Experimental evidence for multivariate stabilizing sexual selection

Stabilizing selection is a fundamental concept in evolutionary biology. In the presence of a single intermediate optimum phenotype (fitness peak) on the fitness surface, stabilizing selection should cause the population to evolve toward such a peak. This prediction has seldom been tested, particularly for suites of correlated traits. The lack of tests for an evolutionary match between population means and adaptive peaks may be due, at least in part, to problems associated with empirically detecting multivariate stabilizing selection and with testing whether population means are at the peak of multivariate fitness surfaces. Here we show how canonical analysis of the fitness surface, combined with the estimation of confidence regions for stationary points on quadratic response surfaces, may be used to define multivariate stabilizing selection on a suite of traits and to establish whether natural populations reside on the multivariate peak. We manufactured artificial advertisement calls of the male cricket Teleogryllus commodus and played them back to females in laboratory phonotaxis trials to estimate the linear and nonlinear sexual selection that female phonotactic choice imposes on male call structure. Significant nonlinear selection on the major axes of the fitness surface was convex in nature and displayed an intermediate optimum, indicating multivariate stabilizing selection. The mean phenotypes of four independent samples of males, from the same population as the females used in phonotaxis trials, were within the 95% confidence region for the fitness peak. These experiments indicate that stabilizing sexual selection may play an important role in the evolution of male call properties in natural populations of T. commodus.     

INBREEDING AND ADVERTISEMENT CALLING IN THE CRICKET TELEOGRYLLUS COMMODUS: LABORATORY AND FIELD EXPERIMENTS

If sexually selected traits reveal a male's heterozygosity or condition to females, then such traits should exhibit declines with inbreeding. We tested this by examining the effect of inbreeding on advertisement calling in male crickets Teleogryllus commodus. We investigated the effect of one generation of full‐sibling mating on calling effort and fine‐scale call structure. Inbreeding reduced calling effort but had no effect on call structure. We then compared the attractiveness of inbred and outbred calls in the field by monitoring how many wild females were attracted to each call type. From the field data, we conducted a selection analysis to identify the major axes of linear and nonlinear multivariate sexual selection on call structure. A comparison of multivariate attractiveness of inbred and outbred calls along each major axis of selection revealed no difference in attractiveness. Our results suggest that inbred male calls have a fine‐scale structure that is no less attractive to females than that of outbred calls. However, because inbred males call less often, and female T. commodus prefer males with a higher calling effort, inbred males will suffer reductions in mating success. Females who base mate choice on call rate are therefore using a signal correlated with male heterozygosity and/or condition.     

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.     

Individual Male Calling Pattern and Male Mating Success in the European Treefrog (Hyla arborea): Is there Evidence for Directional or Stabilizing Selection on Male Calling Behaviour?

In anurans, call properties are commonly classified based on within‐male variability as being either static or dynamic. Numerous playback experiments in the laboratory have indicated that female preferences based on dynamic call properties are usually strongly directional, while female preferences based on static call properties are often stabilizing or weakly directional. However, there are only few studies demonstrating that female preferences for high values of dynamic call properties indeed exert directional selection on male calling behaviour in natural populations. Moreover, field studies investigating whether female preferences for values of static call properties around the mean of the population lead to currently operating stabilizing selection on male calling patterns in natural populations are completely lacking. Here I investigate for two consecutive breeding seasons male calling patterns and male mating success in a population of individually marked European treefrogs (Hyla arborea), a hylid frog with prolonged breeding season and a lek mating system. Individual male calling pattern as analysed in terms of seven temporal and spectral call properties did not differ between males that survived from one breeding season to the next and those not surviving. None of the seven call properties investigated differed significantly between mated and unmated males, indicating that there is no strong directional selection on male calling behaviour in the study population. However, in one study season males that produced calls with a number of pulses around the mean of the population were significantly more likely to obtain matings than males that produced calls with a number of pulses at the low or high end of the distribution. Thus, this study provides preliminary evidence for the operation of stabilizing selection on a static call property (i.e. the number of pulses per call) in a natural population of an anuran amphibian.     

Sexual selection in female perceptual space: how female túngara frogs perceive and respond to complex population variation in acoustic mating signals

Female preferences for male mating signals are often evaluated on single parameters in isolation or small suites of characters. Most signals, however, are composites of many individual parameters. In this study we quantified multivariate traits in the advertisement call of the túngara frog, Physalaemus pustulosus. We represented the calls in multidimensional scaling space and chose nine test calls to represent the range of population variation. We then tested females for phonotactic preference between calls in each pair of the nine test calls. We used statistics developed for paired comparisons in such "round robin" competitions to evaluate the null hypothesis of equal attractiveness, and to examine the degree to which females responded to calls as being different from or similar to one another in attractiveness. We then examined the attractiveness of each test call relative to all other test calls as a function of their location in multivariate acoustic space (the acoustic landscape) to visualize sexual selection on calls. Finally, we used methods from cognitive psychology to illustrate the females' perception of call attractiveness in multivariate space, and compared this perceptual landscape to the acoustic landscape of quantitative call variation. We show that correlations between individual call characters are not strong and thus there are few biomechanical constraints on their independent evolution. Most call variables differed among males, and there was high repeatability of call characters within males. Females often discriminated between pairs of calls from the population, and there were significant differences among calls in their attractiveness. Female preferences for calls were not stabilizing. The region of the acoustic landscape that was most attractive to females included the mean call but was not centered around it. The females' perceptual or preference landscape did not correlate with the call's acoustic landscape, and female perception of calls decreased rather than enhanced call differences.     

The influence of geographic heterogeneity in predation pressure on sexual signal divergence in an Amazonian frog species complex

Sexual selection plays an important role in mating signal divergence, but geographic variation in ecological factors can also contribute to divergent signal evolution. We tested the hypothesis that geographic heterogeneity in predation causes divergent selection on advertisement call complexity within the Engystomops petersi (previously Physalaemus petersi) frog species complex. We conducted predator phonotaxis experiments at two sites where female choice is consistent with call trait divergence. Engystomops at one site produces complex calls, whereas the closely related species at the other site produces simple calls. Bats approached complex calls more than simple calls at both sites, suggesting selection against complex calls. Moreover, bat predation pressure was greater at the site with simple calls, suggesting stronger selection against complex calls and potentially precluding evolution of complex calls at this site. Our results show that geographic variation in predation may play an important role in the evolution and maintenance of mating signal divergence.     

Advertisement Call Variation in the Golden Rocket Frog (Anomaloglossus beebei): Evidence for Individual Distinctiveness

Individual distinctiveness in acoustic signals can allow receivers to respond appropriately to different individuals. The aim of this research was to examine signal variation and to investigate the relative importance of different acoustic properties for coding individual distinctiveness in the advertisement calls of male golden rocket frogs (Anomaloglossus beebei). We examined patterns of within‐individual and among‐individual variability in 760 advertisement calls of 40 males as well as repeatability in 16 additional males that were recorded on two different occasions. We examined eight call properties, and all properties exhibited significant among‐individual variation and moderate to high repeatability across days. We employed discriminant function analysis to examine individual distinctiveness statistically. These analyses assigned 79% of calls to the correct individual. Fine temporal properties, including pulse duration, pulse rate, and pulse interval, as well as the spectral property of dominant frequency, contributed most toward discrimination among individuals. These results indicate that individual male golden rocket frogs can be distinguished statistically by their advertisement calls. We use these findings to evaluate results of previous playback studies in A. beebei and to generate testable predictions regarding the potential for specific call properties to function in mate selection and social recognition.     

Multivariate sexual selection on male song structure in wild populations of sagebrush crickets,Cyphoderris strepitans (Orthoptera: Haglidae)

While a number of studies have measured multivariate sexual selection acting on sexual signals in wild populations, few have confirmed these findings with experimental manipulation. Sagebrush crickets are ideally suited to such investigations because mating imposes an unambiguous phenotypic marker on males arising from nuptial feeding by females. We quantified sexual selection operating on male song by recording songs of virgin and mated males captured from three wild populations. To determine the extent to which selection on male song is influenced by female preference, we conducted a companion study in which we synthesized male songs and broadcast them to females in choice trials. Multivariate selection analysis revealed a saddle‐shaped fitness surface, the highest peak of which corresponded to longer train and pulse durations, and longer intertrain intervals. Longer trains and pulses likely promote greater mate attraction, but selection for longer intertrain durations suggests that energetic constraints may necessitate “time outs”. Playback trials confirmed the selection for longer train and pulse durations, and revealed significant stabilizing selection on dominant frequency, suggesting that the female auditory system is tightly tuned to the species‐specific call frequency. Collectively, our results revealed a complex pattern of multivariate nonlinear selection characterized primarily by strong stabilizing and disruptive selection on male song traits.     

A MULTIVARIATE ANALYSIS OF GENETIC VARIATION IN THE ADVERTISEMENT CALL OF THE GRAY TREEFROG,HYLA VERSICOLOR

Genetic variation in sexual displays is crucial for an evolutionary response to sexual selection, but can be eroded by strong selection. Identifying the magnitude and sources of additive genetic variance underlying sexually selected traits is thus an important issue in evolutionary biology. We conducted a quantitative genetics experiment with gray treefrogs (Hyla versicolor) to investigate genetic variances and covariances among features of the male advertisement call. Two energetically expensive traits showed significant genetic variation: call duration, expressed as number of pulses per call, and call rate, represented by its inverse, call period. These two properties also showed significant genetic covariance, consistent with an energetic constraint to call production. Combining the genetic variance–covariance matrix with previous estimates of directional sexual selection imposed by female preferences predicts a limited increase in call duration but no change in call rate despite significant selection on both traits. In addition to constraints imposed by the genetic covariance structure, an evolutionary response to sexual selection may also be limited by high energetic costs of long‐duration calls and by preferences that act most strongly against very short‐duration calls. Meanwhile, the persistence of these preferences could be explained by costs of mating with males with especially unattractive calls.     

The genetic basis and evolution of acoustic mate recognition signals in a Ribautodelphax planthopper (Homoptera,Delphacidae) 1. The female call

Both sexes of the planthopper Ribautodelphax imitans produce species specific acoustic signals. Earlier experiments have shown that isolation between Ribautodelphax species in captivity is at least partly due to male preference for calls of conspecific females. The genetic basis of the female call is studied by bi-directional artificial selection for large and small interpulse intervals (IPI). This resulted in non-overlapping distributions of IPI after only five generations. The mean of eight realized heritability estimates over five generations was above 80%; estimates over ten generations were generally well above 50%. The character is shown to be of a polygenic nature, determined by at least 6 segregating genetic factors. The other features of the female call, strophe duration, and modulation of pulse repetition frequency within the strophe, showed significantly correlated responses. Sexual isolation tests after 10 generations of selection revealed significant symmetrical assortative mating, but coselected males did not exhibit a significant preference for playback calls of females from their own selection line. In view of the high heritability for the call character, and the considerable ecological isolation among Ribautodelphax species, it seems unlikely that the female call differentiated as an adaptation to prevent hybridization (reinforcement). More likely, call and call preference were shaped by sexual selection during allopatry, and may have (had) incidentally an effect in species isolation.     

Preferences for Call Spectral Properties in Hyla intermedia

We studied the pattern of variation in the spectral properties of male advertisement calls and female preference functions for these same properties in the Italian treefrog (Hyla intermedia). The call spectral properties (fundamental and dominant frequencies) were highly negatively correlated with body size, they showed low within‐individual variation, and, at the population level, they were found to be under weak stabilizing selection. In two‐choice discrimination tests, females did not show preferences between calls with fundamental and dominant frequencies within two standard deviations from the population mean, whereas females significantly discriminated against calls when their frequencies were three (or more) standard deviations above or below the population mean. Consistent with the observed permissiveness in female preference over the call spectral structure, in the natural population, we found no evidence either of directional selection on male body size or of a size‐assortative mating pattern. The pattern of preferences observed at the population level did not mirror the preferences of individual females. In fact, in a multi‐trial discrimination experiment, females showed significant differences in their choice between the average frequency call and a two standard deviation lower than average frequency alternative, for which no significant preferences were observed at the population level. Variations in preference were not found to correlate with female body size.     

Inbreeding and courtship calling in the cricket Teleogryllus commodus

Male field crickets produce two acoustic signals for mating: advertisement calls and courtship calls. While the importance of advertisement calling in mate attraction is well understood, the function of courtship calling is less clear. Here, we tested if the courtship call of male crickets Teleogryllus commodus signals aspects of male quality by comparing the calls of inbred and outbred males. We examined the effect of one generation of full sibling mating on fine‐scale call structure, along with several life history traits. Inbreeding reduced nymph survival but had no significant effect on weight or development time. Inbreeding resulted in a small but significant change in two of the six call parameters measured. We then tested if inbreeding affects call trait combinations that are important to females by using the results of a previous selection analysis to compare the multivariate attractiveness of the calls of inbred and outbred males. There was no difference. We conclude that the courtship call of T. commodus is not a reliable signal of aspects of male quality that are affected by inbreeding (which generally reduces fitness‐enhancing traits). It might, however, signal components of male fitness that are not affected by changes in heterozygosity.     

Dynamic properties of the advertisement calls of gray tree frogs: patterns of variability and female choice

We assessed the potential for several acoustic properties ofthe advertisement calls of male gray tree frogs to affect relativemating success by relating patterns of variation in these propertiesto minimum differences required to elicit female choice. Dynamicproperties (pulse number, PN; call rate, CR; and duty cycle,DC, the ratio of call duration to call period) varied much morewithin bouts of calling than a static property (dominant frequency,DF) but nevertheless exhibited significant between male variationin three of four breeding seasons. Many multiply recorded malesconsistently produced calls with values substantially aboveor below mean values of males recorded on the same nights. Nightlyranges of variation in PN and CR were often greater than theminimum differences required to elicit female preferences inthe laboratory. In most experiments, females chose high-PN orfast-CR calls over low-PN or slow-CR alternatives, respectively,even if the preferred stimuli were farther away or 6-10 dB lowerin sound pressure level (SPL), provided that differences inPN or CR were 100%. Consistent with these results, females didnot always choose the closer of two calling males in the field.Nightly ranges of variation in DF rarely equaled the minimumdifference required to elicit SPL independent preferences. Femalespreferred a stimulus of high-PN and slow-CR over an alternativeof low-PN or fast-CR with the same acoustic on-time; in twoexperiments, females chose calls of high-PN over low-PN alternativeseven though the playback of the high-PN call was interruptedand the low-PN call was broadcast continuously. Thus, femalepreferences were not merely based on the total time of acousticstimulation. Responses of females tested twice in the same experimentsuggest that phenotypic variation in preference was limitedin our study populations.     

Complex multivariate sexual selection on male acoustic signaling in a wild population of Teleogryllus commodus

Mate choice may impose both linear (i.e., directional) and nonlinear (i.e., quadratic and correlational) sexual selection on advertisement traits. Traditionally, mate recognition and sensory tuning have been thought to impose stabilizing (i.e., negative quadratic) sexual selection, whereas adaptive mate choice effects directional selection. It has been suggested that adaptive choice may exert positive quadratic and/or correlational sexual selection. Earlier, we showed that five structural components of the advertisement call of male field crickets (Teleogryllus commodus) were under multivariate stabilizing selection under laboratory conditions. Here we experimentally estimate selection on these five traits plus a measure of calling activity (the number of repeats in a looped bout of calling) in the field. There was general support for multivariate stabilizing selection on call structure, and calling activity was under strong positive directional selection, as predicted for a signal of genetic quality. There was, however, also appreciable correlational selection, suggesting an interaction between male call structure and calling effort. Interestingly, selection for short interbout durations of silence favored longer intercall durations in the field, in contrast to results from continuous looped call playback in the laboratory. We discuss the general importance of nonlinear selection in the honest signaling of genetic quality.     

饰纹姬蛙求偶鸣声特征分析

2012年5月,用SX950录音笔和Praat声音分析软件对浙江丽水繁殖季节饰纹姬蛙(Microhyla ornata)求偶鸣声进行录制和特征分析.结果表明,饰纹姬蛙发出的求偶鸣声具有单一谐波鸣声结构、多脉冲(7、9～16)及纺锤形振幅等特征;所有鸣声主频率范围为1.22～4.09 kHz (n=233),且由不同脉冲数组成的鸣声主频率平均值几近相等;叫声时程随脉冲数的增加而增大,脉冲时程在不同脉冲数鸣声中的大小几近相等,但最后一个脉冲的时程大小≤其他脉冲;脉冲间隔与叫声时程则刚好相反,即叫声时程越短,脉冲间隔就越大.在7个脉冲的鸣声中,其脉冲间隔最大,脉冲率最小;而在16个脉冲的鸣声中,脉冲间隔则最小,脉冲率最大.除7个脉冲和16个脉冲鸣声的脉冲率分别与其他鸣声存在显著性差异以外,随着叫声时程和脉冲数的增加,脉冲率也出现相应变化.在声强方面,除16个脉冲鸣声与其余所有的脉冲鸣声出现显著性差异以外,其他脉冲鸣声之间的两两比较差异不显著.丽水种群与其他5个地理种群(杭州、宣城、Kamoor、Bajipe和Padil)的鸣声特征比较显示饰纹姬蛙在不同地理种群的鸣声结构相似,而鸣声主频率、叫声时程、脉冲时程及脉冲率等在6个地理种群种均出现不同程度的差异.了解不同物种的声信号特征有助于更好地理解动物通讯行为及其进化特点.     

COMMUNITY COMPOSITION AFFECTS THE SHAPE OF MATE RESPONSE FUNCTIONS

The evolution of mate preferences can be critical for the evolution of reproductive isolation and speciation. Heterospecific interference may carry substantial fitness costs and result in preferences where females are most responsive to the mean conspecific trait with low response to traits that differ from this value. However, when male traits are unbounded by heterospecifics, there may not be selection against females that respond to extreme trait values in the unbounded direction. To test how heterospecifics affected the shape of female response functions, I presented female Oecanthus tree crickets with synthetic calls representing a range of male calls, then measured female phonotaxis to construct response functions. The species with the fastest pulse rates in the community consistently responded to pulse rates faster than those produced by their males, whereas in the intermediate and slowest pulse rate species there was no significant difference between the male trait and the female response. This work suggests that species with the most extreme signal in the community respond to a greater range of signals, potentially resulting in a higher probability of hybridization during secondary contact, and revealing interactions between mate recognition and other aspects of sexual selection.     

SPECIES RECOGNITION AND SEXUAL SELECTION AS A UNITARY PROBLEM IN ANIMAL COMMUNICATION

We investigated patterns of mating call preference and mating call recognition by examining phonotaxis of female túngara frogs, Physalaemus pustulosus, in response to conspecific and heterospecific calls. There are four results: females always prefer conspecific calls; most heterospecific calls do not elicit phonotaxis; some heterospecific calls do elicit phonotaxis and thus are effective mate recognition signals; and females prefer conspecific calls to which a component of a heterospecific call has been added to a normal conspecific call. We use these data to illustrate how concepts of species recognition and sexual selection can be understood in a unitary framework by comparing the distribution of signal traits to female preference functions.     

Call Characteristics of Two Sympatric and Morphologically Similar Tree Frogs Species,Polypedates megacephalus and Polypedates mutus(Anura: Rhacophoridae), from Hainan,China

Anuran calls are usually species-specific and therefore valued as a tool for species identification. Call characteristics are a potential honest signal in sexual selection because they often reflect male body size. Polypedates megacephalus and P. mutus are two sympatric and morphologically similar tree frogs, but it remains unknown whether their calls are associated with body size. In this study, we compared call characteristics of these two species and investigated any potential relationships with body size. We found that P. megacephalus, males produced six call types which consisting of three distinct notes, while P. mutus males produced five types consisting of two types of notes. Dominant frequency, note duration, pulse duration, and call duration exhibited significant interspecific differences. In P. megacephalus, one note exhibited a dominant frequency that was negatively correlated with body mass, snout-vent length, head length, and head width. In P. mutus, the duration of one note type was positively correlated with body mass and head width. These differences in call characteristics may play an important role in interspecific recognition. Additionally, because interspecific acoustic variation reflects body size, calls may be relevant for sexual selection. Taken together, our results confirmed that calls are a valid tool for distinguishing between the two tree-frog species in the field.     

Evolution of intraspecific variation in the advertisement call of a cricket frog (Acris crepitans, Hylidae)

Advertisement calls of the cricket frog, Acris crepitans , show statistically significant variation among populations in all call variables measured. Call variables show strong clinal variation resulting in calls of lower frequency, longer duration and slower call rates produced by A. c. blanchardi in open habitat in the west of the range, and calls of higher frequency, shorter calls and faster call rates produced by A. c. crepitans in the pine forests in the eastern part of the range. This clinal variation does not result from pleiotropic effects of body size or any other morphological characters we measured.
The two subspecies usually reside in different habitats, but some A. c. blanchardi reside in an isolated pine forest in central Texas. By comparing the calls of this subspecies in open and forest habitat, and by statistically removing the effects of clinal variation for all populations, we determined that habitat explains some of the variation in call structure; this is not true of subspecies.
Our data reject several hypotheses that purport to explain the evolution of mate recognition signals. (1) We reject the notion of Paterson and others that there is strong stabilizing selection on species-specific mate recognition signals. (2) There is no support for the hypothesis that call variation is primarily due to pleiotropic effects of body size or other morphological characters over the geographic range we examined. (3) There is no evidence for reproductive character displacement. (4) Our data, as well as experimental studies of habitat acoustics, support the hypothesis that some differences in calls among habitats result from environmental selection on call structure to enhance call transmission. We suggest that the latter hypothesis does not explain the strong clinal component of call variation. This might result from the passive effects of gene flow between populations at the extremes of the range under selection generated by habitat acoustics.     

Social calls are subject to stabilizing selection in insular bats

Aim Bats communicate by emitting social calls, and these often elicit reactions in conspecifics. Many such vocalizations are species‐specific so that unambiguous signals can be transmitted and interpreted by conspecifics. In species‐rich assemblages, evolutionary pressures might prompt interspecific diversification of call structure so that communication with heterospecifics is avoided. In species‐poor island communities, where no risk of miscommunication occurs, stabilizing selection should prevail and preserve call structure and function. Call structure in island bats might be inherited from colonizers from the mainland and be maintained with little change in the absence of selection from heterospecifics. To test this hypothesis we studied Pipistrellus maderensis, an insular taxon occurring on the Madeira Archipelago, the Canary Islands and the Azores. It is closely related to one of the most widespread European pipistrelles, Pipistrellus kuhlii. Pipistrellus maderensis most probably evolved from a common ancestor shared with P. kuhlii, or from founders of that taxon that colonized the islands. We hypothesized that on Madeira Island, where no risk of ambiguous communication with heterospecifics exists, the structure and function of social calls should have been preserved by stabilizing selection. Echolocation calls, subject to different selection pressures, may instead show more pronounced differences between P. maderensis and P. kuhlii. Location Madeira Island (Portugal, Atlantic Ocean), central and southern Italy. Methods We recorded social and echolocation calls from allopatric populations of the two pipistrelles and explored interspecific differences in time and frequency characteristics. We also conducted playback experiments by broadcasting recordings of social calls from P. kuhlii and P. maderensis (taken respectively in peninsular Italy and on Madeira) and monitoring the bats’ responses. Results Social call structure showed a strong similarity between species, whereas echolocation calls were markedly different and exhibited a mean divergence of over 6 kHz in their frequency of maximum energy. On Madeira, P. maderensis significantly reduced flight activity when we broadcast P. kuhlii signals, as did P. kuhlii in Italy in response to P. maderensis calls. Main conclusions Reliable interpretation of social calls provides benefits to both the signaller and the receiver because signals help to optimize food exploitation at foraging sites. In the absence of closely related species that can emit similar calls, this advantage may have acted as a strong evolutionary pressure, stabilizing social call structure in P. maderensis in insular ecosystems with limited foraging resources.