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Eli S. Sagor, Martin Ouellet, Evelynne Barten, David M. Green, Skeletochronology and Geographic Variation in Age Structure in the Wood Frog, Rana sylvatica, Journal of Herpetology, Vol. 32, No. 4 (Dec., 1998), pp. 469-474
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Sagor et al. (1998) studied this question.
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