The effect of sex itself was also highly significant ( F 1,515 = 181.90, p < 0.001): females called at higher frequencies than males (females: 113.72 ± 1.84 kHz; males: 111.77 ± 1.85 kHz), although there was no evidence of a silent band that could facilitate sex recognition.
A plot of call frequency difference against pairwise genetic distance also showed a positive trend ( r 2 = 0.133, d.f. = 90, p < 0.01) ( figure 3 b ); however, this was not significant after correcting for the effect of geographical distance (partial Mantel test, r 2 = 0.057, P = 0.094).
In contrast, comparisons between populations 17–20 (from the south and southwest) and the more northerly populations showed a negative trend.