SOME IMPORTANT ASPECTS OF LOGGERHEAD TURTLE BIOLOGY & LIFE HISTORY

Presented by Barbara Schroeder 

Several aspects of the biology and life history of loggerhead turtles are important to understand and keep in mind when developing approaches to recover the species and when evaluating their population status. These aspects include:

• Complex Life History
• Multi-national Habitats/Highly Migratory
• Mixing and Separating of Stocks
• Long Age to Maturity, Long-lived
• Non-annual Reproduction, Multiple Egg Clutches, Nest Site Fidelity
• Temperature Dependent Sex Determination

What follows is a brief narrative on each of these aspects of loggerhead biology or life history and why they are relevant in the context of recovery planning.

Complex Life History: Following their emergence from nests laid on sandy beaches, sea turtles begin a complex life history which will take them to differing habitats, often widely separated in space and time. A generalized life history model of sea turtles can be illustrated as a series of movements (migrations) among and between oceanic (also referred to as pelagic), neritic (also referred to as coastal benthic), and terrestrial habitats. The diagram below illustrates the life history of the loggerhead turtle.

Multi-national Habitats/Highly Migratory: Loggerheads, like all sea turtles, are a highly migratory species, regularly crossing political boundaries throughout all or most of their lifetime. Loggerheads nesting on U.S. Atlantic and Gulf of Mexico beaches may use the waters of the Gulf of Mexico, the north Atlantic Ocean, and the western Mediterranean Sea during their lifetime. While some of these waters are under international jurisdiction, others are within the territorial seas of various nations. The southeast United States hosts one of the two most significant nesting assemblages of loggerheads in the world. The conservation and recovery of sea turtles will require international cooperation and multi-lateral agreements to ensure the survival of this highly migratory species throughout its range.

Mixing and Separating of Stocks: At least five different genetic stocks of loggerhead turtles have been identified in the western Atlantic, through mtDNA analyses. While adult females from each stock are separated while on their nesting beaches, post-hatchlings, immature turtles, adult males, and non-nesting females mix on their foraging grounds. This mixing of stocks away from nesting beaches complicates assessments of the status of the population and requires comprehensive recovery actions to ensure that all stocks are conserved.

Long Age to Maturity, Long-lived: Loggerheads reach maturity at approximately 30 to 35 years of age. The status of loggerhead populations is most commonly evaluated in the context of the number of nesting females, because nests are relatively easily counted and accessible. Both recovery efforts and threats to the population are not likely to be immediately reflected in counts of nesting females (or their nests) and this complicates evaluating the status of the species and understanding population trends. Dr. David Ehrenfeld eloquently commented on this problem and compared looking at sea turtle population data with looking at the light from a star 25 light-years away. Although the star may appear to be shining now, we are in fact looking at the way it was 25 years ago, and there is no way of telling whether that star has increased in brightness or has gone out altogether in the intervening years. Sea turtles are a long-lived species, and while exact longevity figures are not available, we know that once mature, turtles remain reproductively active for many years and this extended reproductive viability is critical to population recovery and population stability.

Non-annual Reproduction and Multiple Clutches: Loggerheads do not nest every year. The average remigration interval (number of years between successive nesting years) is 2½ to 3 years. During a nesting season, loggerheads in the southeast United States lay between 1 and 7 clutches, but the average is approximately 4. These aspects of the species life history are important to keep in mind when thinking about how many adult females are in the population, especially when nest counts are used to analyze population trends. The relationship between number of nests and number of nesting females in the population is not one-to-one. Another important aspect of loggerhead reproductive behavior is that nesting females remain faithful to their nesting beaches. The distance between nest sites (both within a nesting season and between nesting seasons) for an individual turtle is typically less than 5 kilometers. This behavioral trait, termed nest site fidelity, is important to consider in the development of recovery actions to protect and conserve nesting habitat.

Temperature Dependent Sex Determination: Sea turtles do not have the typically understood genetically controlled mechanism of sex determination of offspring. For sea turtles, and many non-marine turtles as well, sex of hatchlings is determined by the incubation temperature of the eggs. At higher temperatures, all females are produced and at lower temperatures, all males are produced. At the pivotal temperature, a 50 percent mix of males and females are produced. The incubation temperature during the middle third of the incubation period is the most critical. This biological constraint is extremely important to keep in mind when developing recovery actions such as those aimed at improving hatching success as well as those actions aimed at maintaining the historical range of the species.

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