Embryonic thermal adaptations of the frogs Rana aurora aurora and Rana pretiosa pretiosa from the Pacific Northwest, are described. Limits of temperature tolerance of young R. aurora embryos are about 4-21 C, both the upper and lower lethals being the lowest for any North American ranid frog. For R. pretiosa, the lethal thermal limits of young embryos are approximately 6-28 C. The tolerance limits broaden as embryos become older, and embryos of both species can survive short-term exposure to normally lethal chronic cold temperatures. The developmental rates for embryos of both species at a wide range of constant temperatures are given. Egg masses of both species are compact and globular. The ova of R. aurora average 3.03 mm, those of R. pretiosa 2.31 mm. R. aurora embryos hatch at stage 21, and R. pretiosa embryos hatch at stage 19, a difference that may reflect the 02 needs of the hatching embryos. The 02 consumption by R. aurora embryos between developmental stages 12-15 at 18.5 C averaged 0.59 cmm 02/egg per hour. R. pretiosa embryos at the same stage and temperature averaged 0.57 cmm 02/egg per hour. Field observations of breeding frogs indicate a correlation between breeding habits -- such as initiation of breeding season, time of daily sexual activity, male calling behavior, and spawn- ing site-and embryonic thermal requirements. High mortality of R. pretiosa embryos in the field often results from freezing temperatures at night and desiccation of egg masses. These factors do not greatly affect R. aurora embryos. These thermal adaptations of the two western species of Rana are compared with those of species from eastern North America, as an aid in broadening our understanding of the evolutionary strategies within the genus in North America.
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