Oregon Spotted Frog (Rana pretiosa) Migration from an Aquatic Overwintering Site: Timing, Duration, and Potential Environmental Cues

Abstract

Relatively few North American anurans overwinter in water and information is sparse on their movement from overwintering habitat to breeding sites. Oregon spotted frogs (Rana pretiosa) breed explosively in early spring and often overwinter submerged at sites that are distanced from breeding habitats. In montane parts of their range, wintering and breeding habitats can remain frozen for months. We investigated timing, duration, and potential cues for R. pretiosa migrations from a wintering lake near the Cascade Mountains in central Oregon, U.S.A. First and median migrant males moved slightly earlier than females. Onset of migration was as early as February 12 (males) and as late as April 4 (females) in years of mild and extended winters, respectively. Frogs were active at water temperatures below those associated with early breeding activities in one lowland R. pretiosa population. Higher proportions of frogs migrated before ice-out in years of prolonged winter conditions. Migrations were temporally compressed in years of later movement. This migration ‘rush’, along with the ability to move at cold temperatures and to vary timing of migrations likely helps montane R. pretiosa deal with colder and more variable spring conditions than lowland populations.