Adaptive Variation in Size of Cricket Frogs

Geographic variation in body size (BL) and relative foot size (F/B) was studied across the ranges of two sibling species of cricket frogs, genus Acris (Hylidae)in North America. Acris crepitans has a large range in eastern and central states extending from mesic woodlands to xeric grasslands, whereas A. gryllus has a limited range in southeastern states in mesic woodlands. Measurements of BL and F/B of 5,286 frogs from 108 localities were divided into 11 biogeographical regions and subjected to discriminant and multiple regression analyses. BL varies primarily in A. crepitans, negatively correlated with annual rainfall, which is its best single predictor, accounting for about .50 of size variance. A combination of three humidity variables explains .60 of BL variance.Desiccation experiments indicate that loss of body weight and vital time limit regress linearly on size and are highly correlated with it. Larger frogs lose less water and reach their limit of tolerance in twice as long a time as do small ones. F/B in both species is positively correlated and best predicted by a combination of temperature, precipitation, and longitude or sympatric anuran species. It is suggested that BL is casually related to humidity, large size being an adaptation to arid climates, and that F/B is casually related to predation pressure and/or competition for food, large F/B being an adaptation for increased jumping efficiency.