Calling behavior and directional hybridization between two toads (Bufo microscaphus x B-woodhousii) in Arizona

Male calling effort and mitochondrial DNA (mtDNA) variation were examined i n a breeding chorus of roads from a hybrid zone between Bufo microscaphus a nd B. woodhousii in central Arizona. The chorus comprised 50 B. microscaphu s and 17 hybrids, identified on the basis of morphology and advertisement c alls; no pure B. woodhousii were observed. Males produced advertisement cal ls throughout the early evening, even when relatively large numbers of male s (>50) were present at the chorus; active searching and satellite tactics were not observed. Calling efforts (call duration X call rate) of hybrids ( 23.9%, n = 8) and B. microscaphus (24.9%, n = 19) were similar and comparab le 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 variab le than that of their parental species. The distribution of mtDNA haplotype s revealed directional introgression is occurring between male B. r,microsc aphus and female B. woodhousii: All 17 hybrids possessed B. woodhousii mtDN A. The proximate mechanism driving hybridization appears to involve common male (B.,microscaphus) and rare female (B. woodhousii) matings as B. woodho usii expands its range.