Male calling effort and mitochondrial DNA (mtDNA) variation were examined in a breeding chorus of toads from a hybrid zone between Bufo microscaphus and B. woodhousii in central Arizona. The chorus comprised 50 B. microscaphus and 17 hybrids, identified on the basis of morphology and advertisement calls; no pure B. woodhousii were observed. Males produced advertisement calls throughout the early evening, even when relatively large numbers of males (>50) were present at the chorus; active searching and satellite tactics were not observed. Calling efforts (call duration × call rate) of hybrids (23.9%, n = 8) and B. microscaphus (24.9%, n = 19) were similar and comparable to call efforts of B. woodhousii (21.9%, n = 10) from a different site. Moreover, repeatabilities of calling effort were significant (r = 0.45) for hybrid males, but not for B. microscaphus and B. woodhousii. Thus, calling behavior of hybrid males was neither significantly reduced nor more variable than that of their parental species. The distribution of mtDNA haplotypes revealed directional introgression is occurring between male B. microscaphus and female B. woodhousii: All 17 hybrids possessed B. woodhousii mtDNA. The proximate mechanism driving hybridization appears to involve common male (B. microscaphus) and rare female (B. woodhousii) matings as B. woodhousii expands its range.
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Vol. 55 • No. 3