Upper thermal tolerances of anuran embryos in relation to stage of development and breeding habits. American Museum novitates ; no. 2617

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New York, N.Y. : American Museum of Natural History
"Embryos of several species of anurans were exposed from two to 10 hours and at various stages of development to higher temperatures than the maximum that early embryos of the same species can tolerate under continuous exposure. The purposes were to determine (1) the extent of increase in temperature tolerance through ontogeny (2) the period(s) of development when the increase takes place, and (3) the influence of duration of exposure to high temperature on survival. Species studied were a forest-dwelling form adapted to breeding in cold ponds in early spring (Rana sylvatica), a stream and pond dweller of southwestern mountains (Rana sp., pipiens complex), and three species of desert and grassland habitats (Bufo cognatus, Scaphiopus bombifrons, and S. couchii). Embryos studied (and presumably of all anurans) increase their temperature tolerance as they grow. A marked increase takes place early in development, during the first several cleavages. The species of Rana, adapted to cooler conditions, do not attain maximum tolerance until gastrulation is well under way or virtually is complete. At the other extreme, S. couchii achieves more than 90 percent of its total tolerance before gastrulation has commenced. The highest tolerance among species studied is that of Bufo cognatus, whose embryos survive six hours at 40.5 C. when exposed as mid-gastrulae (lethal level was not determined). Species of Scaphiopus do almost as well, whereas limits of the two species of Rana are several degrees lower. Among most species, the maximum temperature tolerated and the duration of exposure are inversely related, but in Bufo cognatus duration of exposure had no obvious effect on maximum tolerance. In its extremely rapid embryonic development, early attainment of maximum tolerance, and high level of tolerance, Scaphiopus couchii is the species best adapted to breeding in warm waters. Although adapted to breeding in cold waters, Rana sylvatica nevertheless undergoes sufficient ontogenetic expansion in tolerance quickly enough to protect developing embryos against all but the most unseasonally warm temperatures occurring in its breeding ponds"--P. [1].
21 p. : ill. ; 26 cm.
Includes bibliographical references (p. 20-21).