We tested the effects of ultraviolet-a (uv-a) and ultraviolet-b (uv-b) radiation on life-history stages of the anurans Bufo americanus, Hyla versicolor, Rana clamitans, and Rana sylvatica. No effect of uv-a was found on eggs or larvae even at exposures twice the intensity of normal outdoor levels. Only R. sylvatica was tested in the embryo stage. All embryos exposed to 30 min or more of artificially high intensity uv-b died. After exposure to artificially high intensity uv-b of 15 min or less, or at ecologically relevant levels, there was no effect on hatching success. The proportion of abnormal embryos after exposure to 10 or 15 min of artificially high intensity treatment was higher at 12 °C than at 20 °C. The jelly surrounding the ova can effectively reduce uv-b transmission through the egg mass. Of the four species tested, only larval R. clamitans showed some tolerance of artificially high uv-b levels, but development of the survivors was arrested and metamorphosis was not initiated. Ecologically relevant doses of uv-b had no effect on developmental period, duration of metamorphic climax, or mass at metamorphosis in B. americanus, H. versicolor, or R. sylvatica. Whereas all metamorphosed juvenile R. clamitans and R. sylvatica died after exposure to high-intensity uv-b, some B. americanus survived. Older B. americanus exposed for the same length of time had higher survivorship than younger animals. The role of uv radiation in presumed amphibian population declines is discussed.

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Field survival of embryos, tadpoles, and adults of Rana aurora and Rana pretiosa sympatric in marshes near Vancouver, British Columbia, was studied.Embryonic survival for R. aurora was 90% and better, whereas for R. pretiosa it was about 70%. As well, in dry periods during breeding, R. pretiosa embryos face the danger of desiccation and extensive or complete mortality.Survival of tadpoles of both species in a pond breeding site studied was less than 1% in 1968. By the end of the season of transformation, there was about 5% survival of young-of-the-year frogs of both species from the initial number of eggs deposited in river breeding sites.After the end of the first full year of life (1969), there was a minimal survival of 2.5% for R. aurora and 3.5% for R. pretiosa from the eggs laid the year before. By the end of 1969, there was a 52% survival of R. aurora and 67% of R. pretiosa which metamorphosed in 1968.For R. pretiosa adults, there was a 64% survival between 1968 and 1969; males suffered higher mortality than females. The survival of adult R. aurora was 69% between 1968 and 1969.Factors influencing mortality are discussed, and the conclusion is reached that predation (and chance climatic events for R. pretiosa embryos) on all life-history forms is the strongest factor limiting frog population numbers.

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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--28oC. 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. pretzosa embryos hatch at stage 19, a difference that may reflect the 0 2 needs of the hatching embryos. The 0 2 consumption by R. aurora embryos between developmental stages 12-15 at 18.5oC averaged 0.59 cmm 0 2 /egg per hour. R. pretiosa embryos at the same stage and temperature averaged 0.57 cmm 0 2 /egg per hour.Field observations of breeding frogs indicate a correlation between breeding habits-such ~s ini_tiation of breedin~ season, time of daily sexual activity, male calling behavior, and spawnmg s1te-and embryomc 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.