The effect of past condition on a multicomponent sexual signal

Ecological Constraints on Sexual Selection in a Human-Modified Landscape

Anthropogenic activities are changing the sensory landscape, interfering with transmission and reception of sexual signals. These changes are leading to alterations in mating behaviour with consequences to fitness. In systems where mate-finding involves long-distance signalling by one sex and approach by the other sex, the spatial distribution of signallers can have implications for male and female fitness. Spatial distribution of signallers is typically determined by an interplay of multiple factors, both ecological and evolutionary, including male competition, female choice and resources, such as calling and oviposition sites. We investigated the possible influence of resource distribution (signalling sites) on the strength and direction of sexual selection acting on false-leaf katydid Onomarchus uninotatus males, signalling in a human-modified landscape in the Western Ghats, India, a biodiversity hotspot. The landscape has changed from evergreen forests to plantations owing to human settlements. We first determined the spatial distribution of calling males and of available calling sites, which are trees of the genus Artocarpus, in the landscape. Using the information on male spacing, call transmission and hearing thresholds, the perceptual spaces of male signals were computed to understand the acoustic environment of calling males and females. It was found that both calling males and females could hear calls of males from neighbouring trees with a probability of 0.76 and 0.59, respectively. Although calling males were found to be spaced apart more than predicted by chance, significant overlap was seen in their acoustic ranges. Clustering of males enables females to easily sample multiple males, facilitating mate choice, but is detrimental to males as it increases competition for females. Using simulations, we determined the optimal spatial distributions of O. uninotatus males for female choice, and for reduction of male competition, given the signalling site distribution. The observed distribution of signallers was then compared with the hypothetical optimal distributions to examine the drivers of signaller spacing. Spacing of calling males in the field was found to be not optimal for either males or females. Resource distribution was found to limit the effectiveness of sexual selection drivers in pushing male spacing toward fitness optima of males or females.

Design of ideal vibrational signals for stinkbug male attraction through vibrotaxis experiments

BACKGROUND

Many groups of insects utilize substrate-borne vibrations for intraspecific communication. This characteristic makes them a suitable model for exploring the use of vibrations as a tool for pest control as an alternative to the use of chemicals. Detailed knowledge of species communication is a prerequisite to select the best signals to use. This study explored the use of substrate-borne vibrations for pest control of the brown marmorated stink bug (BMSB), Halyomorpha halys Stål (Heteroptera: Pentatomidae). For this purpose, we first identified the spectral and temporal characteristics that best elicit male responsiveness. Bioassays were conducted with artificial signals that mimicked the natural female calling signal. Second, we used the acquired knowledge to synthesize new signals endowed with different degrees of attractiveness in single- and two-choice bioassays using a wooden custom-made T stand.

RESULTS

The results from this study showed that males were attracted to female signals along a high range of amplitudes, especially starting from a threshold of 100 μm s^-1 , a high pulse repetition time (1 s) and frequency peak corresponding to the first harmonic (76 Hz). This resulted in an "optimal" signal for use to attract males, while the choice test in the T arena showed that this signal elicits searching behavior and attracts BMSB males towards a stimulation point.

CONCLUSION

We confirm the use of vibrational signals as a strong tool for behavioral manipulation of male BMSB and suggest its possible use in the development of field traps and further management of this pest. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Sensitive periods during the development and expression of vertebrate sexual signals: A systematic review

Many sexually selected traits exhibit phenotypic plasticity. Despite a growing appreciation for the ecological context in which sexual selection occurs, and for the role of plasticity in shaping traits associated with local adaptation and divergence, there is an important gap in knowledge about the onset and duration of plasticity in sexual trait expression. Integrating this temporal dimension of plasticity into models of sexual selection informs our understanding of the information conveyed by sexual traits and our predictions related to trait evolution, and is critical in this time of unprecedented and rapid environmental change. We conducted a systematic review of 869 studies to ask how trait modalities (e.g., visual and chemical) relate to the onset and duration of plasticity in vertebrate sexual signals. We show that this literature is dominated by studies of coloration in birds and fish, and most studies take place during the breeding season. Where possible, we integrate results across studies to link physiology of specific trait modalities with the life stage (e.g., juvenile, breeding, or nonbreeding) during which plasticity occurs in well-studied traits. Limitations of our review included a lack of replication in our dataset, which precluded formal analysis. We argue that the timing of trait plasticity, in addition to environmental context, is critical for determining whether and how various communication signals are associated with ecological context, because plasticity may be ongoing or occur at only one point in an individual's lifetime, and determining a fixed trajectory of trait expression. We advocate for careful consideration of the onset and duration of plasticity when analyzing how environmental variation affects sexual trait expression and associated evolutionary outcomes.

Food Limitation but Not Enhanced Rates of Ejaculate Production Imposes Reproductive and Survival Costs to Male Crickets

Estimating costs of ejaculate production is challenging. Metabolic investment in ejaculates may come at the expense of other physiological functions and may negatively affect future reproduction and/or survival. These trade-offs are especially likely to occur under constrained resource pools (e.g., poor nutrition). Here, we investigated costs of ejaculate production via trade-offs in the field cricket Gryllus bimaculatus. We experimentally increased rates of ejaculate production, while keeping an unmanipulated group, in adult males kept at high and low feeding regimes and tested the effects of our treatments on (i) somatic maintenance (i.e., changes in male body mass), (ii) future reproduction (i.e., the likelihood of producing a spermatophore and the viability of its sperm), and (iii) lifetime survival and longevity. We predicted investment in ejaculates to impinge upon all measured responses, especially in low-fed individuals. Instead, we only found negative effects of food limitation, suggesting low or undetectable costs of spermatophore production. High mating rates may select for males to maximize their capacity of ejaculate production, making ejaculate traits less prone to trade-offs with other fitness-related life history traits. Nevertheless, males were impaired due to nutrient deficiency in producing viable ejaculates, suggesting condition-dependent costs for ejaculate production.

Mate choice in fruit flies is rational and adaptive

According to rational choice theory, beneficial preferences should lead individuals to sort available options into linear, transitive hierarchies, although the extent to which non-human animals behave rationally is unclear. Here we demonstrate that mate choice in the fruit fly Drosophila melanogaster results in the linear sorting of a set of diverse isogenic female lines, unambiguously demonstrating the hallmark of rational behaviour, transitivity. These rational choices are associated with direct benefits, enabling males to maximize offspring production. Furthermore, we demonstrate that female behaviours and cues act redundantly in mate detection and assessment, as rational mate choice largely persists when visual or chemical sensory modalities are impaired, but not when both are impaired. Transitivity in mate choice demonstrates that the quality of potential mates varies significantly among genotypes, and that males and females behave in such a way as to facilitate adaptive mate choice.

A characteristic of rational behaviour is that it is transitive, such that preferences are ranked in a strict linear order. Here, Arbuthnott and colleagues show that mate choice in the fruit fly, Drosophila melanogaster, is transitive at the population level and that preferred mates produce more offspring.

Field Crickets Compensate for Unattractive Static Long-Distance Call Components by Increasing Dynamic Signalling Effort

The evolution of multiple sexual signals presents a dilemma since individuals selecting a mate should pay attention to the most honest signal and ignore the rest; however, multiple signals may evolve if, together, they provide more information to the receiver than either one would alone. Static and dynamic signals, for instance, can act as multiple messages, providing information on different aspects of signaller quality that reflect condition at different time scales. While the nature of static signals makes them difficult or impossible for individuals to augment, dynamic signals are much more susceptible to temporary fluctuations in effort. We investigated whether male Texas field crickets, Gryllus texensis, that produce unattractive static signals compensate by dynamically increasing their calling effort. Our findings lend partial support to the compensation hypothesis, as males that called at unattractive carrier frequencies (a static trait) spent more time calling each night (a dynamic trait). Interestingly, this finding was most pronounced in males that called with attractive pulse characteristics (static traits) but did not occur in males that called with unattractive pulse characteristics. Males that signalled with unattractive pulse characteristics (duration and pause) spent less time calling through the night. Our correlative findings on wild caught males suggest that only males that signal with attractive pulse characteristics may be able to afford to pay the costs of both trait exaggeration and increased calling effort to compensate for poor carrier frequencies.

Spatio-Temporal Dynamics of Field Cricket Calling Behaviour: Implications for Female Mate Search and Mate Choice

Amount of calling activity (calling effort) is a strong determinant of male mating success in species such as orthopterans and anurans that use acoustic communication in the context of mating behaviour. While many studies in crickets have investigated the determinants of calling effort, patterns of variability in male calling effort in natural choruses remain largely unexplored. Within-individual variability in calling activity across multiple nights of calling can influence female mate search and mate choice strategies. Moreover, calling site fidelity across multiple nights of calling can also affect the female mate sampling strategy. We therefore investigated the spatio-temporal dynamics of acoustic signaling behaviour in a wild population of the field cricket species Plebeiogryllus guttiventris. We first studied the consistency of calling activity by quantifying variation in male calling effort across multiple nights of calling using repeatability analysis. Callers were inconsistent in their calling effort across nights and did not optimize nightly calling effort to increase their total number of nights spent calling. We also estimated calling site fidelity of males across multiple nights by quantifying movement of callers. Callers frequently changed their calling sites across calling nights with substantial displacement but without any significant directionality. Finally, we investigated trade-offs between within-night calling effort and energetically expensive calling song features such as call intensity and chirp rate. Calling effort was not correlated with any of the calling song features, suggesting that energetically expensive song features do not constrain male calling effort. The two key features of signaling behaviour, calling effort and call intensity, which determine the duration and spatial coverage of the sexual signal, are therefore uncorrelated and function independently.

Sex differences in the effects of juvenile and adult diet on age-dependent reproductive effort

Sexual selection should cause sex differences in patterns of resource allocation. When current and future reproductive effort trade off, variation in resource acquisition might further cause sex differences in age-dependent investment, or in sensitivity to changes in resource availability over time. However, the nature and prevalence of sex differences in age-dependent investment remain unclear. We manipulated resource acquisition at juvenile and adult stages in decorated crickets, Gryllodes sigillatus, and assessed effects on sex-specific allocation to age-dependent reproductive effort (calling in males, fecundity in females) and longevity. We predicted that the resource and time demands of egg production would result in relatively consistent female strategies across treatments, whereas male investment should depend sharply on diet. Contrary to expectations, female age-dependent reproductive effort diverged substantially across treatments, with resource-limited females showing much lower and later investment in reproduction; the highest fecundity was associated with intermediate lifespans. In contrast, long-lived males always signalled more than short-lived males, and male age-dependent reproductive effort did not depend on diet. We found consistently positive covariance between male reproductive effort and lifespan, whereas diet altered this covariance in females, revealing sex differences in the benefits of allocation to longevity. Our results support sex-specific selection on allocation patterns, but also suggest a simpler alternative: males may use social feedback to make allocation decisions and preferentially store resources as energetic reserves in its absence. Increased calling effort with age therefore could be caused by gradual resource accumulation, heightened mortality risk over time, and a lack of feedback from available mates.

The potential influence of morphology on the evolutionary divergence of an acoustic signal

The evolution of acoustic behaviour and that of the morphological traits mediating its production are often coupled. Lack of variation in the underlying morphology of signalling traits has the potential to constrain signal evolution. This relationship is particularly likely in field crickets, where males produce acoustic advertisement signals to attract females by stridulating with specialized structures on their forewings. In this study, we characterize the size and geometric shape of the forewings of males from six allopatric populations of the black field cricket (Teleogryllus commodus) known to have divergent advertisement calls. We sample from each of these populations using both wild-caught and common-garden-reared cohorts, allowing us to test for multivariate relationships between wing morphology and call structure. We show that the allometry of shape has diverged across populations. However, there was a surprisingly small amount of covariation between wing shape and call structure within populations. Given the importance of male size for sexual selection in crickets, the divergence we observe among populations has the potential to influence the evolution of advertisement calls in this species.

Sex differences in ventral side colouration of Alpine newts

Sexual selection based on visual stimuli was recently studied in several amphibian species with permanent or temporary dichromatism. The Alpine newt (Ichthyosaura alpestris) is a Caudate species with prominent sexual dichromatism during the breeding period. We focused on the intersexual differences in the orange, carotenoid-base ventral colouration of this widespread European species. We used an image analysis approach to compare the content of the red colour and saturation of the belly in 80 male and 62 female aquatic adult newts captured during spring migration in two localities in the Czech Republic. Both studied colour parameters of the ventral side of the Alpine newt are connected with sex. Males have significantly higher values of saturation on both localities, but the relationship with red content was not so clear. Individual characteristics (body condition, body length) affect colour variables of males and females similarly. Effect of body condition on colour variables was not demonstrated. Therefore we presume that the pronounced colouration of males is a cue facilitating detection, localisation or interspecies identification, rather than a direct component of male quality as assessed by females.

Adaptive plasticity in wild field cricket's acoustic signaling

Phenotypic plasticity can be adaptive when phenotypes are closely matched to changes in the environment. In crickets, rhythmic fluctuations in the biotic and abiotic environment regularly result in diel rhythms in density of sexually active individuals. Given that density strongly influences the intensity of sexual selection, we asked whether crickets exhibit plasticity in signaling behavior that aligns with these rhythmic fluctuations in the socio-sexual environment. We quantified the acoustic mate signaling behavior of wild-caught males of two cricket species, Gryllus veletis and G. pennsylvanicus. Crickets exhibited phenotypically plastic mate signaling behavior, with most males signaling more often and more attractively during the times of day when mating activity is highest in the wild. Most male G. pennsylvanicus chirped more often and louder, with shorter interpulse durations, pulse periods, chirp durations, and interchirp durations, and at slightly higher carrier frequencies during the time of the day that mating activity is highest in the wild. Similarly, most male G. veletis chirped more often, with more pulses per chirp, longer interpulse durations, pulse periods, and chirp durations, shorter interchirp durations, and at lower carrier frequencies during the time of peak mating activity in the wild. Among-male variation in signaling plasticity was high, with some males signaling in an apparently maladaptive manner. Body size explained some of the among-male variation in G. pennsylvanicus plasticity but not G. veletis plasticity. Overall, our findings suggest that crickets exhibit phenotypically plastic mate attraction signals that closely match the fluctuating socio-sexual context they experience.

Limited plasticity in the phenotypic variance-covariance matrix for male advertisement calls in the black field cricket, Teleogryllus commodus

Phenotypic integration and plasticity are central to our understanding of how complex phenotypic traits evolve. Evolutionary change in complex quantitative traits can be predicted using the multivariate breeders' equation, but such predictions are only accurate if the matrices involved are stable over evolutionary time. Recent study, however, suggests that these matrices are temporally plastic, spatially variable and themselves evolvable. The data available on phenotypic variance-covariance matrix (P) stability are sparse, and largely focused on morphological traits. Here, we compared P for the structure of the complex sexual advertisement call of six divergent allopatric populations of the Australian black field cricket, Teleogryllus commodus. We measured a subset of calls from wild-caught crickets from each of the populations and then a second subset after rearing crickets under common-garden conditions for three generations. In a second experiment, crickets from each population were reared in the laboratory on high- and low-nutrient diets and their calls recorded. In both experiments, we estimated P for call traits and used multiple methods to compare them statistically (Flury hierarchy, geometric subspace comparisons and random skewers). Despite considerable variation in means and variances of individual call traits, the structure of P was largely conserved among populations, across generations and between our rearing diets. Our finding that P remains largely stable, among populations and between environmental conditions, suggests that selection has preserved the structure of call traits in order that they can function as an integrated unit.

Calling, courtship, and condition in the fall field cricket, Gryllus pennsylvanicus

Theoretically, sexual signals should provide honest information about mating benefits and many sexually reproducing species use honest signals when signalling to potential mates. Male crickets produce two types of acoustic mating signals: a long-distance mate attraction call and a short-range courtship call. We tested whether wild-caught fall field cricket (Gryllus pennsylvanicus) males in high condition (high residual mass or large body size) produce higher effort calls (in support of the honest signalling hypothesis). We also tested an alternative hypothesis, whether low condition males produce higher effort calls (in support of the terminal investment hypothesis). Several components of long-distance mate attraction calls honestly reflected male body size, with larger males producing louder mate attraction calls at lower carrier frequencies. Long-distance mate attraction chirp rate dishonestly signalled body size, with small males producing faster chirp rates. Short-range courtship calls dishonestly reflected male residual mass, as chirp rate and pulse rate were best explained by a curvilinear function of residual mass. By producing long-distance mate attraction calls and courtship calls with similar or higher effort compared to high condition males, low condition males (low residual mass or small body size) may increase their effort in current reproductive success at the expense of their future reproductive success, suggesting that not all sexual signals are honest.

The juvenile social environment introduces variation in the choice and expression of sexually selected traits

The juvenile environment provides numerous cues of the intensity of competition and the availability of mates in the near environment. As research demonstrates that the developing individuals can use these cues to alter their developmental trajectories, and therefore, adult phenotypes, we examined whether social cues available during development can affect the expression and the preference of sexually selected traits. To examine this, we used the Australian black field cricket (Telogryllus commodus), a species where condition at maturity is known to affect both male calling effort and female choice. We mimicked different social environments by rearing juveniles in two different densities crossed with three different calling environments. We demonstrate that the social environment affected female response speed but not preference, and male age-specific calling effort (especially the rate of senescence in calling effort) but not the structural/temporal parameters of calls. These results demonstrate that the social environment can introduce variation in sexually selected traits by modifying the behavioral components of male production and female choice, suggesting that the social environment may be an overlooked source of phenotypic variation. We discuss the plasticity of trait expression and preference in reference to estimations of male quality and the concept of condition dependence.

Sex-specific genotype-by-environment interactions for cuticular hydrocarbon expression in decorated crickets, Gryllodes sigillatus: implications for the evolution of signal reliability

Phenotypic traits that convey information about individual identity or quality are important in animal social interactions, and the degree to which such traits are influenced by environmental variation can have profound effects on the reliability of these cues. Using inbred genetic lines of the decorated cricket, Gryllodes sigillatus, we manipulated diet quality to test how the cuticular hydrocarbon (CHC) profiles of males and females respond across two different nutritional rearing environments. There were significant differences between lines in the CHC profiles of females, but the effect of diet was not quite statistically significant. There was no significant genotype-by-environment interaction (GEI), suggesting that environmental effects on phenotypic variation in female CHCs are independent of genotype. There was, however, a significant effect of GEI for males, with changes in both signal quantity and content, suggesting that environmental effects on phenotypic expression of male CHCs are dependent on genotype. The differential response of male and female CHC expression to variation in the nutritional environment suggests that these chemical cues may be under sex-specific selection for signal reliability. Female CHCs show the characteristics of reliable cues of identity: high genetic variability, low condition dependence and a high degree of genetic determination. This supports earlier work showing that female CHCs are used in self-recognition to identify previous mates and facilitate polyandry. In contrast, male CHCs show the characteristics of reliable cues of quality: condition dependence and a relatively higher degree of environmental determination. This suggests that male CHCs are likely to function as cues of underlying quality during mate choice and/or male dominance interactions.

Phenotypic covariance structure and its divergence for acoustic mate attraction signals among four cricket species

The phenotypic variance-covariance matrix (P) describes the multivariate distribution of a population in phenotypic space, providing direct insight into the appropriateness of measured traits within the context of multicollinearity (i.e., do they describe any significant variance that is independent of other traits), and whether trait covariances restrict the combinations of phenotypes available to selection. Given the importance of P, it is therefore surprising that phenotypic covariances are seldom jointly analyzed and that the dimensionality of P has rarely been investigated in a rigorous statistical framework. Here, we used a repeated measures approach to quantify P separately for populations of four cricket species using seven acoustic signaling traits thought to enhance mate attraction. P was of full or almost full dimensionality in all four species, indicating that all traits conveyed some information that was independent of the other traits, and that phenotypic trait covariances do not constrain the combinations of signaling traits available to selection. P also differed significantly among species, although the dominant axis of phenotypic variation (p(max)) was largely shared among three of the species (Acheta domesticus, Gryllus assimilis, G. texensis), but different in the fourth (G. veletis). In G. veletis and A. domesticus, but not G. assimilis and G. texensis, p(max) was correlated with body size, while p(max) was not correlated with residual mass (a condition measure) in any of the species. This study reveals the importance of jointly analyzing phenotypic traits.

The evolution of the multicoloured face of mandrills: insights from the perceptual space of colour vision

Multicomponent signals consist of several traits that are perceived as a whole. Although many animals rely on multicomponent signals to communicate, the selective pressures shaping these signals are still poorly understood. Previous work has mainly investigated the evolution of multicomponent signals by studying each trait individually, which may not accurately reflect the selective pressures exerted by the holistic perception of signal receivers. Here, we study the design of the multicoloured face of an Old World primate, the mandrill (Mandrillus sphinx), in relation to two aspects of signalling that are expected to be selected by receivers: conspicuousness and information. Using reflectance data on the blue and red colours of the faces of 34 males and a new method of hue vectorisation in a perceptual space of colour vision, we show that the blue hue maximises contrasts to both the red hue and the foliage background colouration, thereby increasing the conspicuousness of the whole display. We further show that although blue saturation, red saturation and the contrast between blue and red colours are all correlated with dominance, dominance is most accurately indicated by the blue-red contrast. Taken together our results suggest that the evolution of blue and red facial colours in male mandrills are not independent and are likely driven by the holistic perception of conspecifics. In this view, we propose that the multicoloured face of mandrills acts as a multicomponent signal. Last, we show that information accuracy increases with the conspicuousness of the whole display, indicating that both aspects of signalling can evolve in concert.

Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits

1. A growing number of studies suggest that female ornaments are linked to maternal quality and influence male mate choice. These findings challenge the traditional male-biased view of sexual selection and the hypothesis that female ornaments are the outcome of a genetic correlation with male ornaments. To further test the hypothesis that female traits have a function, it is now essential to investigate their honesty and to determine how signalling and reproduction interact in females. If female traits are honest indicators of quality, then they are likely to have a specific signalling function. 2. We investigated whether carry-over effects of reproduction might ensure the honesty of plumage colour signalling of a bird species with conspicuous UV-blue and yellow coloration, the blue tit Cyanistes caeruleus. Reproductive effort was manipulated by removing clutches, thereby forcing both sexes to reproduce twice and to raise chicks later in the breeding season when food is less abundant. In the year following this manipulation, we investigated the change in plumage in experimental and control males and females. The change was measured in the two putative feather ornaments, the UV-blue cap and the yellow breast, and another feather trait probably less likely to be sexually selected: the wing length. We also tested whether higher-quality females had their coloration less affected by the experiment. 3. We found that control but not manipulated males and females increased their signal towards UV. In addition, in the manipulated group, females that were able to lay more eggs had their UV-blue coloration less affected by the treatment. For yellow coloration, we found that manipulated yearlings but not manipulated adults decreased their yellow chroma in comparison with control. Lastly, our results show that the condition of the manipulated females tended to be positively correlated with yellow chroma. 4. These results show that the trade-offs between reproduction and signalling can ensure the honesty of conspicuous plumage traits in female and male blue tits. In addition, they suggest that female traits have the potential to evolve under sexual selection in this and other bird species.

The role of genotype-by-environment interactions in sexual selection

Genotype-by-environment interactions (GxEs) in naturally selected traits have been extensively studied, but the impact of GxEs on sexual selection has only recently begun to receive attention. Here, we review recent models and consider how GxEs might affect the evolution of sexual traits through influencing sexual signal reliability and also how GxEs may influence variation in sexually selected traits and the process of reproductive isolation. We then assess the current empirical literature on GxEs in sexual selection and conclude by highlighting areas that need additional work. Research on GxEs and sexual selection is an important new area of study for the discipline, which has largely focused on relatively simple mate choice/competition scenarios to date. Investigators now need to apply this knowledge to more complex, but realistic, situations, to more fully explore the evolution of sexual traits, and in this review we suggest potentially useful directions for future research.

No heightened condition dependence of zebra finch ornaments--a quantitative genetic approach

The developmental stress hypothesis offers a mechanism to maintain honesty of sexually selected ornaments, because only high quality individuals will be able to develop full ornamentation in the face of stress during early development. Experimental tests of this hypothesis have traditionally involved the manipulation of one aspect of the rearing conditions and an examination of effects on adult traits. Here, we instead use a statistically powerful quantitative genetic approach to detect condition dependence. We use animal models to estimate environmental correlations between a measure of early growth and adult traits. This way, we could make use of the sometimes dramatic differences in early growth of more than 800 individually cross-fostered birds and measure the effect on a total of 23 different traits after birds reached maturity. We find strong effects of environmental growth conditions on adult body size, body mass and fat deposition, moderate effects on beak colour in both sexes, but no effect on song and plumage characters. Rather surprisingly, there was no effect on male attractiveness, both measured in mate choice trials and under socially complex conditions in aviaries. There was a trend for a positive effect of good growth conditions on the success at fertilizing eggs in males breeding in aviaries whereas longevity was not affected in either sex. We conclude that zebra finches are remarkably resilient to food shortage during growth and can compensate for poor growth conditions without much apparent life-history trade-offs. Our results do not support the hypothesis that sexually selected traits show heightened condition dependence compared to nonsexually selected traits.

Androgens enhance plasticity of an electric communication signal in female knifefish, Brachyhypopomus pinnicaudatus

Sex steroids were initially defined by their actions shaping sexually dimorphic behavioral patterns. More recently scientists have begun exploring the role of steroids in determining sex differences in behavioral plasticity. We investigated the role of androgens in potentiating circadian, pharmacological, and socially-induced plasticity in the amplitude and duration of electric organ discharges (EODs) of female gymnotiform fish. We first challenged female fish with injections of serotonin (5-HT) and adrenocorticotropic hormone (ACTH), and with social encounters with female and male conspecifics to characterize females' pre-implant responses to each treatment. Each individual was then implanted with a pellet containing dihydrotestosterone (DHT) concentrations of 0.0, 0.03, 0.1, 0.3, or 1.0 mg 10 g(-1) body weight. We then repeated all challenges and compared each female's pre- and post-implant responses. The highest implant dose enhanced EOD duration modulations in response to all challenge types, responses to male challenge were also greater at the second highest dose, and responses to ACTH challenge were enhanced in females receiving all but the smallest dose (and blank) implants. Alternatively, amplitude modulations were enhanced only during female challenges and only when females received the highest DHT dose. Our results highlight the differential regulation of EOD duration and amplitude, and suggest that DHT enhanced the intrinsic plasticity of the electrogenic cells that produce the EOD rather than modifying behavioral phenotypes. The relative failure of DHT to enhance EOD amplitude plasticity also implies that factors other than androgens are involved in regulating/promoting male-typical EOD circadian rhythms and waveform modulations displayed in social contexts.

Matched filters, mate choice and the evolution of sexually selected traits

Background

Fundamental for understanding the evolution of communication systems is both the variation in a signal and how this affects the behavior of receivers, as well as variation in preference functions of receivers, and how this affects the variability of the signal. However, individual differences in female preference functions and their proximate causation have rarely been studied.

Methodology/Principal Findings

Calling songs of male field crickets represent secondary sexual characters and are subject to sexual selection by female choice. Following predictions from the “matched filter hypothesis” we studied the tuning of an identified interneuron in a field cricket, known for its function in phonotaxis, and correlated this with the preference of the same females in two-choice trials. Females vary in their neuronal frequency tuning, which strongly predicts the preference in a choice situation between two songs differing in carrier frequency. A second “matched filter” exists in directional hearing, where reliable cues for sound localization occur only in a narrow frequency range. There is a strong correlation between the directional tuning and the behavioural preference in no-choice tests. This second “matched filter” also varies widely in females, and surprisingly, differs on average by 400 Hz from the neuronal frequency tuning.

Conclusions/Significance

Our findings on the mismatch of the two “matched filters” would suggest that the difference in these two filters appears to be caused by their evolutionary history, and the different trade-offs which exist between sound emission, transmission and detection, as well as directional hearing under specific ecological settings. The mismatched filter situation may ultimately explain the maintenance of considerable variation in the carrier frequency of the male signal despite stabilizing selection.

Diel variation in a dynamic sexual display and its association with female mate-searching behaviour

Dynamic sexual signals often show a diel rhythm and may vary substantially with time of day. Diel and short-term fluctuations in such sexual signals pose a puzzle for condition capture models of mate choice, which assume a female preference for male traits that reliably reflect a male's quality. Here we experimentally manipulated the food supply of individual male field crickets Gryllus campestris in their natural habitat in two consecutive seasons to determine (i) the effect of male nutritional condition on the fine-scaled variation of diel investment in acoustic signalling and (ii) the temporal association between the diel variation in male signalling and female mate-searching behaviour. Overall food-supplemented males signalled more often, but the effect was only visible during the daytime. In the evening and the night, signal output was still high but the time spent signalling was unrelated to a male's nutritional condition. Females' mate-searching behaviour also showed a diel rhythm with peak activity during the afternoon, when differences among calling males were highest, and where signal output reliably reflects male quality. These findings suggest that males differing in nutritional condition may optimize their investment in signalling in relation to time of day as to maximize mating success.

A potential resolution to the lek paradox through indirect genetic effects

Females often prefer males with elaborate traits, even when they receive no direct benefits from their choice. In such situations, mate discrimination presumably has genetic advantages; selective females will produce offspring of higher genetic quality. Over time, persistent female preferences for elaborate secondary-sexual traits in males should erode genetic variance in these traits, eventually eliminating any benefit to the preferences. Yet, strong female preferences persist in many taxa. This puzzle is called the lek paradox and raises two primary questions: do females obtain genetic benefits for offspring by selecting males with elaborate secondary-sexual characteristics and, if so, how is the genetic variation in these male traits maintained? We suggest that indirect genetic effects may help to resolve the lek paradox. Maternal phenotypes, such as habitat selection behaviours and offspring provisioning, often influence the condition and the expression of secondary-sexual traits in sons. These maternal influences are commonly genetic based (i.e. they are indirect genetic effects). Females choosing mates with elaborate traits may receive 'good genes' for daughters in the form of effective maternal characteristics. Recognizing the significance of indirect genetic effects may be important to our understanding of the process and consequences of sexual selection.