Nest site selection in loggerhead sea turtles that use different foraging areas: do less fecund oceanic foragers nest at safer sites?

Nest-site choice by loggerhead sea turtles as a risk-management adaptation to offset hatching failure by unpredictable storms and predators

Introduction

Along the coasts of Florida, United States, the nesting season of the loggerhead sea turtle, Caretta caretta, overlaps with the hurricane season. Nesting loggerhead females do not extend parental protection beyond depositing eggs in sandy, excavated nests in locations that provide a viable range of temperatures, moisture, and respiratory gas exchange. Thereafter, a female’s clutches are subjected to the uncertainties of desiccation, predation, flooding, or beach erosion.

Methods

Here, we used data from a 1996-2004 study of 94 tagged loggerhead females nesting on a small barrier island off the Gulf Coast of south Florida, United States. We tested the hypothesis that the distribution of nest sites by loggerhead females was a randomized response to unpredictable hatching failure.

Results

We show that nest site choice accounted for 19.2% of variation in hatching success whereas breeding year and breeding month accounted for the remaining 81.8% of variation in hatching success. We show that the emergence site along the beach-length axis, nest site choice along the beach-width axis, and distances between nest locations did not fit a uniform-random distribution or a normal distribution. Instead, we show that loggerhead females employed a “Goldilocks” distribution in which nest sites were “neither too clustered nor too dispersed.” Moreover, loggerhead females selected nest sites with limited overlap with nest sites from previous breeding seasons.

Discussion

We propose that nest site choice by this population of loggerhead females constitutes a significant maternal risk-management adaptation that deserves thoughtful consideration as we continue to assess the impacts of climate change on the future of loggerhead sea turtles.

Nesting preferences of the green sea turtle (Chelonia mydas L.) and the hawksbill sea turtle (Eretmochelys imbricata L.) in the SW of Mahe Island in the Seychelles

Data concerning 212 turtles emerging on the southwest beaches of Mahe Island in the Seychelles were collected in 2017 and 2018. These data were used to model the probability of eggs being laid in relation to several variables. The probability of successful laying after emergence was highest on certain beaches and in areas of short vegetation, between open sand and trees. We found successful laying was related to the physical properties of the soil, indicating that survivability of embryos and hatchlings is higher in certain areas. The turtles appeared to choose zones where soil had low salinity, good drainage but ability to retain water, and absence of spring tides and extreme temperatures.

Seasonal and annual variations in egg mass and clutch size for Loggerhead Sea Turtles (Caretta caretta): experienced females lay heavier eggs

Organisms modify reproductive traits adaptively or non-adaptively in response to temporal environmental variation. Long-lived iteroparous sea turtles are ideal animals to examine such temporal shifts in resource allocation. We analyzed seasonal shifts in egg mass and clutch size for Loggerhead Sea Turtles (Caretta caretta (Linnaeus, 1758)) nesting at a temperate rookery (Yakushima Island, Japan) over a 2-year period, as well as annual variation in egg mass and clutch size over a 5-year period. Egg mass and clutch size, adjusted for female body size, did not vary seasonally at both the population and individual levels. Adjusted egg mass showed significant annual variation, despite a lack of annual variation in adjusted clutch size. Thus, Loggerhead Sea Turtles are unlikely to exhibit adaptive seasonal variation in reproductive traits, whereas they vary egg size non-adaptively in response to annual environmental conditions. Although experienced Loggerhead Sea Turtles laid heavier eggs, the annual variation in egg mass was not attributable either to breeding experience of the sampled females or to ambient temperature during follicular development, implying that other factors are involved, such as resource availability. Our data show that egg size is a more plastic reproductive trait than clutch size for Loggerhead Sea Turtles inhabiting the North Pacific.