Loggerhead sea turtles lay eggs in multiple locations to improve reproductive success

Although loggerhead sea turtles return to the same beach where they hatched to lay their eggs, a new study by a USF professor finds individual females lay numerous clutches of eggs in locations miles apart from each other to increase the chance that some of their offspring will survive.

A study published in the journal "Scientific Reports" found that some females lay as many as six clutches as far as six miles apart during the same breeding season.

"Nesting females don't lay all their eggs in one basket. Their reproductive strategy is like investing in a mutual fund. Females divide their resources among many stocks rather than investing everything in a single stock," said Deby Cassill, biology professor at USF's St. Petersburg campus and author of the study.

During their 50-year lifetime, a single female loggerhead will produce around 4,200 eggs and scatter them at 40 different sites on the barrier island. This strategy helps reduce the risk of complete reproductive failure by hurricanes and thunderstorms that could wash out or flood all clutches.

"Because females diversify reproduction in unpredictable patterns over time and space, nearly two-thirds of loggerhead sea turtle hatchlings made it into the Gulf of Mexico," said Cassill.

For the study, Cassill analyzed 17 years of data provided by the Conservancy of Southwest Florida on loggerhead females nesting on Keewaydin Island off the southwestern Gulf coast of Florida. For years, conservancy staff and community volunteers tagged turtles and patrolled the island to monitor and record detailed information on the nesting population.

Though the study shows most sea turtle hatchlings reach the Gulf of Mexico, future impacts due to human encroachment and climate change could affect the population. Increased frequency of extreme storms due to warmer waters and sea-level rise may flood or wash away larger portions of clutches, leading to population declines of the threatened species.

"It's important to follow individuals over time to really get a glimpse of how they mate, find food and ensure that some of their young survive to maturity. Without knowledge of the sea turtle's survival and reproductive biology, we cannot develop and implement effective conservation policies," said Cassill.

The study is one in a series of upcoming articles by Cassill pertaining to her "maternal risk management model," which looks at how natural selection pressures, such as predators, storms and resource scarcity, influence how mothers invest in offspring quantity and quality.

She argues turtles and fish invest in large numbers of offspring when the likelihood their offspring will be killed by predators is high. Mammal mothers like whales and elephants provide extensive care to one offspring at a time when the likelihood their young will starve during seasonal droughts is high.

The model, based on the number and size of offspring produced by a mother, extends Darwin's theory of natural selection by explaining the fusion of parents and offspring into family units and societies.