Sexual selection often involves female preference for males of a certain age, and a body of theory predicts preference for old males. We measured a comprehensive set of traits from the acoustic sexual display of male field crickets, Gryllus bimaculatus, and found that nearly all song traits changed predictably as males aged, involving a general slowing down of the wing movements during song production. Our female preference experiments indicated a strong and repeatable preference for the songs of young males, contradicting the existing literature, which argues that female crickets prefer older males on the basis of changes in song carrier frequency. Rather, female preference for young male song was determined by its high energetic quality. We develop the 'old flight muscle' hypothesis, arguing that age-related degradation of stridulatory muscle performance is likely to result in the observed changes with age. Secondary sexual characters may be subject to oxidative somatic degradation suggesting that, when males provide only sperm, females should prefer the sexual displays of young males. Our results support new modelling approaches and a growing body of empirical evidence suggesting that old males are not always preferred by females. Ó

For Gryllus bimaculatus (Orthoptera: Gryllidae), a comparison was made of the prediction from species recognition theory that stabilizing selection acts on mate recognition traits and that of sexual selection that predicts diversifying selection on mate recognition traits. The calling songs and morphometrics of four populations of ®eld crickets from South Africa were compared. Song characteristics generally showed less between-population variation than did morphometric variation. This applied in particular to the song characteristics known to be important in species recognition. Within-population variation of species recognition traits were signi®cantly smaller than for other call traits as well as morphometric traits. This is consistent with the expectation that stabilizing selection occurs for species recognition traits within populations as well as throughout their geographical ranges. No song traits correlated signi®cantly with body size. Our study suggests that the calling song traits involved in mate recognition lack the variation required for sexual selection to operate. However, the amount of variation inherent in some of the other song parameters of ®eld crickets still has the potential to give rise to sexual selection based on traits other than body size.

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We investigate the degree of between-population genetic differentiation in the Mediterranean field cricket Gryllus bimaculatus, as well as the possible causes of such differentiation. Using cytochrome b mtDNA sequences, we estimate genetic variation in G. bimaculatus from seven South African and two Mediterranean populations. Within-population genetic variation in Europe (two haplotypes, one unique to a single individual) suggest low effective population size and strong bottlenecks with associated founder effects, probably due to cold winter environments in Europe that limit reproduction to a short part of the summer. The likely cause for this is the daily maxima in winter temperatures that fall below the critical level of 16 C (enabling normal calling and courtship behaviour) in Mediterranean Europe, whereas the equivalent temperatures in southern Africa are above this limit and enable reproduction over a large part of the year. European genetic variants were either shared with Africa or closely related to African haplotypes. For survival, European populations are probably dependent on immigration from other areas, including Africa. South African populations have low but measurable gene flow with Europe and show significant between-population genetic differentiation (30 haplotypes). Isolation-by-distance is not sufficient to explain the degree of between-population genetic differences observed, and a large degree of dispersal is also required in order to account for the observed patterns. Differences in morphology and calling behaviour among these populations are underlied by these genetic differences.