song). We used an automated radio tracking system to follow the movements of wild great tits (Parus major) to other territories in relation to male dawn song, plumage ornaments, and extrapair paternity. We show that both sexes frequently forayed into others’ territories throughout the breeding period. Movements of both males and females were associated with male song but not with plumage ornaments. Contrary to our expectations, females stayed away from territories where males sang elaborately, whereas males were attracted to those territories. Moreover, neither female nor male forays were associated with the occurrence of extrapair offspring. Our results, thus, suggest that, although forays into other territories are associated with male dawn song, females may not be attracted and males not repelled by dawn song. This sheds a different light on the sex-specific effects of male advertisement signals, expanding the view on the selection pressures shaping such communication systems.

Environmental heterogeneity alters mate choice behavior for multimodal signals

Animals frequently experience changes in their environment, including diel and seasonal shifts in abiotic and biotic factors. In addition to physiological and morphological changes, animals alter their behavior in response to environmental variation. This study examined the impacts of heterogeneous environments on mating behaviors. We examined both male and female tungara frog phonotactic responses to multimodal (audiovisual) and unimodal (acoustic) stimuli. We altered aspects of the physical environment by changing substrate (terrestrial and aquatic) and ambient light levels. Females demonstrated a similar preference for the audiovisual stimulus regardless of substrate but decreased latency to choose in an aquatic environment. When ambient light levels were increased (relative to darker control), females reversed their preference, avoiding the multimodal stimulus, but the latency to choose was unchanged. Males demonstrated no preference for the multimodal signal on either substrate, but like females, male latency was reduced in an aquatic environment. Different environments carry their own associated costs, including varying levels of predation risk. Increased light levels and an aquatic environment likely carry higher predation risk and therefore should lead to changes in female and male responses. Interestingly, these two environments do not cause uniform changes in female responses. The addition of an aquatic environment led to a reduction in latency, whereas an increase in ambient light levels induced a change in female mate preference. These findings demonstrate the importance of the environment on mating responses to multimodal signals. Responses to conspecific mating signals (e.g., mate preferences) have often been assumed to be static. In this study, we tested responses of both male and female tungara frogs to a multimodal (visual + vocal) advertisement signal under varying environmental conditions. Elevated light levels changed female responses to mating signals. When both sexes were required to swim, rather than walk to a signal, their choices did not change. They did respond faster, however. These results indicate that measuring the strength of sexual selection should be considered as a function of both innate preferences and the environmental conditions experienced by the animals.

Dueling frogs: do male green tree frogs (Hyla cinerea) eavesdrop on and assess nearby calling competitors?

Individuals produce advertisement signals with intended purposes and targets. However, these signals can be received by “eavesdroppers,” who may extract information from them and alter their behavior according to the extracted information. In anuran systems, males congregate at breeding sites to produce advertisement calls to attract receptive females and fend off rival males. Both sexes directly assess these calls in dyadic encounters and make decisions based on the call’s characteristics, e.g., frequency. What is unknown is whether bystander males eavesdrop on these same calls to inform their future competitive decisions. Here, we examined whether male green tree frogs (Hyla cinerea) eavesdrop on competing males, assess their competitor’s call frequency, and respond accordingly. We exposed males to playbacks of two competing males that varied in call frequency—high, average, low—and quantified latency to call, time spent calling, and number of calling bouts. We found that males had reduced latency to call and called more when eavesdropping low-frequency competition, but not average or high-frequency competition. Focal male size also influenced how they responded, with larger males being more responsive than smaller males. Our results indicate that male green tree frogs are capable of eavesdropping on nearby male calls and produce behavioral responses accordingly. Further, it appears males are able to alternate between assessment strategies dependent upon the frequency of the eavesdropped competition. These findings indicate that males not only directly assess an opponent’s call in dyadic encounters, but also indirectly through eavesdropping. SIGNIFICANCE STATEMENT: Animals produce signals with intended purposes and targets, but which can be received by nearby eavesdroppers. Eavesdropped signals can elicit complex phenotypic changes in the eavesdropper, and lead to significant fitness consequences for the signal producer and eavesdropper. Thus, examining whether and to what extent individuals eavesdrop is an important step in understanding the evolution of signal production and response. Here, we examined whether male green tree frogs, H. cinerea, eavesdrop on nearby competing males and base their own calling behaviors on eavesdropped male’s calling characteristics. Calling behavior was mediated by focal male body size and eavesdropped male’s call frequency. Larger males reduced response latency and called more to nearby low-frequency males, while smaller males reduced response latency to nearby calls of average frequency. These results indicate that males will extract information from their competitive environment through eavesdropping and produce behavioral responses according to the eavesdropped information.

Male Field Cricket Songs Are Altered After Aggressive Interactions

To address the hypothesis that male acoustic sexual advertisement signals might be indicators of aggressiveness, in addition to chemical signals, we examined the relationship between levels of aggression/dominance status and acoustic sexual advertisement signals in the field cricket Gryllus integer. Males were paired in aggression trials and recorded the night before and the night after the trial. This allowed us to test whether aggression is inherently linked to song phenotypes, or whether aggressive interactions cause males to alter their songs. We found that dominant (winning) males signaled with higher energy, amplitude and power the night after winning an aggressive encounter, but not before the encounter. Time spent calling and the number of calling bouts were unrelated to aggression, whereas whereas winning males increased their bout lengths after winning, and losing males decreased their bout lengths after losing.

Multivariate phenotypic selection on a complex sexual signal.

Animal signals are complex, comprising multiple components that receivers may use to inform their decisions. Components may carry information of differing value to receivers, and selection on one component could modulate or reverse selection on another, necessitating a multivariate approach to estimating selection gradients. However, surprisingly few empirical studies have estimated the strength of phenotypic selection on complex signals with appropriate design and adequate power to detect nonlinear selection. We used phonotaxis assays to measure sexual selection on the advertisement signal of Cope's gray tree frog, Hyla chrysoscelis. Female preferences were assessed for five signal components using single- and two-stimulus behavioral assays. Linear, quadratic, and correlational selection gradients were estimated from the single-stimulus data. Significant directional selection is acting on call duration, call rate, pulse rate, and relative amplitude; stabilizing selection is acting on call duration and call rate. Under the two-stimulus paradigm, conclusions were qualitatively different, revealing nonlinear selection on all components except call duration. For individual subjects, the outcomes of single- and two-stimulus trials were frequently discordant, suggesting that the choice of testing paradigm may affect conclusions drawn from experiments.