Age and Growth in Sea Turtles: Limitations of Skeletochronology for Demographic Studies

Demographic models and management plans for sea turtle populations have been limited by our inability to determine the age of individuals. Skeletochronology-the use of incremental growth marks to estimate age-has been used in several studies to assign ages to sea turtles based on growth marks in the humerus. These growth marks also can be used to estimate body size at earlier ages (back-calculations of lengths-at-age). We test two critical assumptions upon which these techniques are based: (1) growth marks in the humeri of sea turtles are laid down annually; and (2) there is a constant proportional allometry between radial growth of the humerus and longitudinal growth of the carapace. As a result of our study of a tropical population of juvenile green turtles (Chelonia mydas), both assumptions were rejected. With tetracycline as a skeletal marker, no growth marks were visible in biopsies from the humeri of 25 green turtles after periods of either 1.3 or 2.4 years. The proportion of humeral radial growth to carapace length growth was not constant but was significantly correlated with carapace length. Skeletochronology in sea turtles has additional limitations resulting from remodeling of the humerus (endocortical, periosteal, and intracortical) during growth that may destroy growth marks, accumulation of noncyclic growth marks, and uninterpretable patterns of growth marks. These violations of assumptions and limitations dictate that skeletochronology should only be applied to sea turtle populations or life stages for which the assumptions have been validated. Our results underscore the importance of testing the assumptions upon which skeletochronology and back-calculations of lengths-at-age are based. These techniques have been applied widely to vertebrate and invertebrate populations, often without validation.