TEMPORAL STABILITY IN HYBRIDIZATION BETWEEN BUFO-MICROSCAPHUS AND BUFO-WOODHOUSII (ANURA, BUFONIDAE) - BEHAVIOR AND MORPHOLOGY

I analyzed temporal variation in hybridization between the southwester n toad (Bufo microscaphus) and Woodhouse's toad (Bufo woodhousii) alon g the Virgin River in southwestern Utah and northwestern Arizona. Bufo woodhousii is largely restricted to the floodplain of the Virgin Rive r from southern Nevada to the vicinity of St. George, Utah. By contras t, B. microscaphus is confined primarily to the tributaries of the Vir gin River, only occupying the Virgin River proper exclusive of B. wood housii along its upper reaches east of La Verkin Creek. As in all othe r zones of sympatry for these bufonids, behavior and morphology provid e clear evidence of hybridization at a number of sites along the Virgi n River. Analysis of morphology and behavior indicates that the distri bution of these taxa and their hybrids is largely unchanged relative t o that documented forty years ago by Blair (1955). Comparisons of morp hological hybrid index scores reveal slight shifts in relative numbers of parental taxa at only one site. Hybridization between these anuran s has been relatively stable geographically, and may be related to the nature of the riparian habitat available for breeding.