Age determination of loggerhead sea turtles, Caretta caretta, by incremental growth marks in the skeleton

Abstract

Zug, George R., Addison H. Wynn, and Carol Ruckdeschel. Age Determination of Loggerhead Sea Turtles, Caretta caretta, by Incremental Growth Marks in the Skeleton. Smithsonian Contributions to Zoology, number 427, 34 pages, 9 figures, 6 tables, 1 appendix, 1986.—Periosteal growth in the skeleton of Caretta caretta is cyclic and produces a record of distinct bony layers (growth marks). These layers are most apparent in the long bones, particularly the humerus and femur. Correlative evidence supports the production of one layer (i.e., growth increment) each year in some reptiles, and the annual nature of each bony layer is an explicit assumption in our estimation of ages of Caretta from Cumberland Island, Georgia. Sections from the shafts of humeri were prepared with standard histological techniques, and the marks of skeletal growth were counted and measured. Owing to resorption of the early periosteal layers, determination of number of growth marks and, hence, age requires an extrapolation from the remaining marks to total growth marks produced during the life of the turtle. Various age estimates are possible; the most reliable are derived from the narrowest axis of the humerus sections and suggest that, on the average, Georgia Caretta attain sexual maturity in their thirteenth to fifteenth years, with some individuals possibly maturing as early as six years and others as late as their twenty-fifth year. Since the average age of maturity agrees with the results of a mark-recapture study in a neighboring Florida population, the value of the skeletochronological technique and its assumptions are confirmed. The technique is not advocated as a method for the age determination of individual sea turtles, but does provide statistical age estimates for different size classes of turtles. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: The coral Montastrea cavernosa Linnaeus. Library of Congress Cataloging in Publication Data Zug, George R., Addison H. Wynn, and Carol Ruckdeschel Age determination of loggerhead sea turtles, Caretta caretta, by incremental growth marks in the skeleton (Smithsonian contributions to zoology ; no. 427) Bibliography: p. Supt. of Docs. no. : SI 1.27:427 1. Loggerhead turtle—Age determination. 2. Skeleton—Growth. 3. Reptiles—Age determination. I. Wynn, Addison H., 1955. II. Ruckdeschel, Carol. III. Title. IV. Series. QL1.S54 no. 427 591s [597.92] 85-600229 [QL666.C536]