Abstract:
Breeding ecology and behavior were investigated in a field study of three sympatric anuran amphibians in the Oregon Cascade Mountains: the western toad (Bufo
boreas), the Cascades frog (Rana cascadae) and the Pacific treefrog (Hyla regilla). A comprehensive study of the western toad mating system was conducted at three populations during five years (15 explosive-breeding aggregations). Two size-dependent pairing patterns (a large male mating advantage and size assortative mating) occurred variably among aggregations and among breeding days within aggregations. Body size variation and the degree of sexual size dimorphism explained much of the mating pattern variability. Field observations and experiments suggest nonrandom mating resulted from male-male exploitative competition for mates and passive female choice of mates. Density and sex ratio had little affect on pairing behavior. Sporadic mass-predation on breeding toads was associated with pair-separations and may have influenced pairing patterns. Toads showed sex and site differences in multiyear breeding schedules, recapture/survival rates, and yearly and
multiyear cumulative mating success. The opportunity for selection on toads was partitioned into 3 selection episodes: adult survival, mating success, and fertility. The largest contributors to selection opportunity were mating success for males, and clutch size (fertility) for females. At some aggregations, the opportunity for sexual selection on males was the lowest recorded for anurans. Temporal and spatial synchrony of breeding occurred with Cascade frogs and Pacific treefrogs. During two years, interspecific interactions resulted in the
displacement of treefrogs by Cascade frog males from the warmer areas of the ice-covered pond.
