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Responses of Sympatric Acridid Species to Natural and Artificial Sound Signals
Abstract
In behavioral experiments females of acridid species Chorthippus biguttulus, Ch.parallelus, Ch.dorsatus responded not only to conspecific but to heterospecific songs as well. The response frequency to conspecific songs ranged from 75 to 96%, to the heterospecific songs — from 15 to 38%. The investigation of female responses to artificial songs showed, that in order to recognize the conspecific song Ch.parallelus females use echeme structure (the presence of discrete impulses) and amplitude modulation of the echeme; in Ch.dorsatus ffemales, the presence of two parts in the song as well as subdivision of the first part into the echemes with separate pulses are important for recognition. It is supposed that Chorthippus females do not use one single key characteristic, but employ the whole complex of temporal parameters.
... Meaningful differences among crosses are confined to the temporal rather than spectral features of songs. This finding is consistent with the more general observation that timing is more important than pitch in determining mate choice among animals with acoustical signals (Pollack, 1979; Doherty, 1985; Schwartz, 1987; Doolan & Young, 1989; Vedenina, 1990; Allan & Simmons, 1994; Ritchie & Gleason, 1995), and it confirms the results of earlier studies on behavioral choice in green lacewings (Wells & Henry, 1992a). Temporal features are also highly correlated with one another (Table 1; correlations not shown), suggesting epistasis and pleiotropy in gene expression. ...
Speciation often involves incremental responses to natural selection and results in large scale genomic changes, but it may also occur abruptly and with little genetic imprint, as seen in some complexes of cryptic species. Recent attention has focused on sexual selection in rapid speciation, because it can disrupt premating signals that mediate reproductive isolation. Some models require that environmental adaptation assist sexual selection during speciation, while others show that populations can diverge through mate choice alone. We propose that speciation involving environmental adaptation in premating signals is likely to have a polygenic basis, while speciation due to arbitrary changes in premating signals could be accompanied by changes at just a few loci. The sibling species Chrysoperla
plorabunda and Chrysoperla johnsoni belong to a large complex of cryptic species of green lacewings, and meet all criteria for speciation via sexual selection. We perform a genetic analysis of line crosses between them, focusing on their substrate-borne premating songs. Measurements of seven song features and their principal components are compared among parentals, F1 and F2 hybrids, and backcrosses. The distributions of phenotypes are consistent with a model of more than one, but not many, genes. Sex linkage and/or maternal effects are negligible. C. plorabunda is dominant for most traits affecting mate choice. Bayesian analyses of segregation variance show significant additive and epistatic effects on line means. A Bayesian Castle-Wright estimate suggests that relatively few effective factors are responsible for variation in volley period (x = two factors), number of volleys per song (x = 1), and PC-1 (x = 4). Our results are consistent with simple genetic architecture of songs, supporting a role for major genes in premating isolation and strengthening the notion that mate choice alone, without significant environmental adaptation, has been responsible for generating new lacewing species.
The courtship song of the P1 morph of the Chrysoperla plorabunda complex consists of a volley of tremulation about 600 ms long and modulated in frequency from 77 to 40 Hz. It is repeated every second, and a female responds to a male with a volley of her own after each of the latter's volleys. Synthetic songs were used in playback experiments to determine what song features this song morph uses for species recognition during courtship and mating. Females responded to songs that had higher or lower frequencies than their own songs when all other features were held constant. Females did not answer to random noise or to single frequency songs. They gave some answers to songs consisting of partial frequencies, but gave the most responses to frequency-modulated songs with the full range of frequencies found in the normal song. Females responded to frequency-modulated songs even if the song was only half the standard duration, or if the direction of frequency modulation was reversed. They did not respond to songs in which the interval was shorter than the normal song. Hence, frequency modulated sound appears to be necessary to elicit a response, but temporal features of song affect responses of females as well.
1.
Computer generated, artificial songs (Fig. 1) were played to grasshoppers of the gomphocerine speciesOmocestus viridulus L. in a temperature-regulated, echo-reduced chamber (Fig. 2). The animals often responded by singing their response song, and the occurrence of response songs was used to indicate the animals' preference for the artificial songs. At various temperatures, the chirp duration and impulse rate preferred by the animals were determined and compared with the magnitudes of these parameters in the response songs.
2.
The duration of the response songs decreases with temperature (Fig. 5), and so does the chirp duration (Fig. 3). The chirp rate (which in this species is the reciprocal of the chirp duration) decreases during a response song (Fig. 6).
3.
In the first experiments the chirp duration was varied in the artificial songs, while the ratio chirp rate to impulse rate was kept constant (as in the natural songs). The preferred chirp duration changes more with temperature than does the chirp duration of the response songs (Fig. 3). At all temperatures, songs with chirp durations half or double of the preferred one could also release a response song. The animals even responded to chirp durations outside the natural range (Fig. 3).
4.
In the second experiments, the chirp rate (C) and impulse rate (I) at 28 C (C28 and I28) and 38 C were combined into four different songs: C28I28, C38I38, C28I38 and C38I28. The former two songs correspond to the natural songs at 28 and 38 C, whereas the latter ones are artificial. The temperature of the listening animals was also varied. The choices made by femal grasshoppers showed that the chirp rate is the major parameter and the impulse rate the minor parameter in determining the acceptance of a song.
5.
Four different criteria for measuring the female song discrimination were compared: (a) the behavioural threshold (i.e. the sound level of an artificial song which is just sufficient for response songs to be released in the females), (b) response or no response, (c) the number of response songs in a response, and (d) whether the response was given to the 1st, 2nd or 3rd song presentation. (a) was more sensitive than (b), and both gave rather similar results. In (c) and (d) the responses appeared to be independent of the type of songs presented.
6.
The behavioural threshold increased before and decreased after egg deposition (Fig. 7).
Many acridid species have evolved a means of auditory communication, the chief function of which is to bring sexual partners together, and thereby to ensure the genetic isolation of the species. Both males and females produce the acoustic signals, by rubbing a file with many small teeth on the inside of the hindlegs against prominent veins on the elytra. The songs generated in this way in many cases have a highly developed, complex pattern, as described by Faber (1929, 1953), Jacobs (1953) and Elsner (1974).
Song recognition in sympatric acridid grasshoppers
- V Vedenina
- Yu
- R D Zhantiev
Bioacoustics of insects
- R D Zhantiev