The Sounds of Silence as an Alarm Cue in Túngara Frogs,Physalaemus pustulosus

In many species males vocally advertise for mates in choruses and these choruses serve as acoustic beacons to conspecific females as well as to eavesdropping predators and parasites. Chorusing will often cease in response to disturbances, such as the presence of predators. In some cases the cessation is so rapid and over such a large area that it seems improbable that males are all responding directly to the same local disturbance. Here, we demonstrate experimentally in Neotropical túngara frogs, Physalaemus pustulosus, that the cessation of calling by males spreads rapidly through the chorus. The cessation of chorusing in response to the cessation of playbacks of three calling males is more effective in inducing chorus cessation than is the cessation of one male calling. When three males are calling, the cessation of complex calls is more effective in inducing chorus cessation than simple calls. There is no main effect on whether the final call of the male is complete or is interrupted. We thus conclude that the sudden lack of signals—the ‘sounds of silence’—becomes an alarm cue that can explain the rapid cessation of choruses that are common in many chorusing species.     

The transmission of advertisement calls in Central American frogs

Acoustic communication signals change over distance due to lossof amplitude and fidelity, and it is assumed that signal degradationinfluences the receiver's ability to detect and decode signals.The degree of degradation depends on the signal's structureand the environment through which it transmits. We broadcastthe advertisement calls of 22 species of Central American frogsat two heights within forested and open environments in Panama. Werecorded these calls at five distances from the source and estimatedsignal degradation with a cross-correlation analysis, a measurethat combines the effects of decrement in signal amplitude andfidelity. Calls degraded less when broadcast higher above theground compared to on the ground, and less in open habitat comparedto forested habitat; there was an additional interaction betweenheight and environment. Furthermore, calls with low dominant frequenciesexperienced less degradation than calls with high dominant frequencies.There was no evidence, however, that the calls of these frogs haveevolved to maximize habitat-specific transmission.     

A micro-geography of fear: learning to eavesdrop on alarm calls of neighbouring heterospecifics

Many vertebrates eavesdrop on alarm calls of other species, which is a remarkable ability, given geographical variation in community composition and call diversity within and among species. We used micro-geographical variation in community composition to test whether individuals recognize heterospecific alarm calls by: (i) responding to acoustic features shared among alarm calls; (ii) having innate responses to particular heterospecific calls; or (iii) learning specific alarm calls. We found that superb fairy-wrens (Malurus cyaneus) fled to cover to playback of noisy miner (Manorina melanocephala) aerial predator alarm calls only in locations where miners were present, suggesting that learning rather than acoustic structure determines response. Sites with and without miners were well within the dispersal distance of fairy-wrens, and philopatric males and dispersing females showed the same pattern, so that local genetic adaptation is extremely unlikely. Furthermore, where miners were present, fairy-wrens responded appropriately to different miner calls, implying eavesdropping on their signalling system rather than fleeing from miners themselves. Learned eavesdropping on alarm calls enables individuals to harvest ecologically relevant information from heterospecifics on an astonishingly fine spatial scale. Such phenotypic plasticity is valuable in a changing world, where individuals can be exposed to new species.     

Trade-off in short- and long-distance communication in tungara (Physalaemus pustulosus) and cricket (Acris crepitans) frogs

Female phonotaxis in túngara (Physalaemus pustulosus)and cricket (Acris crepitans) frogs is biased toward male advertisementcalls or call components of lower frequency. This behavioralbias might result in part from a mismatch between the spectralcharacteristics of the advertisement call and the most sensitive frequencyof the peripheral end organ implicated in reception of thesesounds. In both species, females are tuned to frequencies lowerthan average for the calls in their population. This mismatch,however, represents the situation during short-distance communication.Female frogs can also use the call to detect choruses at longdistances, and the spectral distribution of call energy canvary with transmission distance. We used computer simulationsto test the hypothesis that there is a better match betweentuning and call spectral energy at long distances from the callingmale than at short distances by comparing the performance (soundenergy received) of the natural tuning curve relative to anoptimal tuning curve (i.e., one centered at the call's dominantfrequency). The relative performance of the natural tuning curveincreased with distance in túngara frogs. For the twosubspecies of cricket frogs, however, the relative performancedecreased at longer distances. The performance did not equalthe optimal tuning curve at the distances tested. The resultsindicate that the relationship between calls and auditory tuningcannot be optimal for both long and short distance reception.The relationship between female tuning and call dominant frequencymay represent a compromise between short and long distance communication,and the bias toward short or long distances may vary among species.     

Eavesdropping on heterospecific alarm calls: from mechanisms to consequences

Animals often gather information from other species by eavesdropping on signals intended for others. We review the extent, benefits, mechanisms, and ecological and evolutionary consequences of eavesdropping on other species' alarm calls. Eavesdropping has been shown experimentally in about 70 vertebrate species, and can entail closely or distantly related species. The benefits of eavesdropping include prompting immediate anti‐predator responses, indirect enhancement of foraging or changed habitat use, and learning about predators. Eavesdropping on heterospecifics can provide more eyes looking for danger, complementary information to that from conspecifics, and potentially information at reduced cost. The response to heterospecific calls can be unlearned or learned. Unlearned responses occur when heterospecific calls have acoustic features similar to that used to recognize conspecific calls, or acoustic properties such as harsh sounds that prompt attention and may allow recognition or facilitate learning. Learning to recognize heterospecific alarm calls is probably essential to allow recognition of the diversity of alarm calls, but the evidence is largely indirect. The value of eavesdropping on different species is affected by problems of signal interception and the relevance of heterospecific alarm calls to the listener. These constraints on eavesdropping will affect how information flows among species and thus affect community function. Some species are ‘keystone’ information producers, while others largely seek information, and these differences probably affect the formation and function of mixed‐species groups. Eavesdroppers might also integrate alarm calls from multiple species to extract relevant and reliable information. Eavesdropping appears to set the stage for the evolution of interspecific deception and communication, and potentially affects communication within species. Overall, we now know that eavesdropping on heterospecific alarm calls is an important source of information for many species across the globe, and there are ample opportunities for research on mechanisms, fitness consequences and implications for community function and signalling evolution.     

Sex‐specific responses to territorial intrusions in a communication network: Evidence from radio‐tagged great tits

Signals play a key role in the ecology and evolution of animal populations, influencing processes such as sexual selection and conflict resolution. In many species, sexually selected signals have a dual function: attracting mates and repelling rivals. Yet, to what extent males and females under natural conditions differentially respond to such signals remains poorly understood, due to a lack of field studies that simultaneously track both sexes. Using a novel spatial tracking system, we tested whether or not the spatial behavior of male and female great tits (Parus major) changes in relation to the vocal response of a territorial male neighbor to an intruder. We tracked the spatial behavior of male and female great tits (N = 44), 1 hr before and 1 hr after simulating territory intrusions, employing automatized Encounternet radio‐tracking technology. We recorded the spatial and vocal response of the challenged males and quantified attraction and repulsion of neighboring males and females to the intrusion site. We additionally quantified the direct proximity network of the challenged male. The strength of a male's vocal response to an intruder induced sex‐dependent movements in the neighborhood, via female attraction and male repulsion. Stronger vocal responders were older and in better body condition. The proximity networks of the male vocal responders, including the number of sex‐dependent connections and average time spent with connections, however, did not change directly following the intrusion. The effects on neighbor movements suggest that the strength of a male's vocal response can provide relevant social information to both the males and the females in the neighborhood, resulting in both sexes adjusting their spatial behavior in contrasting ways, while the social proximity network remained stable. This study underlines the importance of “silent” eavesdroppers within communication networks for studying the dual functioning and evolution of sexually selected signals.     

Mate choice copying as public information

We outline a framework that views mate choice copying as public information. In this framework, all individuals assess the quality of potential mates. Individuals can benefit by copying when they cannot discriminate differences in potential mates, as copying can result in better discrimination. This framework predicts that the discrimination ability of individuals will strongly determine the frequency of copying behaviour observed: individuals with poor discrimination ability should often copy, whereas individuals with superior discrimination ability should rarely copy. Copying behaviour may occur in a wide variety of taxa but only be observed when individuals are presented with difficult discrimination tasks.     

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.     

A biomechanical perspective on the use of forelimb length as a measure of sexual selection in frogs

Differences in forelimb length between male and female frogs and between amplexing and non-amplexing males have been interpreted to be the results of sexual selection on forelimb length. The causal feature of the forelimb that has been posited to cause such selection is the observation that non-amplexing males attempt to disrupt breeding by prying amplexing males from females. A biomechanical model of forelimb function suggests that total length per se may not be the most appropriate measure to use. There are more functionally significant aspects of forelimb morphology, such as lever arm lengths, that should influence amplexing ability and may make measures of overall forelimb length misleading. This example highlights the relevance of functional analysis to current questions in evolutionary biology that rely on postulated roles for morphological structures under selection.     

The genetic architecture of behavioral traits in a spider

The existence of consistent individual differences in behavior has been shown in a number of species, and several studies have found observable sex differences in these behaviors, yet their evolutionary implications remain unclear. Understanding the evolutionary dynamics of behavioral traits requires knowledge of their genetic architectures and whether this architecture differs between the sexes. We conducted a quantitative genetic study in a sexually size‐dimorphic spider, Larinioides sclopetarius, which exhibits sex differences in adult lifestyles. We observed pedigreed spiders for aggression, activity, exploration, and boldness and used animal models to disentangle genetic and environmental influences on these behaviors. We detected trends toward (i) higher additive genetic variances in aggression, activity, and exploration in males than females, and (ii) difference in variances due to common environment/maternal effects, permanent environment and residual variance in aggression and activity with the first two variances being higher in males for both behaviors. We found no sex differences in the amount of genetic and environmental variance in boldness. The mean heritability estimates of aggression, activity, exploration, and boldness range from 0.039 to 0.222 with no sizeable differences between females and males. We note that the credible intervals of the estimates are large, implying a high degree of uncertainty, which disallow a robust conclusion of sex differences in the quantitative genetic estimates. However, the observed estimates suggest that sex differences in the quantitative genetic architecture of the behaviors cannot be ruled out. Notably, the present study suggests that genetic underpinnings of behaviors may differ between sexes and it thus underscores the importance of taking sex differences into account in quantitative genetic studies.     

The evolution of courtship display repertoire size in the dabbling ducks (Anatini)

Many animals possess multiple ornaments or behaviours that seem to have evolved via sexual selection. A complete understanding of sexual selection requires an explanation for such multiple traits. The dabbling ducks (Tribe: Anatini) exhibit considerable variation among species in the number of displays in the male courtship repertoire. I tested five hypotheses concerning the evolution of the variation in display repertoire size of dabbling ducks: (1) species recognition, (2) courtship habitat, (3) sexual selection intensity, (4) display media tradeoff and (5) time constraints on pair formation. I tested these hypotheses, using an explicit phylogenetic hypothesis developed from DNA sequences for the dabbling ducks, with two types of statistical comparative methods (discrete and continuous character). The variation observed in male courtship display repertoire size in dabbling ducks was consistent with the courtship habitat and sexual selection intensity hypotheses. Specifically, the size of the display repertoire was larger in species that exhibit courtship exclusively on water and larger in species with dimorphic plumage. These results suggest that ecological (habitat) as well as social (sexual selection) factors may be important in driving the evolution of displays in the dabbling ducks.     

Effects of an invasive species on refuge‐site selection by native fauna: The impact of cane toads on native frogs in the Australian tropics

Invasive species can induce shifts in habitat use by native taxa: either by modifying habitat availability, or by repelling or attracting native species to the vicinity of the invader. The ongoing invasion of cane toads (Rhinella marina) through tropical Australia might affect native frogs by affecting refuge‐site availability, because both frogs and toads frequently shelter by day in burrows. Our laboratory and field studies in the wet‐dry tropics show that native frogs of at least three species (Litoria tornieri, Litoria nasuta and Litoria dahlii) preferentially aggregate with conspecifics, and with (some) other species of native frogs. However, the frogs rarely aggregated with cane toads either in outdoor arenas or in standardized experimental burrows that we monitored in the field. The native frogs that we tested either avoided burrows containing cane toads (or cane toad scent) or else ignored the stimulus (i.e. treated such a burrow in the same way as they did an empty burrow). Native frogs selected a highly non‐random suite of burrows as diurnal retreat sites, whereas cane toads were less selective. Hence, even in the absence of toads, frogs do not use many of the burrows that are suitable for toads. The invasion of cane toads through tropical Australia is unlikely to have had a major impact on retreat‐site availability for native frogs.     

Ontogenetic colour change signals sexual maturity in a non-territorial damselfly

Conspicuous colouration increases male reproductive success through female preferences and/or male–male competition. Despite the advantages of conspicuous colouration, inconspicuous male morphs can exist simultaneously in a population due to genetic diversity, condition dependence or developmental constraints. We are interested in explaining the male dichromatism in Xanthagrion erythroneurum damselflies. We reared these damselflies in outdoor insectaries under natural conditions and showed that this species undergoes ontogenetic colour changes. The younger males are yellow and change colour to red 6–7 days after their emergence. We took red and yellow male reflectance spectra and found that red males are brighter than yellow males. Next, we aimed to determine whether ontogenetic colour change signals sexual maturity with field observations and laboratory experiments. Our field observational data showed that red males are in higher abundance in the breeding territory, and they have a higher mating frequency than yellow males. We confirmed these field observations by enclosing a red and a yellow male with two females and found that yellow males do not mate in presence of red males. To determine whether colour change signals sexual maturity, we measured mating success of males before and after colour changes by enclosing a single male at different age (day 3-day 7) and colour (yellow, intermediate and red) with a single female in a mating cage. Males did not mate when yellow but the same male mated after it changed colour to red, suggesting the ontogenetic colour change signals sexual maturity in this species. Our study shows that male dichromatism can be age-dependent and ontogenetic colour change can signal age and sexual readiness in non-territorial insects.     

Foraging nine-spined sticklebacks prefer to rely on public information over simpler social cues

Social animals can observe others' behavior and in the processacquire information of varying quality about a given resource.Theoretical models predict that blind copying of others' behavioris more likely when individuals are only able to observe thedecisions (here "social cues") of others rather than the cues(here "public information") on which such decisions are based.We investigated information use by nine-spined sticklebacks(Pungitius pungitius) in a two-patch foraging context. Socialcues were provided by the number of demonstrator fish presentat each patch (two versus six), which either conflicted withthe demonstrators' observed feeding rate at each patch (publicinformation) or was the only information available. Consistentwith predictions, observers preferred the patch previously associatedwith six demonstrators when social cues were the only availablesource of information but preferred the patch previously associatedwith two demonstrators ("rich" patch) when also provided withpublic information. On the bases of these experiments, we arguethat it is because these fish preferentially base decisionson public information rather than social cues that they canpotentially avoid engaging in erroneous informational cascades.Thus, the availability of public information can help socialanimals make adaptive decisions.     

The use and misuse of public information by foraging red crossbills

Group foragers may assess patch quality more efficiently bypaying attention to the sampling behavior of group members foragingin the same patch (i.e., using "public information"). To determinewhether red crossbills (Loxia curvirostra) use public informationto aid their patch departure decisions, we conducted experimentsthat compared the sampling behavior of crossbills foraging ona two-patch system (one patch was always empty, one patch containingseeds) when alone, in pairs, and in flocks of three. When foragingalone, crossbills departed from empty patches in a way thatwas qualitatively consistent with energy maximization. We foundevidence for the use of public information when crossbills werepaired with two flock mates, but not when paired with one flockmate. When foraging with two flock mates, crossbills sampledapproximately half the number of cones on the empty patch beforedeparting as compared to when solitary. Furthermore, as expected ifpublic information is used, the variance in both the numberof cones and time spent on the empty patch decreased when crossbillsforaged with two flock mates as compared to when alone. Althoughhigh frequencies of scrounging reduce the availability of publicinformation, scrounging is usually uncommon in crossbills, apparentlybecause they exploit divisible patches. Consequently, publicinformation is likely to be important to crossbills in the wild.We also show that feeding performance is greatly diminishedwhen the feeding performances of flock mates differ. This providesa mechanism that will favor assortative grouping by phenotypewhen phenotypes affect feeding performance, which may in turnpromote speciation in some groups of animals.     

Carotenoid‐based bill coloration functions as a social,not sexual,signal in songbirds (Aves: Passeriformes)

Many animals use coloration to communicate with other individuals. Although the signalling role of avian plumage colour is relatively well studied, there has been much less research on coloration in avian bare parts. However, bare parts could be highly informative signals as they can show rapid changes in coloration. We measured bill colour (a ubiquitous bare part) in over 1600 passerine species and tested whether interspecific variation in carotenoid‐based coloration is consistent with signalling to potential mates or signalling to potential rivals in a competitive context. Our results suggest that carotenoid bill coloration primarily evolved as a signal of dominance, as this type of coloration is more common in species that live in social groups in the nonbreeding season, and species that nest in colonies; two socio‐ecological conditions that promote frequent agonistic interactions with numerous and/or unfamiliar individuals. Additionally, our study suggests that carotenoid bill coloration is independent of the intensity of past sexual selection, as it is not related to either sexual dichromatism or sexual size dimorphism. These results pose a significant challenge to the conventional view that carotenoid‐based avian coloration has evolved as a developmentally costly, condition‐dependent sexual signal. We also suggest that bare part ornamentation may often signal different information than plumage ornaments.