Thermal Habitat Use and Evidence of Seasonal Migration by Rocky Mountain Tailed Frogs, Ascaphus montanus, in Montana
All life stages of Rocky Mountain Tailed Frogs (Ascaphus montanus) occurred in a reach of Moore Creek. Montana, where water temperatures exceeded those previously reported for Ascaphus in the wild. However, relative density of Ascaphus in the wannest reach, immediately downstream of a lake outlet, was lower than in cooler reaches downstream. Although we observed larvae and frogs in water temperatures up to 21°C. cold groundwater seeps contributed to a spatially complex thermal structure in the warmest stream reach. Frogs congregating near a cold seep and nesting in a groundwater-influenced sile were likely using behavioral thermoregulalion. At a stream weir in the warmest reach, we captured 32 Tailed Frogs moving downstream and none upstream, in September and October 1997. Because no migration was evident at five other weirs where summer water temperatures remained below 16°C, we propose that the frogs moving through upper Moore Creek migrated seasonally lo avoid the high temperatures. The mature frogs may spend summers in the small, cold lake inlet streams, moving downstream in the fall lo overwinter. Behavioral studies would be necessary lo determine the extent to which individuals limit their overall thermal exposure in such spatially complex environments. Migration in response to local, seasonally changing habitat suitability could explain the diverse, and apparently contradictory, movement patterns (or lack thereot) among Ascaphus populations reported in the literature. Future studies of Ascaphus movements could benefit by accounting for seasonal changes in habitat suitability and by quantifying in-stream movements.