Threshold to maturity in a long-lived reptile: interactions of age, size, and growth

Thresholds to sexual maturity—either age or size—are critical life history parameters. Usually investigated in short-lived organisms, these thresholds and interactions among age, size, and growth are poorly known for long-lived species. A 34-year study of captive green turtles (Chelonia mydas) that followed individuals from hatching to beyond maturity provided an opportunity to evaluate these parameters in a long-lived species with late maturity. Age and size at maturity are best predicted by linear growth rate and mass growth rate, respectively. At maturity, resource allocation shifts from growth to reproductive output, regardless of nutrient availability or size at maturity. Although captive turtles reach maturity at younger ages than wild turtles, the extensive variation in captive turtles under similar conditions provides important insights into the variation that would exist in wild populations experiencing stochastic conditions. Variation in age/size at maturity should be incorporated into population models for conservation and management planning. Introduction Age and size at sexual maturity are critical components of life history studies because of their importance in determining an organism’s fitness (Stearns 1992; Roff 2002). The timing of maturation involves classic life history tradeoffs that balance the fitness benefits of early maturation (increased survival to first reproduction and decreased generation time) and late maturation (increased body size and enhanced size-mediated processes such as reproductive output and competitive ability). Maturity is controlled by a number of complex processes that are influenced by a variety of factors, and the relative contributions of these influences on age and size at maturity can vary considerably among individuals in a population (Bernardo 1993). The thresholds to maturation have been the subject of many studies, particularly in exploited fish stocks in which age and size at maturity change with exploitation intensity and the size and maturity status of the fish being targeted (Ernande et al. 2004; Dieckmann and Heino 2007). Although most studies of maturity thresholds have focused on age, size, and growth rates, other relevant phenotypic traits, such as body condition, should be considered (UusiHeikkilä et al. 2011), as well as developmental processes (Berner and Blanckenhorn 2007; Kingsolver et al. 2012) because maturation rate does not necessarily follow the same trajectory as somatic growth rate (Bernardo 1993). Naturally, most experimental studies designed to address maturation thresholds have involved organisms with short maturation times (e.g., Uusi-Heikkilä et al. 2011; Kingsolver et al. 2012). Our study explores variation in age and size at maturity and in growth rates before and after maturity in a long-lived species, the green turtle, Chelonia mydas, under controlled conditions. Like most species of sea turtles, green turtles in the wild reach Communicated by R. Lewison. Electronic supplementary material The online version of this article (doi:10.1007/s00227-012-2116-1) contains supplementary material, which is available to authorized users. K. A. Bjorndal (&) A. B. Bolten Department of Biology, Archie Carr Center for Sea Turtle Research, University of Florida, Gainesville, FL 32611, USA e-mail: [email protected] J. Parsons W. Mustin Cayman Turtle Farm, Grand Cayman KY1-1301, Cayman Islands