Estimation of Age for Trachemys scripta and Deirochelys reticularia by Counting Annual Growth Layers in Claws

The need for accurate methods of age determination in life-history studies is well recognized. Techniques used to estimate the age of turtles include analysis of body size (Balazs, 1982; Frazer and Ehrhart, 1985; Frazer and Ladner, 1986), weighing eye lenses (. Frazier,J. Ballon, and S. Salas, unpubl.), counting growth zones on plastral scutes (Cagle, 1946; Sexton, 1959; Gibbons, 1983), and skeletochronology (Mattox, 1935; Zug, 1990; Klinger and Musick, 1992). Each technique has its disadvantages, and finding a technique that is accurate and can be applied objectively has been difficult (Zug, 1991). An ideal age estimation technique would not cause stress or mortality to the individual being aged, would be valid for all age/size classes, would be based on a clearly recognizable structure that is permanent throughout the life of the individual, and would not require subjective judgment. The use of claws could potentially satisfy three of these requirements for turtle age estimation. Zug (1991) suggested that because claws grow continuously throughout the life of a turtle and because growth is affected by environmental factors, the formation of seasonal growth layers might be expected. The objective of our study was to determine whether annual layers could be observed in sections of claws from Trachemys scripta and Deirochelys reticularia and whether counts based on such layering could provide reliable data for age estimation.