Temporal stability in hybridization between Bufo microscaphus and Bufo woodhousii (Anura: Bufonidae): Behavior and morphology

I analyzed temporal variation in hybridization between the southwestern toad (Bufo microscaphus) and Woodhouse's toad (Bufo woodhousii) along the Virgin River in southwestern Utah and northwestern Arizona. Bufo woodhousii is largely restricted to the floodplain of the Virgin River from southern Nevada to the vicinity of St. George, Utah. By contrast, B. microscaphus is confined primarily to the tributaries of the Virgin River, only occupying the Virgin River proper exclusive of B. woodhousii along its upper reaches east of La Verkin Creek. As in all other zones of sympatry for these bufonids, behavior and morphology provide clear evidence of hybridization at a number of sites along the Virgin River. Analysis of morphology and behavior indicates that the distribution of these taxa and their hybrids is largely unchanged relative to that documented forty years ago by Blair (1955). Comparisons of morphological hybrid index scores reveal slight shifts in relative numbers of parental taxa at only one site. Hybridization between these anurans has been relatively stable geographically, and may be related to the nature of the riparian habitat available for breeding.