Addition of Annual Growth Lines in Adult Snapping Turtles Chelydra serpentina

Abstract

Persistent grooves (growth-lines) in keratin formed during periods of slowed growth have been used to estimate age and growth rates of turtles since the midnineteenth century (Agassiz, 1857). Since then, many studies have used the growth lines (also termed growth rings or annuli) on the surface of plastral or carapacial scutes (reviewed by Gibbons, 1976), or rings of varying calcium matrix density in bones (reviewed by Castanet, 1982). Age has been estimated by enumeration of these growth lines, while growth of the carapace has been reconstructed by relating the past length of scutes (growth-line length) to past carapace length (Sergeev, 1937). The periodicity of growth-line addition should be tested wherever possible by directly comparing numbers of growth lines added with time elapsed (Dapson, 1980). Despite the great number of studies using growth lines for age estimates or growth studies in turtles (e.g., Chelydra serpentina: Gibbons, 1968a; Hammer, 1969; Graham and Perkins, 1976; Christiansen and Burken, 1979; Chrysemys picta: Sexton, 1959; Gibbons, 1967, 1968b; Wilbur, 1975a, b; MacCulloch and Secoy, 1983; Clemmys guttata: Ernst, 1975; Clemmys muhlenbergii: Ernst, 1977; Emydoidea blandingii: Graham and Doyle, 1977; Graptemys pseudogeographica: Moll, 1976; Macroclemys temmincki: Dobie, 1971; Testudo graeca: Lambert, 1982), only three previous studies of turtles have directly tested the periodicity of growthline deposition using recaptures of the same individuals. In Illinois, sliders (Pseudemys scripta: Cagle, 1946), and in Michigan, wood turtles (Clemmys insculpta: Harding, 1985) added single growth lines each year as juveniles, but deposited lines less frequently as they aged. However, a slider population in Panama added multiple growth lines each year (Moll and Legler, 1971). Apart from comparisons of recaptured individuals, periodicity of growth lines has been 'verified' by comparisons with growth rings in bones for snapping turtles (Hammer, 1969) and painted turtles (Chrysemys picta: MacCulloch and Secoy, 1983). Age estimtes based on growth rings in bones have been successful for many species of reptiles (Castanet, 1982), but the technique must be calibrated carefully for each species, and preferably for each population (Groupe d'Etude Practique de Squelettochronologie, 1977). However, one non-calibrated technique should not be used to 'confirm' another, as such comparisons provide nothing reliable about age, but describe the relationship between the two dependent variables (Dapson, 1980). The present study was undertaken to test whether growth lines were deposited annually on the fourth vertebral scutes of snapping turtles (Chelydra serpentina serpentina) in a northern population. This scute was selected as it appeared to suffer less from wear than others, and because it was possible to reconstruct growth from this scute since it lay along the line of measurement of carapace length. This study was conducted in Algonquin Provincial Park, Ontario, from May 1976 through July 1985, in the Ontario Ministry of Natural Resources Wildlife Research Area (45?35'N, 78?30'W). Turtles were captured in the North Madawaska River watershed in lakes of 45 ha and smaller. The river itself is small