Anurans that hibernate at or near the ground surface can survive prolonged exposure to low winter temperatures of northern latitudes by tolerance to freezing. An accumulation of glycerol during winter was correlated with frost tolerance, indicating that this compound is associated with natural tolerance to freezing in a vertebrate.

The water economies of nine species of anuran amphibians have been examined for (1) response to controlled desiccation, (2) response to prolonged hydration, and (3) some physiological factors related to the observed interspecific differences in these responses.The nine species exhibited a marked variation in habitat preferences with regard to the availability of water, and correlations were observed between habitats and responses to experimental conditions. Species from more aquatic habitats exhibited lower tolerances to desiccation; however, they were able to endure hydration for significantly longer periods than terrestrial species. Physiological specialization was observed which may be primarily responsible for the observed interspecific differences in response to prolonged hydration. The skin of aquatic species was less permeable to water and the effective osmolarity of blood plasma was less than that of terrestrial species. It has been suggested that species from terrestrial habitats, especially those of the families Bufonidae and Hylidae, may be limited from wintering in water because of their intolerance to hydration. INTRODUCTIONEcology is concerned not only with the question of what the relationships are between organisms and their environment, but also how these relationships are taking place. The latter question may be approached by studying responses to controlled conditions in the laboratory. These responses, and the physiological bases of these responses, provide information which is fundamental to the concept of the ecological niche. This approach was used in the present study in which the water economies of nine species of anuran amphibians have been examined for (1) response to desiccation , (2) response to hydration, and (3) some physiological factors related to the observed interspecific differences in these responses.The amphibians included in this study were found to have widely differing habitat preferences with respect to the availability of water. The habitat preferences of the nine species of frogs and toads are summarized as follows. (1) Rana septentrionalis Baird, the mink frog, is probably the most aquatic of the species studied. It was never found far from permanent bodies of water. All specimens were collected in Lake Itasca, Clearwater County, Minnesota. (2) Rana clamitans Latreille, the green frog, is also a distinctly aquatic species, and like the mink frog, was never encountered far from water. This frog is scarce in the Itasca area, and specimens were collected in the St. Croix River near Never's Dam, Pine County, Minnesota. (3) Rana pipiens Schreber, the leopard frog, is semiaquatic and has successfully invaded a number of habitats. Specimens were collected in grassy fields, along banks of streams and lakes, and in the brushy edges of deciduous woods. (4) Rana sylvatica LeConte, the wood frog, might be classified as semiaquatic, but seems to be more terrestrial than the leopard frog for it was often collected far from water. All specimens were collected in deciduous woods of th...

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The aquatic vegetation of Long Lake (T 144 N, R 36, 37 W) in northwestern Minnesota was surveyed by means of line transects run underwater with SCUBA apparatus. Eight stations were established about the lake and 12 transects were run from each station. The transects were run along depth contours at ½—m depth and 1—m intervals from 1 through 11 m depth. No vegetation was observed deeper than 11 m. Each transect was 20 m long, and the presence of plant species was recorded at intervals of ⅓ m. The data were recorded underwater by writing on a sheet of aluminum foil held in a mental clipboard. The aquatic vegetation was more abundant on the southwest—facing slope than on the northeast—facing slope of the lake bottom. This difference has been attributed to a difference in solar insolation. Some of the significant interspecies correlations which were observed appeared to be related to water depth and the texture of the substrate.

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