Ability of Oregon Spotted Frog (Rana pretiosa) Embryos from Central Oregon to Tolerate Low Temperatures

Abstract

minima, egg mass, oviposition More than half of the known remaining populations of the Oregon Spotted Frog (Rana pretiosa) are at elevations .1200 m along the Cascade Range and its eastern flank in Oregon (Pearl and Hayes 2005). Rana pretiosa in this region typically breed soon after thaw and often lay eggs in water ,20 cm deep (Pearl and others 2009). Egg masses in shallow microhabitats experience broad temperature fluctuations, and ice formation on the surface is common (Bull and Shepherd 2003). At .20 oviposition sites around Sunriver in central Oregon (elevation 1270 m), we have observed high survival of R. pretiosa embryos after exposure to subfreezing air temperatures and ice cover for up to several days. These observations and the characteristics of oviposition sites across much of the species’ range along the Cascade Range suggest that R. pretiosa eggs in this region may be more tolerant of low temperatures than previously reported. Information on thermal limits of R. pretiosa embryos derives from studies of 1 population in the Fraser River Valley of southwestern British Columbia. The low elevation of this site (approximately 50 m above sea level) and its proximity to the Strait of Georgia and Pacific Ocean make its climate more moderate than much of the extant range of R. pretiosa. Licht (1971) reported a lethal minimum, the temperature at which egg survival is ,50%, near 66C for eggs from that site at a range of stable temperature treatments in the laboratory. A subset of embryos survived lower temperatures for up to 8 h, after which trials were terminated (Licht 1971), but data were not presented on presence or timing of develop