Morphological variation and age‐class determination in overwintering tadpoles of the tailed frog, Ascaphus truei

Abstract

The morphological variation and development of different age classes of Ascaphus tadpoles is described for a subalpine population in the North Cascade Mountains. Adult females lay eggs in mid July, and embryos develop into hatchling tadpoles in late August. The torrent‐adapted tadpoles develop in cold waters, overwinter in ice‐covered streams and complete metamorphosis in four years. In September/October there may be five age‐classes of tadpoles: cohort 0, hatchling tadpoles (total length (ToL) = 11 mm); cohort I, one‐year tadpoles (ToL = 32 mm); cohort 2, two‐year tadpoles (ToL = 36 mm); cohort 3, three‐year tadpoles (ToL = 42 mm); and cohort 4, metamorphosing tadpoles or recently transformed frogs (ToL = 20 mm). Determination of these age‐classes is based upon total length, body weight and hind limb and forelimb differentiation. The limb buds appear in the first year of tadpole development, but the forelimbs are hidden by the body wall and branchial chamber, and the hind limbs are concealed by the anal fold. Arguments are presented from the data of total size‐frequency and morphological differentiation and a review of the literature that the larval life period in montane populations is not two or three years, as is commonly accepted, but is approximately four years. Copyright © 1990, Wiley Blackwell. All rights reserved