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For many animals, options abound when choosing a mate in socially complex environments like a breeding chorus or lek. In such environments, receivers often choose their mate based on individual differences in signal repetition rate. However, signallers also differ in the regularity with which they produce repeated signals. Irregularity in signalling introduces uncertainty in decision-making by masking the among-individual variation in signalling rate that is a target of mate choice. At present, we know little about how the complexity of the choice environment affects selection on rate and regularity, two signalling behaviours that receivers can only compare after sampling series of signals produced by multiple signallers. In this study of female grey treefrogs (Hyla chrysoscelis), we measured multivariate sexual selection on the rate and regularity of male calling behaviour using two-, four- and eight-choice tests. Receivers overwhelmingly chose faster, more regular calling rates in two-choice tests, but did so markedly less often when they chose among four or eight stimuli. Sexual selection imposed by female choice became markedly weaker and differently shaped as the complexity of the social environment increased, suggesting noise and choice overload effects may allow relatively unattractive males to mate. 

Anuran amphibians exhibit the greatest diversity of reproductive modes among tetrapod vertebrates. The Andamanese Charles Darwin's frog, Minervarya charlesdarwini, is the only species of the family Dicroglossidae that is known to naturally deposit eggs in water-filled cavities of tree holes or buttresses, where they then undergo exotrophic development. We describe the reproductive behavior in this species that involves a unique combination of traits: (1) Males produce complex advertisement calls comprising at least three different call types, in addition to a type of aggressive call. (2) Unpaired males exhibit agonistic interactions with each other and with mated pairs. (3) Mate selection, amplexus, and oviposition take place inside water-filled cavities. (4) During axillary amplexus, mating pairs synchronously switch between head-up and head-down positions above and below the water surface using both forward and backward movements. (5) At the time of egg laying, amplectant pairs are in an upside-down position on the cavity walls with their bodies completely outside the water. (6) Eggs are deposited over multiple bouts on the inner walls of the cavities and terrestrially above the water surface. Upside-down spawning in M. charlesdarwini is a unique trait among phytotelm-breeding terrestrial frogs. The combination of terrestrial oviposition sites in water-filled phytotelmata and the upside-down egg-laying posture is a novel report for the family Dicroglossidae and perhaps all anurans. This specialized behavior is also likely derived for a species that is embedded in a group of largely aquatic-breeding minervaryan frogs. Although M. charlesdarwini appears to be an obligate phytotelm breeder, individuals were often observed breeding inside cylindrical, water-filled plastic sapling bags in plant nurseries adjacent to fragmented forest patches, or in rain-filled discarded plastic, glass, or metal containers left as trash at the forest edge. Use of such unnatural breeding sites is likely a forced behavioral shift in response to rapidly changing forest landscapes associated with recent habitat loss and fragmentation. Our findings call for conservation attention to this habitat specialist, which, although locally abundant, is an endemic and threatened species of the Andaman Islands. 

ABSTRACT Sexual traits that promote species recognition are important drivers of reproductive isolation, especially among closely related species. Identifying neural processes that shape species differences in recognition is crucial for understanding the causal mechanisms of reproductive isolation. Temporal patterns are salient features of sexual signals that are widely used in species recognition by several taxa, including anurans. Recent advances in our understanding of temporal processing by the anuran auditory system provide an opportunity to investigate the neural basis of species-specific recognition. The anuran inferior colliculus consists of neurons that are selective for temporal features of calls. Of potential relevance are auditory neurons known as interval-counting neurons (ICNs) that are often selective for the pulse rate of conspecific advertisement calls. Here, we tested the hypothesis that ICNs mediate acoustic species recognition by exploiting the known differences in temporal selectivity in two cryptic species of gray treefrog (Hyla chrysoscelis and Hyla versicolor). We examined the extent to which the threshold number of pulses required to elicit behavioral responses from females and neural responses from ICNs was similar within each species but potentially different between the two species. In support of our hypothesis, we found that a species difference in behavioral pulse number thresholds closely matched the species difference in neural pulse number thresholds. However, this relationship held only for ICNs that exhibited band-pass tuning for conspecific pulse rates. Together, these findings suggest that differences in temporal processing of a subset of ICNs provide a mechanistic explanation for reproductive isolation between two cryptic treefrog species. 

Synopsis Most animals experience reproductive transitions in their lives; for example, reaching reproductive maturity or cycling in and out of breeding condition. Some reproductive transitions are abrupt, while others are more gradual. In most cases, changes in communication between the sexes follow the time course of these reproductive transitions and are typically thought to be coordinated by steroid hormones. We know a great deal about hormonal control of communication behaviors in birds and frogs, as well as the central neural control of these behaviors. There has also been significant interest in the effects of steroid hormones on central nervous system structures that control both the production and reception of communication signals associated with reproductive behaviors. However, peripheral sensory structures have typically received less attention, although there has been growing interest in recent years. It is becoming clear that peripheral sensory systems play an important role in reproductive communication, are plastic across reproductive conditions, and, in some cases, this plasticity may be mediated by steroid hormones. In this article, we discuss recent evidence for the role of peripheral auditory structures in reproductive communication in birds and frogs, the plasticity of the peripheral auditory system, and the role of steroid hormones in mediating the effects of the peripheral auditory system on reproductive communication. We focus on both seasonal and acute reproductive transitions, introduce new data on the role of hormones in modulating seasonal patterns, and make recommendations for future work.