Individual variation in male vocal traits and female mating preferences in Bufo americanus

Vocal repertoire of Microhyla nilphamariensis from Delhi and comparison with closely related M. ornata populations from the western coast of India and Sri Lanka

Advertisement calls in frogs have evolved to be species-specific signals of recognition and are therefore considered an essential component of integrative taxonomic approaches to identify species and delineate their distribution range. The species rich genus Microhyla is a particularly challenging group for species identification, discovery and conservation management due to the small size, conserved morphology and wide distribution of its members, necessitating the need for a thorough description of their vocalization. In this study, we provide quantitative description of the vocal behaviour of Microhyla nilphamariensis, a widely distributed south Asian species, from Delhi, India, based on call recordings of 18 individuals and assessment of 21 call properties. Based on the properties measured acrossed 360 calls, we find that a typical advertisement call of M. nilphamariensis lasts for 393.5 ±  57.5 ms, has 17 pulses on average and produce pulses at rate of 39 pulses/s. The overall call dominant frequency was found to be 2.8 KHz and the call spectrum consisted of two dominant frequency peaks centered at 1.6 KHz and 3.6 KHz, ranging between 1.5-4.1 KHz. Apart from its typical advertisement call, our study also reveals the presence of three 'rare' call types, previously unreported in this species. We describe variability in call properties and discuss their relation to body size and temperature. We found that overall dominant frequency 1 (spectral property) was found to be correlated with body size, while first pulse period (temporal property) was found to be correlated with temperature. Further, we compare the vocal repertoire of M. nilphamariensis with that of the congener Microhyla ornata from the western coast of India and Sri Lanka and also compare the call properties of these two populations of M. ornata to investigate intra-specific call variation. We find statistically significant differentiation in their acoustic repertoire in both cases. Based on 18 call properties (out of 20), individuals of each locality clearly segregate on PCA factor plane forming separate groups. Discriminant function analysis (DFA) using PCA factors shows 100% classification success with individuals of each locality getting classified to a discrete group. This confirms significant acoustic differentiation between these species as well as between geographically distant conspecifics. The data generated in this study will be useful for comparative bioacoustic analysis of Microhyla species and can be utilized to monitor populations and devise conservation management plan for threatened species in this group.

The Increasing Role of Short-Term Sperm Storage and Cryopreservation in Conserving Threatened Amphibian Species

Multidisciplinary approaches to conserve threatened species are required to curb biodiversity loss. Globally, amphibians are facing the most severe declines of any vertebrate class. In response, conservation breeding programs have been established in a growing number of amphibian species as a safeguard against further extinction. One of the main challenges to the long-term success of conservation breeding programs is the maintenance of genetic diversity, which, if lost, poses threats to the viability and adaptive potential of at-risk populations. Integrating reproductive technologies into conservation breeding programs can greatly assist genetic management and facilitate genetic exchange between captive and wild populations, as well as reinvigorate genetic diversity from expired genotypes. The generation of offspring produced via assisted fertilisation using frozen-thawed sperm has been achieved in a small but growing number of amphibian species and is poised to be a valuable tool for the genetic management of many more threatened species globally. This review discusses the role of sperm storage in amphibian conservation, presents the state of current technologies for the short-term cold storage and cryopreservation of amphibian sperm, and discusses the generation of cryo-derived offspring.

Effects of operational sex ratio and male density on size-dependent mating in Minshan’s toads, Bufo minshanicus, on the Tibetan Plateau of China

In many animal species, an increase in the operational sex ratio (OSR), density or a combination of both should lead to more intensive competition among individuals of the more abundant sex. To test this, we examined pairing patterns of Minshan’s toad (Bufo minshanicus) from six populations between 2008 and 2015 along the eastern Tibetan Plateau in south-west China. OSRs in breeding aggregations of Minshan’s toad are normally male biased and males actively compete with each other for acquisition and retention of mates. We found evidence that deviations from random mating by size varied between populations and between years according to the magnitude of the OSR and male density. Larger males were generally more successful in pairing than smaller males when the OSR was slightly male biased and male density was high. However, the resulting size-disproportionate mating was more evident when OSR was closer to 1.99, indicating a positive correlation with the intensity of aggressive scramble competition. Thus, the intensity of male-male competition may partly explain variation in size-disproportionate mating among populations.

Endurance rivalry and female choice jointly influence male mating success in the emerald treefrog (Zhangixalus prasinatus), a lek-chorusing anuran

Background

Endurance rivalry and female choice are two important mechanisms of sexual selection in lek-breeding species. Endurance rivalry is when males compete for opportunities to mate by spending more time in leks than others (interaction-independent male-male competition). Because high-quality males can afford to have high lek attendance, females have a higher chance of mating with good-quality males even when they mate randomly. The good gene hypothesis proposes that females can pass good genes on to their offspring by choosing males that display elaborate morphological and/or behavioral traits that reflect the males’ genetic quality. The relative importance of lek attendance and female choice to males’ mating success in anurans is rarely evaluated. In this study, we investigated how these two mechanisms might jointly shape males’ morphological traits in the lek-chorusing emerald treefrog Zhangixalus prasinatus.

Results

Our results show that (1) male lek attendance is positively correlated with body size and condition, and males with higher lek attendance have higher mating success, (2) the dominant frequency of males’ advertisement calls are negatively correlated with body size and males producing lower frequency calls have higher mating success, (3) male body size, but not body condition, has a non-significant positive relationship with mating success and (4) females show preference for calls with lower dominant frequencies in two-choice playback.

Conclusions

Overall, both endurance rivalry and female choice play an important role in the mating success of male emerald treefrogs in the field and both are influenced by male body size/condition. By mating with males that have higher lek attendance and produce lower frequency calls, selection may indirectly favor larger males.

Mate Preference Plasticity in a Critically Endangered Frog: Implications for Conservation Breeding

Variation in female mate preferences for male traits remains poorly understood (both among and within females), despite having important evolutionary and conservation implications, particularly for captive breeding. Here, we investigate female mate preferences for male advertisement call frequency, and determine whether preferences vary over repeated trials, in the critically endangered southern corroboree frog, Pseudophryne corroboree. We conducted a series of phonotaxis trials in a six-speaker arena where naïve, captive-bred, virgin females were offered a choice between low, average and high frequency male advertisement calls, with a subset of females tested repeatedly. In the first trial, we found no evidence for a population-level preference for call frequency, but females spent less time in the low call zone than expected by chance. However, our results showed that female mate preferences changed over sequential trials. Females spent significantly more time in the low frequency call zone in the third trial compared to the first trial, and, in the last trial, females exhibited a significant population-level preference for low frequency calls. Subsequently, repeatability estimates of female preferences were low and did not significantly deviate from zero. Our results indicate that female P. corroboree mate preferences can exhibit temporal variation, and suggest that females are more attracted to low call frequencies after repeated exposure. These findings imply that female P. corroboree may become choosier over time, and highlight the potential for mate preferences to exhibit phenotypic plasticity within a single reproductive cycle. Overall, these findings provide the first information on mate preferences in P. corroboree, and emphasize the importance of considering individual variation in mate choice studies. From a conservation perspective, knowledge of individual variation in female mate preferences may be used to conduct behavioral manipulations in captivity that facilitate the breeding of genetically valuable individuals, and improve the success of conservation breeding programs.

Meta-analysis reveals that animal sexual signalling behaviour is honest and resource based

Animals often need to signal to attract mates and behavioural signalling may impose substantial energetic and fitness costs to signallers. Consequently, individuals often strategically adjust signalling effort to maximize the fitness payoffs of signalling. An important determinant of these payoffs is individual state, which can influence the resources available to signallers, their likelihood of mating and their motivation to mate. However, empirical studies often find contradictory patterns of state-based signalling behaviour. For example, individuals in poor condition may signal less than those in good condition to conserve resources (ability-based signalling) or signal more to maximize short-term reproductive success (needs-based signalling). To clarify this relationship, I systematically searched for published studies examining animal sexual signalling behaviour in relation to six aspects of individual state: age, mated status, attractiveness, body size, condition and parasite load. Across 228 studies and 147 species, individuals (who were predominantly male) invested more into behavioural signalling when in good condition. Overall, this suggests that animal sexual signalling behaviour is generally honest and ability-based. However, the magnitude of state-dependent plasticity was small and there was a large amount of between-study heterogeneity that remains unexplained.

Tail display is regulated by anaerobic metabolism in an Asian agamid lizard

Understanding the mechanism underlying signal variation is an important goal in the study of animal communication. Several potential causes have been proposed for signal variation, including environmental noise (e.g. wind, sound), energy limitation, and predation risk, among others, but the physiological control of many signals are often unclear. Here, we examined the correlation between tail display signal variation and energy metabolic activity using an Asian agamid lizard Phrynocephalus vlangalii. Individual tail display signals were observed in the field, and blood lactate concentration as well as 2 energy metabolic enzymes was assayed. Our results showed that average tail coil speed was positively associated with blood lactate concentration, while tail coil duration was negatively associated with LDH activity. We also found that average tail lash speed was positively associated with blood lactate concentration, suggesting that the tail display behavior of P. vlangalii was regulated by anaerobic metabolism. Furthermore, the correlation between tail display behavior and energy metabolism was not sex-dependent. Taken together, our research provides insight into the physiological mechanisms underlying tail display variation in lizards, and suggests that tail display variation likely transmits important information on individual body condition and resource holding potential.

Variability in anuran advertisement call: a multi-level study with 15 species of monkey tree frogs (Anura, Phyllomedusidae)

Understanding the variability of acoustic signals is a first important step for the comprehension of the evolutionary processes that led to current diversity. Herein, we evaluate the variability of the advertisement call of the phyllomedusid species from the genera Phyllomedusa Wagler, 1830 and Pithecopus Cope, 1866 at different levels: intra-individual, intra-population, inter-population, intra-species, and inter-specific. An analysis of coefficients of variation showed a continuum of variability between the acoustic parameters analyzed, from static to highly dynamic. The majority of the variation was attributed to the inter-specific level, while call parameters at the intra-individual level varied the least. However, each parameter behaved differently with call interval being the most variable across all levels. Most temporal acoustic parameters were affected by environmental temperature, while pulse rate and dominant frequency were strongly influenced by body size. Only pulse rate was correlated to the geographic distance between populations, while all parameters presented a significant phylogenetic signal. Based on these results, we discuss the possible importance of different evolutionary forces and the usage of vocalizations for taxonomic purposes.

Acoustic divergence in advertisement calls among three sympatric Microhyla species from East China

Background

Species-specific advertisement calls are the main mechanism of transmitting information between individuals in anuran amphibians and are therefore indispensable for anuran survival and reproduction. Survey methods that monitor these calls can be used for rapid species recognition, behavioral experiments, and conservation monitoring. In this study, we described in detail 10 call parameters from three sympatric species in the genus Microhyla and analyzed the differences in call parameter among these species to provide a basis for systematic monitoring, acoustic analysis and taxonomic study of this genus.

Methods

The quantitative analyses of temporal and spectral call parameters were used in our study for the advertisement calls of three sympatric Microhyla species (M. beilunensis, M. fissipes and M. heymonsi) in Zhejiang Province, East China.

Results

Our results showed the following: (1) Significant differences existed among the three sympatric Microhyla species in call duration (CD), call interval (CI), number of pulses (NP), pulse rate, call intensity (CIT), dominant frequency (DF) and frequency of the first to fourth formants (F1, F2, F3 and F4). (2) Some spectral parameters (DF, F1 and F3) were negatively correlated with the body size of the vocalizing individuals in each species. (3) The coefficients of variation within individuals (CV_w) for CIT, DF and F1-F4 were smaller than 5%, whereas the CV_W for CI was larger than 10% in each species. (4) The principal component analysis and discriminant function analysis showed that call parameters could distinguish the three Microhyla species. (5) The phylogenetic generalized least squares analysis showed that phylogenetic relationships affected CD and NP against snout-vent length (SVL), DF and NP against CD, and NP against DF, but not of DF against SVL; based on the phylogenetic analysis, CD and NP were not related to SVL, but DF was negatively related to SVL.

Intraspecific call variation in a Neotropical gladiator frog with a complex advertisement call

Vocalisation is one of the most conspicuous behavioural traits in different animals, but its emission can be variable within species, depending on environmental, morphological, and/or social factors. Understanding how acoustic parameters vary can provide information about sexual selection mechanisms that might have shaped the species’ evolutionary history. We quantified and compared the variation of five call parameters within and among the males ofBokermannohyla ibitiguara, a Neotropical treefrog with complex reproduction and advertisement calls. The parameters were classified as static (dominant frequency), intermediate (note durations) or dynamic (pulse rate of the long note and call duration). Despite these differences, all of them varied more among individuals than within individuals, and most were associated with individual discrimination, showing potential acoustic recognition by males and females. A multiple regression analysis showed that all temporal parameters were affected by abiotic factors, except pulse rate of the long note; body size affected dominant frequency, duration of the long notes and call duration. This high variability in parameters, and the strong support for individual discrimination, indicate the existence of sexual selection mechanisms operating on calls; however, the individual recognition system is very complex and not limited to a single characteristic of the call. This is supported by the reproductive behavior of this species, including choosy females, elaborate courtships, and male-male vocal contests and physical combats. Fine-scale playback experiments with males and females will help us further understand sexual selection mechanisms in this and other acoustically oriented animals.

Designing better frog call recognition models

Advances in bioacoustic technology, such as the use of automatic recording devices, allow wildlife monitoring at large spatial scales. However, such technology can produce enormous amounts of audio data that must be processed and analyzed. One potential solution to this problem is the use of automated sound recognition tools, but we lack a general framework for developing and validating these tools. Recognizers are computer models of an animal sound assembled from "training data" (i.e., actual samples of vocalizations). The settings of variables used to create recognizers can impact performance, and the use of different settings can result in large differences in error rates that can be exploited for different monitoring objectives. We used Song Scope (Wildlife Acoustics Inc.) to build recognizers and vocalizations of the wood frog (Lithobates sylvaticus) to test how different settings and amounts of training data influence recognizer performance. Performance was evaluated using precision (the probability of a recognizer match being a true match) and sensitivity (the proportion of vocalizations detected) based on a receiver operating characteristic (ROC) curve-determined score threshold. Evaluations were conducted using recordings not used to build the recognizer. Wood frog recognizer performance was sensitive to setting changes in four out of nine variables, and small improvements were achieved by using additional training data from different sites and from the same recording, but not from different recordings from the same site. Overall, the effect of changes to variable settings was much greater than the effect of increasing training data. Additionally, by testing the performance of the recognizer on vocalizations not used to build the recognizer, we discovered that Type I error rates appear idiosyncratic and do not recommend extrapolation from training to new data, whereas Type II errors showed more consistency and extrapolation can be justified. Optimizing variable settings on independent recordings led to a better match between recognizer performance and monitoring objectives. We provide general recommendations for application of this methodology with other species and make some suggestions for improvements.

Wild cricket social networks show stability across generations

BACKGROUND

A central part of an animal's environment is its interactions with conspecifics. There has been growing interest in the potential to capture these interactions in the form of a social network. Such networks can then be used to examine how relationships among individuals affect ecological and evolutionary processes. However, in the context of selection and evolution, the utility of this approach relies on social network structures persisting across generations. This is an assumption that has been difficult to test because networks spanning multiple generations have not been available. We constructed social networks for six annual generations over a period of eight years for a wild population of the cricket Gryllus campestris.

RESULTS

Through the use of exponential random graph models (ERGMs), we found that the networks in any given year were able to predict the structure of networks in other years for some network characteristics. The capacity of a network model of any given year to predict the networks of other years did not depend on how far apart those other years were in time. Instead, the capacity of a network model to predict the structure of a network in another year depended on the similarity in population size between those years.

CONCLUSIONS

Our results indicate that cricket social network structure resists the turnover of individuals and is stable across generations. This would allow evolutionary processes that rely on network structure to take place. The influence of network size may indicate that scaling up findings on social behaviour from small populations to larger ones will be difficult. Our study also illustrates the utility of ERGMs for comparing networks, a task for which an effective approach has been elusive.

Comparing pre- and post-copulatory mate competition using social network analysis in wild crickets

In many animals, males compete for fertilizations both before and after mating. But do males specialize in 1 type of competition? And do physical fights between males lead to less competition between their ejaculates within females? We studied competitions between wild crickets by building networks of interactions. We found that males that had more fights were more likely to meet in sperm competition, suggesting that evolution will not favor specialists in one of the 2 types of competition.

Twitter: @DFofFreedom

Sexual selection results from variation in success at multiple stages in the mating process, including competition before and after mating. The relationship between these forms of competition, such as whether they trade-off or reinforce one another, influences the role of sexual selection in evolution. However, the relationship between these 2 forms of competition is rarely quantified in the wild. We used video cameras to observe competition among male field crickets and their matings in the wild. We characterized pre- and post-copulatory competition as 2 networks of competing individuals. Social network analysis then allowed us to determine 1) the effectiveness of precopulatory competition for avoiding postcopulatory competition, 2) the potential for divergent mating strategies, and 3) whether increased postcopulatory competition reduces the apparent reproductive benefits of male promiscuity. We found 1) limited effectiveness of precopulatory competition for avoiding postcopulatory competition; 2) males do not specifically engage in only 1 type of competition; and 3) promiscuous individuals tend to mate with each other, which will tend to reduce variance in reproductive success in the population and highlights the trade-off inherent in mate guarding. Our results provide novel insights into the works of sexual competition in the wild. Furthermore, our study demonstrates the utility of using network analyses to study competitive interactions, even in species lacking obvious social structure.

Long recording sequences: how to track the intra-individual variability of acoustic signals

Recently developed acoustic technologies - like automatic recording units - allow the recording of long sequences in natural environments. These devices are used for biodiversity survey but they could also help researchers to estimate global signal variability at various (individual, population, species) scales. While sexually-selected signals are expected to show a low intra-individual variability at relatively short time scale, this variability has never been estimated so far. Yet, measuring signal variability in controlled conditions should prove useful to understand sexual selection processes and should help design acoustic sampling schedules and to analyse long call recordings. We here use the overall call production of 36 male treefrogs (Hyla arborea) during one night to evaluate within-individual variability in call dominant frequency and to test the efficiency of different sampling methods at capturing such variability. Our results confirm that using low number of calls underestimates call dominant frequency variation of about 35% in the tree frog and suggest that the assessment of this variability is better by using 2 or 3 short and well-distributed records than by using samples made of consecutive calls. Hence, 3 well-distributed 2-minutes records (beginning, middle and end of the calling period) are sufficient to capture on average all the nightly variability, whereas a sample of 10 000 consecutive calls captures only 86% of it. From a biological point of view, the call dominant frequency variability observed in H. arborea (116Hz on average but up to 470 Hz of variability during the course of the night for one male) challenge about its reliability in mate quality assessment. Automatic acoustic recording units will provide long call sequences in the near future and it will be then possible to confirm such results on large samples recorded in more complex field conditions.

A test of the Energetics-Hormone Vocalization model in the green treefrog

Male courtship displays may be regulated by, and affect the production of, circulating hormones. The Energetics-Hormone Vocalization (EHV) model, for example, posits that interactions among chorusing male anuran amphibians stimulate androgen production that then mediates an increase in vocal effort. Increased vocal effort is expected to deplete energy reserves and increase glucocorticoid levels that, in turn, negatively affect androgen levels and vocalization. Androgen levels, glucocorticoid levels, and vocal effort are thus expected to increase across and within nights of chorus activity and should be positively correlated in calling males; energy reserves should decline temporally and be inversely related to glucocorticoid levels. We tested predictions of the EHV model in the green treefrog, Hyla cinerea. Consistent with the model, both testosterone and dihydrotestosterone levels increased across the breeding season in calling males. However, testosterone levels decreased and dihydrotestosterone levels did not change within nights of chorus activity, suggesting that chorusing behavior did not drive the seasonal elevation in androgens. Corticosterone (CORT) level remained relatively stable across the breeding season and decreased within nights of chorus activity, contrary to model predictions. Body condition, the proxy for energetic state, was inversely correlated with CORT level but discrepancies between model predictions and temporal patterns of CORT production arose because there was no evidence of a temporal decrease in body condition or increase in vocal effort. Moreover, androgen and CORT levels were not positively correlated with vocal effort. Additional ecological and physiological measures may be needed to support predictions of the EHV model.

Effects of glyphosate-based herbicides on survival, development, growth and sex ratios of wood frog (Lithobates sylvaticus) tadpoles. II: agriculturally relevant exposures to Roundup WeatherMax® and Vision® under laboratory conditions

Glyphosate-based herbicides are currently the most commonly used herbicides in the world. They have been shown to affect survival, growth, development and sexual differentiation of tadpoles under chronic laboratory exposures but this has not been investigated under more environmentally realistic conditions. The purpose of this study is (1) to determine if an agriculturally relevant exposure to Roundup WeatherMax®, a relatively new and understudied formulation, influences the development of wood frog tadpoles (Lithobates sylvaticus) through effects on the mRNA levels of genes involved in the control of metamorphosis; (2) to compare results to the well-studied Vision® formulation (containing the isopropylamine salt of glyphosate [IPA] and polyethoxylated tallowamine [POEA] surfactant) and to determine which ingredient(s) in the formulations are responsible for potential effects on development; and (3) to compare results to recent field studies that used a similar experimental design. In the present laboratory study, wood frog tadpoles were exposed to an agriculturally relevant application (i.e., two pulses) of Roundup WeatherMax® and Vision® herbicides as well as the active ingredient (IPA) and the POEA surfactant of Vision®. Survival, development, growth, sex ratios and mRNA levels of genes involved in tadpole metamorphosis were measured. Results show that Roundup WeatherMax® (2.89 mg acid equivalent (a.e.)/L) caused 100% mortality after the first pulse. Tadpoles treated with a lower concentration of Roundup WeatherMax® (0.21 mg a.e./L) as well as Vision® (2.89 mg a.e./L), IPA and POEA had an increased condition factor (based on length and weight measures in the tadpoles) relative to controls at Gosner stage (Gs) 36/38. At Gs42, tadpoles treated with IPA and POEA had a decreased condition factor. Also at Gs42, the effect on condition factor was dependent on the sex of tadpoles and significant treatment effects were only detected in males. In most cases, treatment reduced the normal mRNA increase of key genes controlling development in tadpoles between Gs37 and Gs42, such as genes encoding thyroid hormone receptor beta in brain, glucocorticoid receptor in tail and deiodinase enzyme in brain and tail. We conclude that glyphosate-based herbicides have the potential to alter mRNA profiles during metamorphosis. However, studies in natural systems have yet to replicate these negative effects, which highlight the need for more ecologically relevant studies for risk assessment.

Acoustic characteristics of eight common Chinese anurans during the breeding season

Anurans often have species-specific vocalizations. To quantify and compare the characteristics of anuran calls in Gutianshan National Nature Reserve, Zhejiang Province, we recorded the advertisement calls of eight species belonging to four families (Ranidae, Microhylidae, Megophryidae and Bufonidae) from June to September 2012 using Sony ICD-FX8 IC recorders. All recordings were analyzed using the "Praat" software. Five acoustics parameters were measured, including temporal traits (call duration, number of notes or pulse number/call) and spectral traits (fundamental frequency, the first three formants and dominant frequency). The characteristic parameters of Microhyla ornate and Fejervarya limnocharis calls were different as were the calls of some populations of the same species recorded in different regions. The advertisement calls of the eight species were specific. Our study has provided a useful reference for identifying the calls of some common Chinese anurans.

Radiographic analysis of vocal tract length and its relation to overall body size in two canid species

Body size is an important determinant of resource and mate competition in many species. Competition is often mediated by conspicuous vocal displays, which may help to intimidate rivals and attract mates by providing honest cues to signaler size. Fitch proposed that vocal tract resonances (or formants) should provide particularly good, or honest, acoustic cues to signaler size because they are determined by the length of the vocal tract, which in turn, is hypothesized to scale reliably with overall body size. There is some empirical support for this hypothesis, but to date, many of the effects have been either mixed for males compared with females, weaker than expected in one or the other sex, or complicated by sampling issues. In this paper, we undertake a direct test of Fitch's hypothesis in two canid species using large samples that control for age- and sex-related variation. The samples involved radiographic images of 120 Portuguese water dogs Canis lupus familiaris and 121 Russian silver foxes Vulpes vulpes. Direct measurements were made of vocal tract length from X-ray images and compared against independent measures of body size. In adults of both species, and within both sexes, overall vocal tract length was strongly and significantly correlated with body size. Effects were strongest for the oral component of the vocal tract. By contrast, the length of the pharyngeal component was not as consistently related to body size. These outcomes are some of the clearest evidence to date in support of Fitch's hypothesis. At the same time, they highlight the potential for elements of both honest and deceptive body signaling to occur simultaneously via differential acoustic cues provided by the oral versus pharyngeal components of the vocal tract.

Only distance matters - non-choosy females in a poison frog population

Background

Females have often been shown to exhibit preferences for certain male traits. However, little is known about behavioural rules females use when searching for mates in their natural habitat. We investigated mate sampling tactics and related costs in the territorial strawberry poison frog (Oophaga pumilio) possessing a lek-like mating system, where both sequential and simultaneous sampling might occur. We continuously monitored the sampling pattern and behaviour of females during the complete period between two successive matings.

Results

We found no evidence that females compared males by visiting them. Instead females mated with the closest calling male irrespective of his acoustic and physical traits, and territory size. Playback experiments in the natural home ranges of receptive females revealed that tested females preferred the nearest speaker and did not discriminate between low and high call rates or dominant frequencies.

Conclusions

Our results suggest that females of O. pumilio prefer the closest calling male in the studied population. We hypothesize that the sampling tactic in this population is affected by 1) a strongly female biased sex ratio and 2) a low variance in traits of available males due to strong male-male competition, preventing low quality males from defending a territory and mating.

Steroid hormone levels in calling males and males practicing alternative non-calling mating tactics in the green treefrog, Hyla cinerea

Circulating glucocorticoids and androgens often figure prominently in mating tactic expression in vertebrates. In anuran amphibians (frogs and toads), for example, recent models predict that the depletion of energy reserves during vocalization will result in increased glucocorticoid levels; high glucocorticoids are expected to negatively affect androgen level to mediate transitions from calling to non-calling behavior. Consistent with these predictions, we show that male green treefrogs (Hyla cinerea) adopting an alternative non-calling "satellite" mating tactic were in poorer condition and had higher circulating corticosterone levels and lower androgen levels than calling males. Body condition was inversely related to corticosterone level and positively related to testosterone, but not dihydrotestosterone, level. Corticosterone level was inversely related to testosterone level but not dihydrotestosterone level. Lastly, we show that calling males that were involved in aggressive bouts had higher corticosterone levels than calling males that were not involved in aggressive bouts. Our results are thus consistent with the prediction that aggressive interactions with conspecific males contribute to high corticosterone levels in satellite males that were observed to lose aggressive contests with larger calling males.

Female preferences for spectral call properties in the western genetic lineage of Cope's gray treefrog (Hyla chrysoscelis)

Female frogs discriminate among potential mates based on individual variation in male advertisement calls. While considerable data have accumulated allowing comparisons of female preference functions among species, we still lack fundamental knowledge about how and why the shapes of preference functions for particular call properties vary among populations within all but a few species. Here, we report results from a study aimed at describing female preference functions for spectral call properties in Cope's gray treefrog (Hyla chrysoscelis). Widespread throughout the eastern half of North America, Cope's gray treefrog is the diploid member of the cryptic diploid-tetraploid Hyla versicolor species complex, and its populations are divided into two distinct genetic lineages (eastern and western). In this study of a western lineage population, we recorded and analyzed the spectral properties of 1000 advertisement calls from 50 males and conducted two-choice phonotaxis experiments to estimate a population-level preference function. Females preferred calls with average frequencies over calls with frequencies that were 2 or 3 semitones (1.4 or 2.1 standard deviations, respectively) lower than the population mean. We observed no behavioral discrimination between calls with average and higher-than-average frequencies. Preferences discriminating against low-frequency calls were weak and were abolished by attenuating the preferred average call by 3 dB. We discuss these results in light of previous studies of eastern lineage populations, geographic variation in female preference functions, and the potential adaptive value of discriminating against calls with low frequencies.