Tropical Cricket Research Articles

Anthropogenic calling sites boost the sound amplitude of advertisement calls produced by a tropical cricket

Acoustic signals are widely used as sexual displays to attract mates. The choice of display site can strongly influence the male call characteristics and their attractiveness to females. Males can improve the conspicuousness of their signals by increasing sound amplitude and expanding their broadcast area thereby reaching more potential mating partners and influencing female choice. Insects possess a rather limited degree of signal plasticity. To increase the intensity of their calls, insects have been known to manipulate and engineer their display locations. Mole crickets, for example, build burrows that are shaped like horns, designing the burrow such that its resonance closely matches that of their call frequency, thereby boosting sound amplitude. Tree crickets manipulate plant leaves, using them as baffles for optimal sound radiation. In this study, we show for the first time that an insect species can enhance its advertisement calls by using novel, anthropogenic environments as singing sites. During this study, the tropical cricket Anurogryllus muticus was frequently found singing beside the walls of houses, on concrete stairs or in storm drains. These locations significantly increased the sound amplitude of its calls, by an average of 13 and 7 dB SPL, respectively, as measured above and in front of the singing animals compared to the sound amplitude measured in their natural grassland habitat. To evaluate the effect of calling site choice on the male's life span, we conducted a 35-day population survey using the mark and recapture method. Our results revealed that those males that called in grassland habitat without occupying burrows had the shortest minimum life spans.

No phenotypic signature of acoustic competition in songs of a tropical cricket assemblage

Efficient acoustic communication in multispecies assemblages is challenging due to the presence of heterospecific signals. Masking interference and signal confusion of similarly structured signals can impose fitness costs and, thus, drive evolutionary processes that shape acoustic signals to reduce their overlap in signal space. Although the partitioning of signal space has been frequently studied in frog and bird communities, this topic has received much less attention with reference to insects that communicate acoustically. In this study, we examined the role of acoustic competition in a tropical cricket community and tested the following hypotheses: 1) cosignaling species are expected to exhibit more dissimilar calling songs as compared with species that are spatially and/or temporally separated and 2) species signaling with similar song frequencies, who are thereby subject to energetic masking, are more likely to differ in the temporal domain of their calling songs. Using non-metric multidimensional scaling and pairwise comparison methods to measure acoustic dissimilarity, we found no evidence for either hypothesis: the acoustic signals of species pairs that cosignaled or used similar calling frequencies did not significantly differ from those of species that were spatially/temporally segregated or had large song frequency differences. In conclusion, for the acoustically communicating cricket community investigated, no supportive evidence for the partitioning hypothesis and the widespread belief that acoustic competition has led to divergent selection pressures on signal structure to avoid masking interference was found. Instead, we argue that selection pressures on sensory/neuronal mechanisms seem to more strongly drive reliable communication.

Spectral niche segregation and community organization in a tropical cricket assemblage

In species-rich biomes such as tropical rainforests, the efficiency of intraspecific acoustic communication will strongly depend on the degree of signal overlap. Signal interference deteriorates detection, recognition, and localization of conspecific signals. Thus, the communication space should be partitioned sufficiently to reduce masking interference and to promote intraspecific communication. Here, we studied the community organization of a tropical cricket assemblage with respect to its multidimensional niche axes, such as song frequencies, space (horizontal and vertical), and time, affecting acoustic communication. We used the null model approach to test whether observed community patterns differed from those expected by chance. The assemblage clearly showed partitioning in the spectral domain of calling frequencies of their songs. Furthermore, the range of song frequencies occupied by species is positively correlated with the distance to the average calling frequency of its adjacent neighbors. Thus, species tended to use a greater range of frequency channels for intraspecific communication if the frequency space is available. Our results support the idea that competition for the acoustic communication channel may have resulted in niche segregation along the frequency axes. Concerning the spatiotemporal organization at the community level the spatial (horizontal) distribution appeared to be randomly structured, whereas we found a significant vertical stratification between species. At a temporal scale, the assemblage aggregated their calling activity with an observed niche overlap significantly greater than expected by chance. However, combining the spatial and temporal distribution resulted in low co-occurrence of pairwise species association, consequently reducing chances for masking events.

Solutions to the Cocktail Party Problem in Insects: Selective Filters, Spatial Release from Masking and Gain Control in Tropical Crickets

BackgroundInsects often communicate by sound in mixed species choruses; like humans and many vertebrates in crowded social environments they thus have to solve cocktail-party-like problems in order to ensure successful communication with conspecifics. This is even more a problem in species-rich environments like tropical rainforests, where background noise levels of up to 60 dB SPL have been measured.Principal FindingsUsing neurophysiological methods we investigated the effect of natural background noise (masker) on signal detection thresholds in two tropical cricket species Paroecanthus podagrosus and Diatrypa sp., both in the laboratory and outdoors. We identified three ‘bottom-up’ mechanisms which contribute to an excellent neuronal representation of conspecific signals despite the masking background. First, the sharply tuned frequency selectivity of the receiver reduces the amount of masking energy around the species-specific calling song frequency. Laboratory experiments yielded an average signal-to-noise ratio (SNR) of −8 dB, when masker and signal were broadcast from the same side. Secondly, displacing the masker by 180° from the signal improved SNRs by further 6 to 9 dB, a phenomenon known as spatial release from masking. Surprisingly, experiments carried out directly in the nocturnal rainforest yielded SNRs of about −23 dB compared with those in the laboratory with the same masker, where SNRs reached only −14.5 and −16 dB in both species. Finally, a neuronal gain control mechanism enhances the contrast between the responses to signals and the masker, by inhibition of neuronal activity in interstimulus intervals.ConclusionsThus, conventional speaker playbacks in the lab apparently do not properly reconstruct the masking noise situation in a spatially realistic manner, since under real world conditions multiple sound sources are spatially distributed in space. Our results also indicate that without knowledge of the receiver properties and the spatial release mechanisms the detrimental effect of noise may be strongly overestimated.

Does acoustic adaptation drive vertical stratification? A test in a tropical cricket assemblage

In species-rich assemblages, differential utilization of vertical space can be driven by resource availability. For animals that communicate acoustically over long distances under habitat-induced constraints, access to an effective transmission channel is a valuable resource. The acoustic adaptation hypothesis suggests that habitat acoustics imposes a selective pressure that drives the evolution of both signal structure and choice of calling sites by signalers. This predicts that species-specific signals transmit best in native habitats. In this study, we have tested the hypothesis that vertical stratification of calling heights of acoustically communicating species is driven by acoustic adaptation. This was tested in an assemblage of 12 coexisting species of crickets and katydids in a tropical wet evergreen forest. We carried out transmission experiments using natural calls at different heights from the forest floor to the canopy. We measured signal degradation using 3 different measures: total attenuation, signal-to-noise ratio (SNR), and envelope distortion. Different sets of species supported the hypothesis depending on which attribute of signal degradation was examined. The hypothesis was upheld by 5 species for attenuation and by 3 species each for SNR and envelope distortion. Only 1 species of 12 provided support for the hypothesis by all 3 measures of signal degradation. The results thus provided no overall support for acoustic adaptation as a driver of vertical stratification of coexisting cricket and katydid species.

High background noise shapes selective auditory filters in a tropical cricket

SUMMARYBecause of call frequency overlap and masking interference, the airborne sound channel represents a limited resource for communication in a species-rich cricket community like the tropical rainforest. Here we studied the frequency tuning of an auditory neuron mediating phonotaxis in the rainforest cricket Paroecanthus podagrosus, suffering from strong competition, in comparison with the same homologous neuron in two species of European field crickets, where such competition does not exist. As predicted, the rainforest species exhibited a more selective tuning compared with the European counterparts. The filter reduced background nocturnal noise levels by 26 dB, compared with only 16 and 10 dB in the two European species. We also quantified the performance of the sensory filter under the different filter regimes by examining the representation of the species-specific amplitude modulation of the male calling song, when embedded in background noise. Again, the filter of the rainforest cricket performed significantly better in terms of representing this important signal parameter. The neuronal representation of the calling song pattern within receivers was maintained for a wide range of signal-to-noise ratios because of the more sharply tuned sensory system and selective attention mechanisms. Finally, the rainforest cricket also showed an almost perfect match between the filter for sensitivity and the peripheral filter for directional hearing, in contrast to its European counterparts. We discuss the consequences of these adaptations for intraspecific acoustic communication and reproductive isolation between species.