Intracellular activity in cricket neurons during generation of song patterns

Summary During production of song patterns by the semi-isolated CNS of Gryllus campestris, intracellullar recordings were made in fibers of the mesothoracic ganglion, including synaptic areas of identified wing opener and closer motor neurons. The normal calling song pattern and some transitional songs toward courtship and toward aggression were generated by the CNS in the absence of any phasic sensory timing (Figs. 1, 4). Intracellular activity of the opener motor neurons was characterized by an absence of events in the interchirp interval, an EPSP underlying each burst, and an IPSP following the burst if the closer motor neurons were to be activated (Fig. 1). Intracellular activity of the closer motor neurons was characterized by an absence of events in the interchirp interval, an IPSP immediately following the onset of the opener motor neuron burst, and an EPSP after the IPSP (Figs. 2, 3). Units were found which fired in a burst during the period when both the opener and closer motor neurons were inhibited (Fig. 5). Complementary sets of units were found which displayed an oscillation of activity at the chirp rhythm but not at the pulse rhythm (Fig. 6). Gaps in the calling song were observed whose characteristics indicated that motor neuron activity was neither required for, nor effective in, resetting the chirp timing oscillator (Fig. 8). A possible model for the song generating mechanism is outlined.     

The importance of calling song and courtship song in female mate choice in the variable field cricket

Male field crickets produce calling songs, courtship songs, tactile signals and chemical signals. Although calling songs are known to play an important role in female mate choice, the importance of the other signals in mate choice is poorly understood. In the variable field cricket, Gryllus lineaticeps, females select mates, in part, based on variation in male calling song. Females prefer higher chirp rates, a trait which is partially dependent on male nutrient intake, and females prefer longer chirp durations, a trait which appears to be independent of male nutrient intake. We tested whether females also have preferences based on variation in male courtship song, and whether the structure of male courtship song varies with nutrient intake. First, we reexamined female preference for calling song chirp rate. Then, we examined: (1) female preference based on courtship song chirp rate; (2) the relative importance of calling song and courtship song chirp rate; (3) the nutrition dependence of courtship song chirp rate; and (4) the correlation between calling song and courtship song chirp rate. As reported previously, females preferred higher calling song chirp rates, and in addition, preferred higher courtship song chirp rates. Females were more likely to switch from a speaker broadcasting more attractive calling song to a speaker broadcasting less attractive calling song when the attractive calling song was associated with an unattractive courtship song than when it was associated with an attractive courtship song. Preferences based on courtship song may thus cause females to alter the choices that they made based on calling song. Males that received greater nutrients did not produce higher courtship song chirp rates. There was no correlation between calling song and courtship song chirp rate. As a result, the two traits may provide information to females about different aspects of male quality. Copyright 2000 The Association for the Study of Animal Behaviour.     

Acoustic communication in Okanagana rimosa (Say) (Homoptera: Cicadidae)

The cicada Okanagana rimosa (Say) has an acoustic communication system with three types of loud timbal sounds: (i) A calling song lasting several seconds to about 1 min which consists of a sequence of chirps at a repetition rate of 83 chirps per second. Each chirp of about 6 ms duration contains 4-5 pulses. The sound level of the calling song is 87-90 dB SPL at a distance of 15 cm. (ii) An amplitude modulated courtship song with increasing amplitude and repetition rate of chirps and pulses. (iii) A protest squawk with irregular chirp and pulse structure. The spectra of all three types are similar and show main energy peaks at 8-10 kHz. Only males sing, and calling song production is influenced by the songs of other males, resulting in an almost continuous sound in dense populations. In such populations, the calling songs overlap and the temporal structure of individual songs is obscured within the habitat. The calling song of the broadly sympatric, closely related species O. canadensis (Provander) is similar in frequency content, but distinct in the temporal pattern (24 chirps per second, 24 ms chirp duration, eight pulses per chirp) which is likely important for species separation in sympatric populations. The hearing threshold of the auditory nerve is similar for females and males of O. rimosa and most sensitive at 4-5 kHz. Experiments in the field show that female phonotaxis of O. rimosa depends on parameters of the calling song. Most females are attracted to calling song models with a 9 kHz carrier frequency (peak frequency of the calling song), but not to models with a 5 kHz carrier frequency (minimum hearing threshold). Phonotaxis depends on temporal parameters of the conspecific song, especially chirp repetition rate. Calling song production is influenced by environmental factors, and likelihood to sing increases with temperature and brightness of the sky. Correspondingly, females perform phonotaxis most often during sunny conditions with temperatures above 22 degrees C. Non-mated and mated females are attracted by the acoustic signals, and the percentage of mated females performing phonotaxis increases during the season.     

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.     

Plurisegmental neurones of the cricket Gryllus campestris L. that discharge in the rhythm of its own song

Male crickets (Gryllus campestris L.) mounted so that their wings and abdomen could move freely were induced to stridulate by brain lesion. During the song the activity of single neurones was recorded extracellularly in a cervical connective. Nine distinct spike patterns were observed. Patterns I and II tend to copy the chirp as a whole rather than the onset of the syllables (the recorded potentials of the wing-opener muscle M99 marked the syllable onset). The other patterns reflect the syllabic structure. Each, in its own way, marks the various syllables with different numbers of spikes. The delay of the spike response is different for each pattern. Some patterns, but not others, also reflect the beginning or end of the song, or the abdominal expiratory activity. One neurone also responds in correlation with muscle discharges typical of the courtship song. In some of the patterns it is evident that there is a stronger correlation with the closer muscle (M90) discharge than with the opener muscle discharge. Activation by auditory self-stimulation by way of the tympanal organs can be ruled out for all patterns. It is possible that patterns I–V are induced by afferent activity coupled to the wing movement. Patterns VI–VIII are probably copies of motor signals ascending from the thoracic song-pattern generators to the head ganglia. It is evident that the head ganglia have detailed information as to the motor output for stridulation and abdominal expiration.     

Brain projections and information processing of biologically significant sounds by two large ventral-cord neurons ofGryllus bimaculatus DeGeer (Orthoptera,Gryllidae)

Summary Two ventral-cord neurons in the auditory system ofGryllus bimaculatus were studied electrophysiologically by stimulation with pulses of sound at a single frequency (sine-wave pulses), stridulatory songs, and artificial sounds constructed to imitate the conspecific songs. The sine-wave pulses were varied in frequency, sound intensity, duration, and repetition rate. The stridulatory songs were the conspecific calling, aggressive, and courtship songs and the calling songs of 8 sympatric gryllids (played back at different sound intensities). The artificial songs were varied in carrier frequency, pulse rate, chirp rate, and sound intensity.The LF₁ neuron precisely duplicates the temporal structure of the conspecific calling (and aggressive) song over the whole intensity range (Figs. 7, 8, 10). It is sharply tuned to the carrier frequency of the song (5 kHz) and shows little or no response above 10 kHz and below 3 kHz (Figs. 1, 2). By variation of the calling song's temporal structure it can be demonstrated that the LF₁ neuron is particularly suited to respond to the pulse duration and the pulse and chirp repetition rates of this song pattern (Figs. 6, 9).On the other hand, the HF₁ neuron is a broad-band neuron with a maximal sensitivity at 16 kHz (Figs. 1, 4); it is tuned to the conspecific courtship song with respect to carrier frequency, the short pulse duration, and the very low pulse repetition rate (Figs. 6, 7, 8).The results demonstrate that the two ventral-cord neurons represent highly evolved channels of the auditory pathway in gryllids, each of which transmits important features of the corresponding conspecific songs to several areas of the brain (Fig. 11). But they are not ideal filters for these conspecific songs, since they also respond to many other sound signals (Fig. 10).Supported by the Deutsche Forschungsgemeinschaft as part of the program Sonderforschungsbereich 114 (Bionach), BochumUnder the auspices of the scientist exchange program of the Deutsche Forschungsgemeinschaft and the Academy of Sciences, USSRWe thank Prof. Dr. Schwartzkopff for his help and support; it was due to his initiative and organization that this work could be done in collaboration between the Sechenov Institute, Leningrad, and the Lehrstuhl für Allgemeine Zoologie, Ruhr University, Bochum. We are grateful to Mrs. I. Klotz and Mrs. B. Brücher for technical assistance.     

Responses to features of the calling song by ascending auditory interneurones in the cricket Gryllus campestris

ABSTRACT. In female Gryllus campestris L., three functional types of ascending auditory intemeurones have been studied by recording from them extracellularly in the split cervical connectives using suction electrodes. Type 1 neurones are characterized by an optimal sensitivity to the carrier frequency of the species calling song (4–5 kHz). They copy the syllable and pause structure of the call at all intensities. The patterned spike discharge is observable at least 8 dB above absolute threshold. With suprathreshold stimulation, the neurones exhibit maximal responses (number of spikes/chirp) around the carrier frequency. The intensity response curves are approximately linear in the range of 40–90 dB SPL. The envelope of each syllable is reflected by a corresponding change in the firing rate, and syllable periods of 24ms and longer are resolved. This type can be considered as a neural correlate for phonotactic behaviour of the female where the syllable period has been found to be the most important temporal parameter. Type 2 neurones are most sensitive in the range of 4–6 and 11–13 kHz. They copy the syllable and pause structure of the species calling song at low and moderate intensities. However, the spikes invade the intersyllable pauses, when stimulated with the calling song at higher intensities (above 85 dB). This is particularly apparent at the onset of a chirp series. The slope of the intensity—response curve mimics that of type 1 units. The neurones cannot follow syllable periods shorter than 32 ms. Type 3 neurones differ from types 1 and 2 by a rather broad-band sensitivity in the range of 3–16 kHz, and in copying the chirp as a whole. Even at low stimulus intensities, the intersyllable pauses are filled with spikes, and information about the syllable—pause structure is lost. Stimulation with suprathreshold intensities gives rise to a rather uniform, broad-band response without distinctive peaks. The intensity—response curve is characterized by a higher absolute threshold, and by the reduction in the response magnitude starting above 70–80 dB. These units are not suitable for copying the calling song temporal structure in detail, but would indicate the chirping rhythm. Their strong response in the range of the species courtship song carrier frequency make them suitable to copy the courtship song.     

北方两种常见蝈螽鸣声与发声器的比较研究

研究了北方常见的优雅蝈螽Gampsocleis gratiosa和暗褐蝈螽Campsocleis sedakovii雄性鸣声特征和发声器结构.优雅蝈螽鸣声规则,脉冲组序列由2种类型的脉冲组组成,第1种类型的脉冲组持续时间约0.09 s,脉冲持续和间隔时问约0.01 5;第2类型的脉冲组持续时间约0,04 s,脉冲持续和间隔时间均约0.003 s;鸣声的主能峰频率约7 kHz.暗褐蝈螽雄性鸣声包含短促的开翅鸣声和由2种类型的脉冲组组成的脉冲组序列构成的闭翅鸣声,第1种脉冲组持续时间约0.012 s,间隔时间约0.002 s;第2种脉冲组持续时间约0.013 s,间隔时间极短;鸣声主能峰频率约9.1kHz.2种蝈螽镜膜的形状、发声锉的形状和长度、发声齿的形状具显著差异.     

Temporal pattern changes in the calling song of the katydidMygalopsis marki Bailey in response to conspecific song (Orthoptera: Tettigoniidae)

Males of Mygalopsis markiBailey (Tettigoniidae: Orthoptera) alter the temporal structure of their song in response to other competing males. The song of males calling in aggregations has a high variance in the number of syllables per chirp, with short intervals between each chirp. In contrast, the temporal pattern of the song of isolated males is more evenly spaced, with an increase in length of the interchirp intervals and low variance in the number of syllables per chirp. In order to simulate a calling male moving closer to a male in an aggregation, a playback technique was adopted whereby the recorded calling song of a male was presented to itself via a loudspeaker in increments of 2dB. The change in song pattern of the resident male involved a reduction in the number of syllables per chirp and an increase in the interchirp interval, with the number of chirps per second remaining constant. This reduction in the output of the song, instead of not calling as a result of an acoustic contest, may still allow males to continue calling for females.     

The structure and function of songs emitted by southern green stink bugs from Brazil, Florida, Italy and Slovenia

Nezara viridula (L.) (Pentatomidae: Heteroptera) from Brazil, Florida, Italy and Slovenia, communicate by vibratory songs associated with long‐range calling and close‐range courting, rivalry and repelling. Each song is composed of spectrally and temporally different units. Spectrally different pulses of duration less than 300 ms are present in the male calling song. The female calling song is characterized by pulse trains composed of pulses shorter than 150 ms and pulse trains composed of a longer (> 700 ms) and shorter (< 250 ms) pulse. Shorter and longer pulses have different spectral characteristics. The male and female courtship songs are characterized by fusion of shorter (< 150 ms) pulses into a pulse train usually followed by a shorter (< 200 ms) postpulse in the case of the male courtship song. The female repelling song is a several seconds long vibration of irregular temporal structure. The short (< 400 ms) male rival song pulses are frequency modulated. The dominant frequency peaks of the songs investigated lie between 70 and 130 Hz. The dominant frequency and the microstructure of song spectra show no population specificity. The average duration varies more in calling than in courtship songs. The repetition time varies extensively in songs of different populations. Normal communication followed by copulation was observed between mates from Slovenia and Brazil and between mates from Florida and Italy. The potential role of different temporal and spectral parameters for species recognition and mate location is discussed in view of the expected distortion of the characteristic signal structure during transmission through plants.     

Pharmacological Activation of Stridulation in the Grasshopper Chorthippus albomarginatus (Orthoptera: Gomphocerinae)

Calling and courtship stridulatory behavior of Chorthippus albomarginatus was induced by injections of acetylcholine agonists into the protocerebrum. Pharmacologically induced stridulation, in many parameters, was quite similar to the natural behavior. However, the order of the courtship element alternation was different from that of the natural song. In some cases the pharmacologically induced stridulation included only one or two courtship elements. Based on the exclusive stimulation of a particular element of courtship songs and similarity of its movement pattern with the calling song, both patterns of Ch. albomarginatus stridulation appear to be homologous. The results obtained on this species confirm the idea of a hierarchic organization of the central nervous control of stridulation in gomphocerine grasshoppers and indicate participation of certain protocerebrum structures in this control.     

The unusual inheritance pattern of the courtship songs in closely related grasshopper species of the Chorthippus albomarginatus-group (Orthoptera: Gomphocerinae)

Sibling species of the Chorthippus albomarginatus-group are exceptional among all European grasshopper species because they have an extremely elaborate courtship behaviour. Here, we present a genetic analysis of the courtship song differences between two closely related grasshopper species Ch. albomarginatus and Ch. oschei. Measurements of seven courtship song characters and one parameter of courtship visual display were compared among parentals, F(1) and F(2) hybrids, and backcrosses. Variation in one morphological character, the number of stridulatory pegs, was studied as well. The distributions of song phenotypes were more consistent with a type-III genetic architecture, which involves complementary or duplicate pairs of loci. We suggest that this type of architecture may provide the novel elements of the hybrid songs, even more complex than the parental ones, which may offer a new starting point for sexual selection. One parameter of the visual display appeared to be controlled by a single locus, but differences in other characters of visual display could be explained by a more complex inheritance pattern. Inheritance of the peg number differences was consistent with a simple polygenic additive model. Sex linkage and/or maternal effects were not detected for any trait. We found a disproportionate influence of the Ch. albomarginatus parent on most hybrid song characteristics and the visual display, and this may support a previously suggested idea that Ch. albomarginatus is an ancestral form and Ch. oschei is a derived species.     

Auditory nerve and interneurone responses to natural sounds in several species of cicadas

ABSTRACT. The calling and courtship songs of 17-year cicadas and of Say's cicadas differ both in the sound frequency spectrum and in temporal pattern. Multiunit recordings with hook electrodes from the whole auditory nerve show that the hearing organs are especially sensitive to transient stimuli occurring in natural sounds. Artificially produced clicks elicit bursts of spikes synchronized among various primary sensory fibres. These fibres respond to natural calling and courtship songs with a specificity dependent on carrier frequency, rhythm and transient content of the sound, following sound pulses (i.e. tymbal actions) up to repetition rates of 200 Hz. An ascending, plurisegmental interneurone was characterized by intracellular recording and simultaneously stained with cobalt. Its main arborization spatially overlaps the anterior part of the sensory auditory neuropile, and the axon was traced as far as the prothoracic ganglion. Direct input from primary auditory fibres was suggested by latency measurements. Intracellular recordings from such neurons in different species show distinct auditory input, with phasic-tonic spike responses to tones. In general, the interneurone response is more species-specific to calling than to courtship songs, and the preferential response to the conspecific calling song is based primarily upon sound frequency content.     

Stridulatory pattern generation in acridid grasshoppers: metathoracic interneurons in Stenobothrus rubicundus (Germar 1817)

Stridulation was elicited in tethered gomphocerine grasshoppers of the species Stenobothrus rubicundus in order to identify interneurons of the stridulation pattern generator, and describe their morphological and physiological properties. Nine types of such neurons could be found and characterized; eight of those could additionally be compared to corresponding neuron types previously known from other species. As shown in detail for one selected type, the neurons of the stridulation pattern generator are very similar in their anatomical appearance, and possess similar physiological qualities at least in two species with similar stridulation patterns. Stridulation interneurons of species with largely different stridulatory motor patterns have a similar morphology, but show a different activation timing throughout the stridulation. Nevertheless, special properties such as resetting or initiation capability of certain stridulation interneurons seem to be conserved throughout the species. The results suggest that the stridulation pattern generator of different species consists of a uniform set of interneurons that change their activity pattern to produce species-specific song movements.     

Identified descending brain neurons control different stridulatory motor patterns in an acridid grasshopper

During courtship sequences male grasshoppers of the species Omocestus viridulus successively perform with their hindlegs three different stridulatory movement patterns: ordinary stridulation, hindleg shaking and precopulatory movements. Microinjection of acetylcholine into protocerebral neuropil regions can either elicit complete courtship sequences or evoke one of the three motor patterns. Intracellular recordings and stainings revealed three types of descending brain neurons: B-DC-3, B-DC-4 and B-DC-5. All three types of interneurons have a medial axon position in the connectives. They cross the midline of the protocerebrum and exhibit a profuse arborization pattern within the medial dorsal protocerebral neuropil. Stimulation of each type of interneuron specifically elicits one particular motor pattern of courtship behaviour. Courtship of the grasshopper O. viridulus may therefore be controlled by successive activation of these descending brain neurons. Accepted: 27 September 1996     

Neuromuscular coordination and proprioceptive control of rhythmical abdominal ventilation in intactLocusta migratoria migratorioides

Summary Within the fifth abdominal segment of intact locusts a group of dorso-ventral expiratory muscles and one inspiratory antagonist display alternating ventilatory patterns of three basic types. Accelerated movements in the dorso-ventral plane are supported by isometric activity of the intersegmental muscles which prevent extensions in the longitudinal axis.The intersegmental coupling of ventilatory motor patterns is strict during strong ventilation and loose and more metachronal with weaker pumping movements.In resting animals ventilatory rhythms are discontinuous and the long intervening pauses are interrupted by miniature inspirations only. Pumping series have a tendency to prolong the later ventilatory cycles, and interfering rhythms of different pumping types occur. Low concentrations of atmospheric CO₂ up to 3 % do not accelerate ventilatory rhythms.Afferent activity from proprioceptors could be related to ventilatory motor bursts and stimulation of the sensory nerve produces inspiratory bursts via the segmental ganglion.The neuronal mechanisms of synergistic and antagonistic muscle control as well as the segmental and intersegmental coordination and the effect of autonomous ganglionic oscillators in ventilation are discussed.     

Filtering of temporal parameters of the calling song by cricket females of two closely related species: a behavioral analysis

The phonotactic response of cricket females was investigated on a locomotion compensator to determine the temporal parameters of the male's calling song which are important for species recognition. Two sympatric species (Teleogryllus commodus, T. oceanicus) that show different syllable periods in the chirp and trill parts of their calling songs were used. By their responses T. commodus females exhibited two temporal filters for syllable periods, which were tuned to the species-specific syllable periods occurring during chirp and trill. For song recognition both filters had to be activated and for both a minimum number of three to five consecutive syllable periods was necessary. In contrast, T. oceanicus females showed only one sharply tuned filter corresponding to the chirp part of the male's calling song. This filter was sufficient for calling song recognition. Syllable periods of the trill part also influenced calling song recognition, but these played only a minor role. Carrier frequency was also important for positive phonotaxis. Calling song recognition by T. commodus females is largely based on central nervous processing, while for T. oceanicus both peripheral frequency filtering and central temporal filtering is important. Accepted: 17 January 1997     

Phonotaxis in flying crickets

The steering responses of three species of field crickets, Teleogryllus oceanicus, T. commodus, and Gryllus bimaculatus, were characterized during tethered flight using single tone-pulses (rather than model calling song) presented at carrier frequencies from 3-100 kHz. This range of frequencies encompasses the natural songs of crickets (4-20 kHz, Fig. 1) as well as the echolocation cries of insectivorous bats (12-100 kHz). The single-pulse stimulus paradigm was necessary to assess the aversive nature of high carrier frequencies without introducing complications due to the attractive properties of repeated pulse stimuli such as model calling songs. Unlike the natural calling song, single tone-pulses were not attractive and did not elicit positive phonotactic steering even when presented at the calling song carrier frequency (Figs. 2, 3, and 9). In addition to temporal pattern, phonotactic steering was sensitive to carrier frequency as well as sound intensity. Three discrete flight steering behaviors positive phonotaxis, negative phonotaxis and evasion, were elicited by appropriate combinations of frequency, temporal pattern and sound intensity (Fig. 12). Positive phonotactic steering required a model calling song temporal pattern, was tuned to 5 kHz and was restricted to frequencies below 9 kHz. Negative phonotactic steering, similar to the 'early warning' bat-avoidance behavior of moths, was produced by low intensity (55 dB SPL) tone-pulses at frequencies between 12 and 100 kHz (Figs. 2, 3, and 9). In contrast to model calling song, single tone-pulses of high intensity 5-10 kHz elicited negative phonotactic steering; low intensity ultrasound (20-100 kHz) produced only negative phonotactic steering, regardless of pulse repetition pattern. 'Evasive', side-to-side steering, similar to the 'last-chance' bat-evasion behavior of moths was produced in response to high intensity (greater than 90 dB) ultrasound (20-100 kHz). Since the demonstration of negative phonotactic steering did not require the use of a calling song temporal pattern, avoidance of ultrasound cannot be the result of systematic errors in localizing an inherently attractive stimulus when presented at high carrier frequencies. Unlike attraction to model calling song, the ultrasound-mediated steering responses were of short latency (25-35 ms) and were produced in an open loop manner (Fig. 4), both properties of escape behaviors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Divergence between the Courtship Songs of the Field Crickets Gryllus texensis and Gryllus rubens (Orthoptera, Gryllidae)

Acoustic mating signals are often important as both interspecific prezygotic isolating mechanisms and as sexually selected traits in intraspecific mate choice. Here, we investigate the potential for cricket courtship song to act as an isolating mechanism by assessing divergence between the courtship songs of Gryllus texensis and Gryllus rubens , two broadly sympatric cryptic sister species of field crickets with strong prezygotic isolation via the calling song and little or no postzygotic isolation. We found significant species-level differences in the courtship song, but the song has not diverged to the same extent as the calling song, and considerable overlap remains between these two species. Only two related courtship song characters are sufficiently distinct to play a possible role in prezygotic species isolation.