Chorthippus (Glyptobothrus) mollis (Charpentier, 1825)

Females of both species showed a significantly lower re- sponse probability to a heterospecific calling song model (C. biguttulus χ2 1 = 62.1, p < 0.001; C. mollis χ2 1 = 14.6, p < 0.001). The difference between the average response rate to the conspecific and heterospecific test stimulus was sub- stantial with 55.7% for C. biguttulus and 49.2% for C. mollis responding to the respective conspecific stimuli and with 2.4 and 6.9%, respectively, to the heterospecific test stimuli. Thus, the calling song preference of females has a strong impact on the reproductive isolation between the two species in both crossing directions (RI = 0.92 for C. biguttulus × C. mollis and RI = 0.75 for C. mollis × C. biguttulus, respectively, Table 1).

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The first barrier in the sequence of reproductive barriers was female preference for male acoustic signals. This prezygotic barrier acts first in long-range mate attraction, and females showed strong preferences for the conspecific male song models, resulting in high RI values (0.92 and 0.75, Table 1). In general, these results are in line with previous reports claiming that female preference profiles do not overlap between the two species and that acoustic signals are a strong component of reproductive isolation (von Helversen and von Helversen 1975a, b). Similar findings have also been reported for other gomphocerine species (Mayer et al. 2010; von Helversen 1997; von Helversen and von Helversen 1994). However, in many grasshopper species, the reproductive be- havior of females is characterized by phases of passive recep- tiveness, which means that female response stridulation is not essential for a successful mating (Riede 1983; Butlin and Hewitt 1986; Wirmer et al. 2010). Butlin and Hewitt (1986) argued that in dense populations, most matings under natural conditions result from chance encounters; this view is support- ed by the finding that intact and mute C. biguttulus males had the same frequency of chance encounters in a mixed popula- tion (Kriegbaum and von Helversen 1992). In these cases, acoustic signals might be less important for mate recognition and additional communication signals, for example, chemical cues or visual signals, must be involved in the prevention of heterospecific matings (Butlin et al. 1985; Ritchie 1990; Kriegbaum and von Helversen 1992; Butlin 1998). However, we considered female preferences for acoustic sig- nals as the first reproductive barrier since long-range mate attraction and localization via acoustic signaling are probably important at low population densities or for the colonization of new habitats. [1]

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