Exceptionally high natal homing precision in hawksbill sea turtles to insular rookeries of the Caribbean

Marine turtles migrate back to their natal region during reproduction, but the precision of this homing behavior and how the precision varies among populations and across biogeographic regions are unclear. We hypothesize that marine turtles nesting on insular landmasses navigate to their rookeries with greater precision than those nesting on continuous coastlines. We analyzed new mitochondrial and microsatellite marker data from hawksbill turtles Eretmochelys imbricata at nesting sites across Antigua and Barbuda, West Indies, to assess the scale of natal homing in the highly insular Leeward Islands. We then used published data from 15 western Atlantic rookeries to examine regional patterns of rookery structure. Mitochondrial control region data showed weak to no partitioning among nesting sites within Antigua and strong partitioning between Antigua and Barbuda, suggesting natal homing at a scale of 50 km. Microsatellite data showed weak to no partitioning between sites, indicating male-mediated gene flow. Regionally, we found stronger population structuring among rookeries of insular landmasses than among those of larger landmasses with continuous coastlines, despite shorter average rookery separation for the former. We also found a positive relationship between a rookery's isolation index (a metric incorporating distances from larger landmasses) and its genetic divergence from proximate rookeries. These findings support our hypothesis, and we caution that insular rookeries that host marine turtles with extreme homing behavior have limited ability to colonize new nesting habitat. The unprecedented rates of development and increasing instability of present-day nesting habitat might therefore pose a greater and increasing threat to insular rookeries.