Evidence for individual vocal recognition in a pair-bonding poison frog, Ranitomeya imitator

Individually distinctive vocalizations are widespread in nature, although the ability of receivers to discriminate these signals has only been explored through limited taxonomic and social lenses. Here, we asked whether anuran advertisement calls, typically studied for their role in territory defense and mate attraction, facilitate recognition and preferential association with partners in a pair-bonding poison frog (Ranitomeya imitator). Combining no- and two-stimulus choice playback experiments, we evaluated behavioral responses of females to male acoustic stimuli. Virgin females oriented to and approached speakers broadcasting male calls independent of caller identity, implying that females are generally attracted to male acoustic stimuli outside the context of a pair bond. When pair-bonded females were presented with calls of a mate and a stranger, they showed significant preference for calls of their mate. Moreover, behavioral responses varied with breeding status: females with eggs were faster to approach stimuli than females that were pair bonded but did not currently have eggs. Our study suggests a potential role for individual vocal recognition in the formation and maintenance of pair bonds in a poison frog and raises new questions about how acoustic signals are perceived in the context of monogamy and biparental care.

Evolution of acoustic signals associated with cooperative parental behavior in a poison frog

The emergence of complex social interactions is predicted to be an important selective force in the diversification of communication systems. Parental care presents a key social context in which to study the evolution of novel signals, as care often requires communication and behavioral coordination between parents and is an evolutionary stepping-stone toward increasingly complex social systems. Anuran amphibians (frogs and toads) are a classic model of acoustic communication and the vocal repertoires of many species have been characterized in the contexts of advertisement, courtship, and aggression, yet quantitative descriptions of calls elicited in the context of parental care are lacking. The biparental poison frog, Ranitomeya imitator, exhibits a remarkable parenting behavior in which females, cued by the calls of their male partners, feed tadpoles unfertilized eggs. Here, we characterized and compared calls across three social contexts, for the first time including a parental care context. We found that egg-feeding calls share some properties with both advertisement and courtship calls but also had unique properties. Multivariate analysis revealed high classification success for advertisement and courtship calls but misclassified nearly half of egg feeding calls as either advertisement or courtship calls. Egg feeding and courtship calls both contained less identity information than advertisement calls, as expected for signals used in close-range communication where uncertainty about identity is low and additional signal modalities may be used. Taken together, egg-feeding calls likely borrowed and recombined elements of both ancestral call types to solicit a novel, context-dependent parenting response.

Female preferences for the spectral content of advertisement calls in Cope's gray treefrog (Hyla chrysoscelis)

Amphibians have inner ears with two sensory papillae tuned to different frequency ranges of airborne sounds. In frogs, male advertisement calls possess distinct spectral components that match the tuning of one or both sensory papillae. Female preferences for the spectral content of advertisement calls can depend on signal amplitude and can vary among closely related lineages. In this study of Cope's gray tree frog (Hyla chrysoscelis), we investigated the amplitude dependence of female preferences for the spectral content of male advertisement calls, which have a "bimodal" spectrum with separate low-frequency (1.25 kHz) and high-frequency (2.5 kHz) components. In two-alternative choice tests, females generally preferred synthetic calls with bimodal spectra over "unimodal" calls having only one of the two spectral components. They also preferred unimodal calls with a high-frequency component over one with the low-frequency component. With few exceptions, preferences were largely independent of amplitude across both a 30 dB range of overall signal amplitude and an 11 dB range in the relative amplitudes of the two spectral components. We discuss these results in the context of evolutionary lability in female preferences for the spectral content of advertisement calls in North American tree frogs in the genus Hyla.

Sensory biases in response to novel complex acoustic signals in male and female grey treefrogs, Hyla chrysoscelis

The sensory bias hypothesis proposes that female preferences for male sexual signalling traits evolved in contexts other than mating. Individuals of both sexes may experience similar selection pressures in these contexts; thus males may have similar biases to females for variation in signal traits. We tested this prediction in the grey treefrog, Hyla chrysoscelis, in which males produce simple advertisement calls, but females are more attracted to certain novel complex stimuli. We recorded males' responses to playbacks of both simple advertisement calls and complex calls consisting of the advertisement call with an acoustic appendage (filtered noise, or heterospecific call pulses) either leading or following the call. We tested females' preferences for the same stimuli in phonotaxis tests. We found evidence for a sensory bias in both sexes: males gave more aggressive calls in response to complex stimuli and females sometimes preferred complex over simple calls. These biases were not universal and depended on both temporal order and appendage characteristics, but how these effects manifested differed between the sexes. Ultimately, our approach of studying biases of both sexes in response to novel mating signals will shed light on the origin of mating preferences, and the mechanisms by which sensory biases operate.

Population genomics and sexual signals support reproductive character displacement in Uperoleia (Anura: Myobatrachidae) in a contact zone

When closely related species come into contact via range expansion, both may experience reduced fitness as a result of the interaction. Selection is expected to favour traits that minimize costly interspecies reproductive interactions (such as mismating) via a phenomenon called reproductive character displacement (RCD). Research on RCD frequently assumes secondary contact between species, but the geographical history of species interactions is often unknown. Population genomic data permit tests of geographical hypotheses about species origins and secondary contact through range expansion. We used population genomic data from single nucleotide polymorphisms (SNPs), mitochondrial sequence data, advertisement call data and morphological data to investigate a species complex of toadlets (Uperoleia borealis, U. crassa, U. inundata) from northern Australia. Although the three species of frogs were morphologically indistinguishable in our analysis, we determined that U. crassa and U. inundata form a single species (synonymized here) based on an absence of genomic divergence. SNP data identified the phylogeographical origin of U. crassa as the Top End, with subsequent westward invasion into the range of U. borealis in the Kimberley. We identified six F1 hybrids, all of which had the U. borealis mitochondrial haplotype, suggesting unidirectional hybridization. Consistent with the RCD hypothesis, U. borealis and U. crassa sexual signals differ more in sympatry than in allopatry. Hybrid males have intermediate calls, which probably reduces attractiveness to females. Integrating population genomic data, mitochondrial sequencing, morphology and behavioural approaches provides an unusually detailed collection of evidence for reproductive character displacement following range expansion and secondary contact.

Experimental evolution of a pheromone signal

Sexual signals are important in speciation, but understanding their evolution is complex as these signals are often composed of multiple, genetically interdependent components. To understand how signals evolve, we thus need to consider selection responses in multiple components and account for the genetic correlations among components. One intriguing possibility is that selection changes the genetic covariance structure of a multicomponent signal in a way that facilitates a response to selection. However, this hypothesis remains largely untested empirically. In this study, we investigate the evolutionary response of the multicomponent female sex pheromone blend of the moth Heliothis subflexa to 10 generations of artificial selection. We observed a selection response of about three-quarters of a phenotypic standard deviation in the components under selection. Interestingly, other pheromone components that are biochemically and genetically linked to the components under selection did not change. We also found that after the onset of selection, the genetic covariance structure diverged, resulting in the disassociation of components under selection and components not under selection across the first two genetic principle components. Our findings provide rare empirical support for an intriguing mechanism by which a sexual signal can respond to selection without possible constraints from indirect selection responses.

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

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

Detecting patterns of correlational selection with sampling error: A simulation study

The adoption of a multivariate perspective of selection implies the existence of multivariate adaptive peaks and pervasive correlational selection that promotes co-adaptation between traits. However, to test for the ubiquity of correlational selection in nature, we must first have a sense of how well can we estimate multivariate nonlinear selection (i.e., the γ-matrix) in the face of sampling error. To explore the sampling properties of estimated γ-matrices, we simulated inidividual traits and fitness under a wide range of sample sizes, using different strengths of correlational selection and of stabilizing selection, combined with different number of traits under selection, different amounts of residual variance in fitness, and distinct patterns of selection. We then ran nonlinear regressions with these simulated datasets to simulate γ-matrices after adding random error to individual fitness. To test how well could we detect the imposed pattern of correlational selection at different sample sizes, we measured the similarity between simulated and imposed γ-matrices. We show that detection of the pattern of correlational selection is highly dependent on the total strength of selection on traits and on the amount of residual variance in fitness. Minimum sample size needs to be at least 500 to precisely estimate the pattern of correlational selection. Furthermore, a pattern of selection in which different sets of traits contribute to different functions is the easiest to diagnose, even when using a large number of traits (10 traits), but with sample sizes in the order of 1000 individuals. Consequently, we recommend working with sets of traits from distinct functional complexes and fitness proxies less prone to effects of environmental and demographic stochasticity to test for correlational selection with lower sample sizes.

Arginine vasotocin affects motivation to call, but not calling plasticity, in Cope's gray treefrog Hyla chrysoscelis

The ability to respond to competition is critical for social behaviors involved in mating, territoriality and foraging. Physiological mechanisms of competitive social behaviors may determine not only baseline behavior, but possibly also the plasticity of the response to competition. We examined the effects of the neuropeptide arginine vasotocin (AVT), which is implicated in social behavior in non-mammalian vertebrates, on both spontaneous acoustic advertisement calling behavior and the plastic response to a simulated competitive challenge in Cope's gray treefrogs, Hyla chrysoscelis. We injected males either with AVT or a saline control and then analyzed recordings of spontaneous calling prior to playback, playback of average advertisement calls, playback of highly competitive advertisement calls, and spontaneous calling after playback. We found a tendency for AVT-treated males to be more likely to resume calling, and AVT males had higher call rates than control males, although they did not differ in pulse number or call effort. There were no differences between the AVT and control treatments in the plasticity of calling behavior in response to simulated competitors. Our results generally align with other studies on how AVT affects anuran vocalizations, and suggest that its primary effect is on motivation to call, with less of an effect on plasticity in response to competition. Nevertheless, these effects on call motivation are significant, because mating success is often determined more by participation in the chorus than by the values of specific call characteristics.

Customizable Recorder of Animal Kinesis (CRoAK): A multi-axis instrumented enclosure for measuring animal movements

Accurately quantifying animal activity and movements is of fundamental importance in a broad range of disciplines, from biomedical research to behavioral ecology. In many instances, it is desirable to measure natural movements in controlled sensory environments in which the animals are not physically or chemically restrained, but their movements are nevertheless constrained to occur within a fixed volume. Here, we describe a novel device to quantify the movements of small animals in response to sensory stimulation. The device consists of an Arduino controlled inertial measurement unit that senses angular velocity (along three axes) of a suspended mesh enclosure that temporarily houses the animal subject. We validated the device by measuring the phonotaxis behavior of gravid female frogs in response to acoustic broadcasts of male mating calls. The system, as designed, proved effective at measuring natural movements made in response to acoustic stimulation.

Inconsistent sexual signaling degrades optimal mating decisions in animals

Like political stump speeches and product advertisements, animal signals are highly repetitive and function to persuade receivers to adopt behaviors benefiting the signaler. And like potential constituents and consumers, receivers assess signals to inform their behavioral decisions. However, inconsistency in sexual signals is widespread and potentially injects uncertainty into mating decisions. Here, we show that females fail to make optimal mating decisions based on assessments of signal quality due to inconsistency in signal production. Natural levels of inconsistency markedly reduced female preference expression for a nonarbitrary signal of male quality. Inconsistency reshaped preferences even more profoundly than the better-known impediment of ambient noise. To our knowledge, this is the first demonstration of how inconsistent messaging degrades optimal decision-making in animals, with implications for understanding signal evolution.

Females prefer the calls of better fathers in a Neotropical frog with biparental care

Male secondary sexual traits potentially function as indicators of direct or indirect fitness benefits to females. Direct benefits, such as paternal care, may be especially important to females in species with biparental care. In an experimental field study of the golden rocket frog (Anomaloglossus beebei), a Neotropical species with biparental care, we tested predictions from four hypotheses proposed to explain the evolutionary relationship between male secondary sexual traits and paternal care quality (the “good parent,” “differential allocation,” “trade-off,” and “essential male care” hypotheses). We examined: 1) the influence of paternal care on offspring survival, 2) the relationships between male calls and paternal care, maternal care, and opportunities for males to acquire multiple mates, and 3) female preferences for three acoustic properties of male advertisement calls. Our results reveal that paternal care positively impacts offspring survival, that males producing longer calls also provide higher-quality paternal care in the form of greater egg attendance and territory defense, and that females prefer longer calls. Females did not discriminate among potential mates based on differences in dominant frequency or call rate. These findings, which suggest male advertisement calls are indicators of potential direct benefits to females in the form of paternal care, are consistent with the good parent hypothesis and inconsistent with the trade-off, differential allocation, and essential male care hypotheses.

Lifetime Fitness in Wild Female Baboons: Trade-Offs and Individual Heterogeneity in Quality

Understanding the evolution of life histories requires information on how life histories vary among individuals and how such variation predicts individual fitness. Using complete life histories for females in a well-studied population of wild baboons, we tested two nonexclusive hypotheses about the relationships among survival, reproduction, and fitness: the quality hypothesis, which predicts positive correlations between life-history traits, mediated by variation in resource acquisition, and the trade-off hypothesis, which predicts negative correlations between life-history traits, mediated by trade-offs in resource allocation. In support of the quality hypothesis, we found that females with higher rates of offspring survival were themselves better at surviving. Further, after statistically controlling for variation in female quality, we found evidence for two types of trade-offs: females who produced surviving offspring at a slower rate had longer life spans than those who produced surviving offspring at a faster rate, and females who produced surviving offspring at a slower rate had a higher overall proportion of offspring survive infancy than females who produced surviving offspring at a faster rate. Importantly, these trade-offs were evident even when accounting for (i) the influence of offspring survival on maternal birth rate, (ii) the dependence of offspring survival on maternal survival, and (iii) potential age-related changes in birth rate and/or offspring survival. Our results shed light on why trade-offs are evident in some populations while variation in individual quality masks trade-offs in others.

The difference a day makes: Breeding remodels hearing, hormones and behavior in female Cope's gray treefrogs (Hyla chrysoscelis)

In seasonal breeders, there are behavioral, endocrine, and neural adaptations that promote the sexual receptivity of females and tune their sensory systems to detect and discriminate among advertising males and to successfully copulate. What happens immediately after this key life history event is unclear, but this transitional moment offers a window into the mechanisms that remodel sexual phenotypes. In this study of wild female Cope's gray treefrogs (Hyla chrysoscelis), we tested the hypothesis that oviposition results in a suite of coordinated changes in the sexual phenotype. Specifically, we predicted that sexual receptivity and discrimination behaviors would decline along with circulating concentrations of steroid hormones (corticosterone, estradiol, testosterone) and auditory sensitivity to the acoustic frequencies emphasized in male advertisement calls. We conducted these trait measurements before and after oviposition (ca. 24-h period). There was a 100% decrease in behavioral responsiveness after oviposition, and the concentrations of all three steroids plummeted during this brief window of time, especially testosterone. Moreover, higher concentrations of corticosterone-an important component of the endocrine stress response-were associated with longer response latencies, suggesting that adrenal hormones should be considered in future studies on the hormonal basis of mate choice. Counter to our prediction, auditory sensitivity increased following oviposition, and the amplitude of the auditory brainstem response was influenced by concentrations of estradiol. In pre-oviposition females auditory sensitivity diminished with increasing estradiol concentrations, while sensitivity increased with increasing estradiol concentrations in post-oviposition females, suggesting non-linear estrogenic modulation of peripheral auditory neural recruitment. Overall, our results indicate that there is considerable remodeling of behavioral output following oviposition that co-occurs with changes in both endocrine and sensory physiology.

Phenotypic integration and the evolution of signal repertoires: A case study of treefrog acoustic communication

Animal signals are inherently complex phenotypes with many interacting parts combining to elicit responses from receivers. The pattern of interrelationships between signal components reflects the extent to which each component is expressed, and responds to selection, either in concert with or independently of others. Furthermore, many species have complex repertoires consisting of multiple signal types used in different contexts, and common morphological and physiological constraints may result in interrelationships extending across the multiple signals in species' repertoires. The evolutionary significance of interrelationships between signal traits can be explored within the framework of phenotypic integration, which offers a suite of quantitative techniques to characterize complex phenotypes. In particular, these techniques allow for the assessment of modularity and integration, which describe, respectively, the extent to which sets of traits covary either independently or jointly. Although signal and repertoire complexity are thought to be major drivers of diversification and social evolution, few studies have explicitly measured the phenotypic integration of signals to investigate the evolution of diverse communication systems. We applied methods from phenotypic integration studies to quantify integration in the two primary vocalization types (advertisement and aggressive calls) in the treefrogs Hyla versicolor, Hyla cinerea, and Dendropsophus ebraccatus. We recorded male calls and calculated standardized phenotypic variance-covariance (P) matrices for characteristics within and across call types. We found significant integration across call types, but the strength of integration varied by species and corresponded with the acoustic similarity of the call types within each species. H. versicolor had the most modular advertisement and aggressive calls and the least acoustically similar call types. Additionally, P was robust to changing social competition levels in H. versicolor. Our findings suggest new directions in animal communication research in which the complex relationships among the traits of multiple signals are a key consideration for understanding signal evolution.