Temporal resolution for calling song signals by female crickets, Gryllus bimaculatus

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.

Does the choosiness of female crickets change as they age?

For crickets, which approach singing males by phonotaxis, the female choosiness hypothesis postulates that young females should be more selective of male calling song patterns than older individuals. However, there is no information about the behavioural preferences of females over their complete adulthood. We analysed phonotaxis in female Gryllus bimaculatus throughout their entire adult lifetime and measured the impact of sound amplitude, carrier frequency and the temporal pattern of test songs on their auditory response. Females of all ages demonstrated their best responses to male calling songs with a pulse period of 34-42 ms, a carrier frequency of 4.5 kHz and a sound pressure level of 75 dB. The response profile to somewhat less optimal song types did vary with age, but not in a manner consistent with a simple loosening of selectiveness in older females. Age, however, had an effect on the overall strength of phonotaxis, as very old females showed an overall diminishing response to all song types. Our data suggest that although there are minor changes in the relative preferences of crickets to individual song elements as they age, the breadth of song patterns to which they will perform phonotaxis remains similar across age groups.

Computational themes of peripheral processing in the auditory pathway of insects

Hearing in insects serves to gain information in the context of mate finding, predator avoidance or host localization. For these goals, the auditory pathways of insects represent the computational substrate for object recognition and localization. Before these higher level computations can be executed in more central parts of the nervous system, the signals need to be preprocessed in the auditory periphery. Here, we review peripheral preprocessing along four computational themes rather than discussing specific physiological mechanisms: (1) control of sensitivity by adaptation, (2) recoding of amplitude modulations of an acoustic signal into a labeled-line code (3) frequency processing and (4) conditioning for binaural processing. Along these lines, we review evidence for canonical computations carried out in the peripheral auditory pathway and show that despite the vast diversity of insect hearing, signal processing is governed by common computational motifs and principles.

Critical song features for auditory pattern recognition in crickets

Many different invertebrate and vertebrate species use acoustic communication for pair formation. In the cricket Gryllus bimaculatus, females recognize their species-specific calling song and localize singing males by positive phonotaxis. The song pattern of males has a clear structure consisting of brief and regular pulses that are grouped into repetitive chirps. Information is thus present on a short and a long time scale. Here, we ask which structural features of the song critically determine the phonotactic performance. To this end we employed artificial neural networks to analyze a large body of behavioral data that measured females' phonotactic behavior under systematic variation of artificially generated song patterns. In a first step we used four non-redundant descriptive temporal features to predict the female response. The model prediction showed a high correlation with the experimental results. We used this behavioral model to explore the integration of the two different time scales. Our result suggested that only an attractive pulse structure in combination with an attractive chirp structure reliably induced phonotactic behavior to signals. In a further step we investigated all feature sets, each one consisting of a different combination of eight proposed temporal features. We identified feature sets of size two, three, and four that achieve highest prediction power by using the pulse period from the short time scale plus additional information from the long time scale.

Computational principles underlying the recognition of acoustic signals in insects

Many animals produce pulse-like signals during acoustic communication. These signals exhibit structure on two time scales: they consist of trains of pulses that are often broadcast in packets-so called chirps. Temporal parameters of the pulse and of the chirp are decisive for female preference. Despite these signals being produced by animals from many different taxa (e.g. frogs, grasshoppers, crickets, bushcrickets, flies), a general framework for their evaluation is still lacking. We propose such a framework, based on a simple and physiologically plausible model. The model consists of feature detectors, whose time-varying output is averaged over the signal and then linearly combined to yield the behavioral preference. We fitted this model to large data sets collected in two species of crickets and found that Gabor filters--known from visual and auditory physiology--explain the preference functions in these two species very well. We further explored the properties of Gabor filters and found a systematic relationship between parameters of the filters and the shape of preference functions. Although these Gabor filters were relatively short, they were also able to explain aspects of the preference for signal parameters on the longer time scale due to the integration step in our model. Our framework explains a wide range of phenomena associated with female preference for a widespread class of signals in an intuitive and physiologically plausible fashion. This approach thus constitutes a valuable tool to understand the functioning and evolution of communication systems in many species.

Selective phonotaxis to high sound-pulse rate in the cricket Gryllus assimilis

Calling song of the cricket Gryllus assimilis is unusual among Gryllus spp. in the high sound-pulse rate, ca. 80 Hz, within its chirps. We asked whether, as in other cricket species, females were able to analyze such a high pulse rate. In phonotaxis experiments, females failed to respond to stimuli with pulse rates substantially higher or lower than the species-typical value, demonstrating that they are indeed selective for this parameter. We also examined how pulse rate was represented by modulation in firing rate of the neuron AN1, the main carrier of information about cricket-song-like stimuli to the brain. For attractive stimuli, i.e. with high pulse rates, modulation of AN1 firing rate through time was surprisingly modest. This suggests that the brain circuits that analyze AN1 spike trains might be more sensitive to slight variations in AN1 firing rate than their counterparts in more slowly singing species.

The Steppengrille (Gryllus spec./assimilis): selective filters and signal mismatch on two time scales

In Europe, several species of crickets are available commercially as pet food. Here we investigated the calling song and phonotactic selectivity for sound patterns on the short and long time scales for one such a cricket, Gryllus spec., available as “Gryllus assimilis”, the Steppengrille, originally from Ecuador. The calling song consisted of short chirps (2–3 pulses, carrier frequency: 5.0 kHz) emitted with a pulse period of 30.2 ms and chirp rate of 0.43 per second. Females exhibited high selectivity on both time scales. The preference for pulse period peaked at 33 ms which was higher then the pulse period produced by males. Two consecutive pulses per chirp at the correct pulse period were already sufficient for positive phonotaxis. The preference for the chirp pattern was limited by selectivity for small chirp duty cycles and for chirp periods between 200 ms and 500 ms. The long chirp period of the songs of males was unattractive to females. On both time scales a mismatch between the song signal of the males and the preference of females was observed. The variability of song parameters as quantified by the coefficient of variation was below 50% for all temporal measures. Hence, there was not a strong indication for directional selection on song parameters by females which could account for the observed mismatch. The divergence of the chirp period and female preference may originate from a founder effect, when the Steppengrille was cultured. Alternatively the mismatch was a result of selection pressures exerted by commercial breeders on low singing activity, to satisfy customers with softly singing crickets. In the latter case the prominent divergence between male song and female preference was the result of domestication and may serve as an example of rapid evolution of song traits in acoustic communication systems.

Calling song signals and temporal preference functions in the cricket Teleogryllus leo

The acoustic display of many cricket species consists of trains of pulses (chirps) with intermittent pauses. Here, we investigated the temporal cues that females of the cricket Teleogryllus leo used to detect a pulse and a chirp pattern on two different time scales. For both patterns, females accepted a wide range of combinations that covered the respective pulse and chirp parameters in the songs of males. In tests with a continuous series of pulses at different modulation frequencies, the transfer function of pattern discrimination was also determined. Females exhibited two ranges of high response scores indicating two temporal filters with an inhibitory interaction. For the modulation frequency of the pulse pattern, the peak of the preference function was rather sharply tuned and at a lower pulse rate than produced by males. These results show that the combined output of both filters did not increase selectivity, but rather enlarged the accepted range of signals.

Auditory processing at two time scales by the cricket Gryllus bimaculatus

The acoustic display of many cricket species consists of series of pulses grouped into chirps, and thus information is distributed over both short and long time scales. Here we investigated the temporal cues that females of the cricket Gryllus bimaculatus used to detect a chirp pattern on a longer time scale than the fast pulse pattern. First, over a range of chirp and pause durations (100-400 ms), the duty cycle of the chirp pattern emerged as the most important cue for detection. The songs of males showed a distribution at lower duty cycles than preferred by females. The duty cycle also limited the responses of females at very short durations and pauses (below 80 ms). Second, by systematic variation of pulse and chirp periods of stimuli, an intermediate response field emerged that revealed the best responses of female crickets to patterns with amplitude modulations on both short and long time scales. On average, females also responded weakly to stimuli that contained amplitude modulations of only one time scale. Third, test patterns were constructed by addition of modulation frequencies rather than rectangular pulses. These tests showed that female crickets processed the chirp pattern in the time domain and tolerated noise levels up to a modulation depth of 50%. The combined evidence from all three approaches indicated inhibitory effects of unattractive patterns on both time scales. The fusion of short and long time scales during auditory processing by female crickets corresponded to a weighted AND-like operation of two processing modules, the pulse and the chirp filter.