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.

Behind the mask(ing): how frogs cope with noise

Albert Feng was a pioneer in the field of auditory neuroethology who used frogs to investigate the neural basis of spectral and temporal processing and directional hearing. Among his many contributions was connecting neural mechanisms for sound pattern recognition and localization to the problems of auditory masking that frogs encounter when communicating in noisy, real-world environments. Feng's neurophysiological studies of auditory processing foreshadowed and inspired subsequent behavioral investigations of auditory masking in frogs. For frogs, vocal communication frequently occurs in breeding choruses, where males form dense aggregations and produce loud species-specific advertisement calls to attract potential mates and repel competitive rivals. In this review, we aim to highlight how Feng's research advanced our understanding of how frogs cope with noise. We structure our narrative around three themes woven throughout Feng's research-spectral, temporal, and directional processing-to illustrate how frogs can mitigate problems of auditory masking by exploiting frequency separation between signals and noise, temporal fluctuations in noise amplitude, and spatial separation between signals and noise. We conclude by proposing future research that would build on Feng's considerable legacy to advance our understanding of hearing and sound communication in frogs and other vertebrates.

Variations in advertisement call modulations do not influence vocal interactions in bullfrog choruses

Chorusing male bullfrogs naturally vary the number of modulations within their advertisement call notes. A field playback experiment investigated whether these variations affect males' evoked vocal responses. Vocal responses were quantified manually and automatically by quantifying acoustic energy. The numbers of calls, number of notes, latency of response, and detected-note acoustic energy did not vary significantly across playback stimuli for focal males or the entire chorus, suggesting that variations in modulation number do not carry relevant information to males. Future work can determine whether modulation cues may function in sexual selection and affect female response.

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.

Animal trait variation at the within-individual level: erythrocyte size variation and malaria infection in a tropical lizard

High levels of within-individual variation (WIV) in reiterative components in plants such as leaves, flowers, and fruits have been shown to increase individual fitness by multiple mechanisms including mediating interactions with natural enemies. This relationship between WIV and fitness has been studied almost exclusively in plant systems. While animals do not exhibit conspicuous reiterative components, they have traits that can vary at the individual level such as erythrocyte size. It is currently unknown if WIV in animals can influence individual fitness by mediating the outcome of interactions with natural enemies as it has been shown in plants. To address this issue, we tested for a relationship between WIV in erythrocyte size, hemoparasite infection status, and body condition (a proxy for fitness) in a Caribbean anole lizard. We quantified the coefficient of variation of adult erythrocytes size in $n = 95$ infected and $n = 107$ non-infected lizards. We found higher degrees of erythrocyte size variation in infected lizards than in non-infected individuals. However, we found no significant relationship between infection status or erythrocyte size variation, and lizard body condition. These results suggest that higher WIV in erythrocyte size in infected lizards is not necessarily adaptive but likely a consequence of the host response to infection. Many hemoparasites destroy their host cells as part of their life cycle. To compensate, the host lizard may respond by increasing production of erythrocytes resulting in higher WIV. Our results emphasize the need to better understand the role of within-animal variation as a neglected driver or consequence of ecological and evolutionary interactions.

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.

Lung mediated auditory contrast enhancement improves the Signal-to-noise ratio for communication in frogs

Environmental noise is a major source of selection on animal sensory and communication systems. The acoustic signals of other animals represent particularly potent sources of noise for chorusing insects, frogs, and birds, which contend with a multi-species analog of the human "cocktail party problem" (i.e., our difficulty following speech in crowds). However, current knowledge of the diverse adaptations that function to solve noise problems in nonhuman animals remains limited. Here, we show that a lung-to-ear sound transmission pathway in frogs serves a heretofore unknown noise-control function in vertebrate hearing and sound communication. Inflated lungs improve the signal-to-noise ratio for communication by enhancing the spectral contrast in received vocalizations in ways analogous to signal processing algorithms used in hearing aids and cochlear implants. Laser vibrometry revealed that the resonance of inflated lungs selectively reduces the tympanum's sensitivity to frequencies between the two spectral peaks present in conspecific mating calls. Social network analysis of continent-scale citizen science data on frog calling behavior revealed that the calls of other frog species in multi-species choruses can be a prominent source of environmental noise attenuated by the lungs. Physiological modeling of peripheral frequency tuning indicated that inflated lungs could reduce both auditory masking and suppression of neural responses to mating calls by environmental noise. Together, these data suggest an ancient adaptation for detecting sound via the lungs has been evolutionarily co-opted to create auditory contrast enhancement that contributes to solving a multi-species cocktail party problem.

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.

Moderately elevated glucocorticoids increase mate choosiness but do not affect sexual proceptivity or preferences in female gray treefrogs

Glucocorticoids (GCs) are rarely studied in the context of female mate choice, despite the expression of receptors for these products in sexual, sensory and decision-making brain areas. Here we investigated the effects of GC concentrations on three aspects of female sexual behavior in breeding Cope's gray treefrogs (Hyla chrysoscelis): proceptivity-a measure of sexual motivation, intraspecific mate preferences, and mate choosiness. To our knowledge this is the first experimental study on the endocrine basis of mate choosiness. We predicted that mate choosiness-forfeiting an initial mate preference to pursue a suddenly more attractive mate-would be particularly impacted by elevated GCs with moderate GC levels associated with greater choosiness. We found support for this predicted inverted-U relationship. Females in the control group (no injection) showed no change in choosiness across timepoints. In contrast, females in the vehicle, Low (20 ng g^-1) and High (180 ng g^-1) corticosterone groups exhibited a nominal decline in choosiness after injection, suggesting that the experience of injection has little or perhaps slightly suppressive effects on female choosiness. Females in the moderate dose group (60 ng g^-1), however, exhibited a significant increase (>100%) in choosiness. Further, we found no effect of elevated GCs on sexual proceptivity or the species-typical preference for longer calls. These findings may reflect a buffering of primary sensory areas in the brain against elevated GCs. The recruitment of other cognitive processes during active decision-making, however, may facilitate GC modulation of mate choosiness, thereby promoting tactical plasticity at this critical life history juncture.

Do male Boana goiana (Lutz, 1968) (Anura: Hylidae) discriminate between the calls of neighbours and strangers?

Neighbour-stranger discrimination is an important mechanism in social behaviour, as it allows the individual to avoid the high energetic costs of repeated agonistic interactions with neighbours. In some anuran species, the owners of territories respond more aggressively towards strangers than neighbours. Here, we investigated whether gladiator frog, Boana goiana, males discriminate between neighbours and strangers. This is a lek-breeding species; therefore, it does not defend fixed territories which are associated with a specific resource. We conducted a playback field experiment with a repeated-measures design, based on three treatments: (i) neighbour’s call in initial position, (ii) stranger’s call in initial position, and (iii) neighbour’s call in new position. We did not observe any systematic modification in the acoustic properties (call duration, dominant frequency, repetition rate) of the advertisement and short aggressive calls of B. goiana males given in response to our playback treatments. Our results indicate that B. goiana males do not exhibit vocally mediated ‘dear enemy’ relationships. We discuss the possible ecological and social factors that may have impeded the evolution of ‘dear enemy’ relationships in this species.

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.