Spatial and temporal variation in three call traits and preferences of the tree cricket Oecanthus forbesi

Delineating the calling pattern of Oecanthus indicus from native and non-native plant species

The study attempted to understand the effect of the host plants on the call parameters of native tree cricket, Oecanthus indicus (Order: Orthoptera, Sub-order: Ensifera, Family: Gryllidae) while calling from native (Justicia adhatoda) and non-native host plant species (Lantana camara and Hyptis suaveolens). The study was conducted at four locations across India. Calls of O. indicus were recorded on these host plants in the field and spectral and temporal parameters of calls were analysed. The results suggested that the peak frequency varied among the two non-native plant species while the difference in temporal pattern between the native and non-native host plants was observed only in the syllable period. The study also quantified the choice of calling positions of insects from the three-host species. The native O. indicus chose non-native H. suaveolens leaves extensively as a preferable site to baffle (37%). Differences in the call parameters and choice of the host plant by insects may ultimately affect the preference and performance of insects on invasive plants. The study would aid in exploring the underlying evolutionary and ecological processes of adaptive success of insects on non-native plants.

Sexual selection and 'species recognition' revisited: serial processing and order-of-operations in mate choice

Following the modern synthesis, mating signals were thought of principally as species recognition traits, a view later challenged by a burgeoning interest in sexual selection-specifically mate choice. In the 1990s, these different signal functions were proposed to represent a single process driven by the shape of female preference functions across both intra- and interspecific signal space. However, the properties of reliable 'recognition' signals (stereotyped; low intraspecific variation) and informative 'quality' signals (condition dependent; high intraspecific variation) seem at odds, perhaps favouring different signal components for different functions. Surprisingly, the idea that different components of mating signals are evaluated in series, first to recognize generally compatible mates and then to select for quality, has never been explicitly tested. Here I evaluate patterns of (i) intraspecific signal variation, (ii) female preference function shape and (iii) phylogenetic signal for male cricket call components known to be processed in series. The results show that signal components processed first tend to have low variation, closed preference functions and low phylogenetic signal, whereas signal components processed later show the opposite, suggesting that mating signal evaluation follows an 'order-of-operations'. Applicability of this finding to diverse groups of organisms and sensory modalities is discussed.

Neural basis of acoustic species recognition in a cryptic species complex

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.