Differential Nesting in the Leatherback Sea Turtle (Dermochelys coriacea) at Pacuare Nature Reserve, Costa Rica

Leatherback sea turtles (Dermochelys coriacea) at Pacuare Nature Reserve have been observed over the past ten years to preferentially nest in certain beach sections. The aim of our study was to test this general observation by comparing nesting activities and false crawls within and among sections of the 6km beach that have been historically identified as high(HDNA) and low density nesting areas (LDNA). Additionally, we relate physical beach variables (e.g., slope, width, sand grain composition, sand color composition, substrate water content, and temperature), near-shore parameters (e.g., bathymetry) and nest characteristics (e.g., nest depth, distance to vegetation, and number of eggs) to the hypothesized difference in nest preference using non parametric statistics and indirect gradient analyses. Between May 6 and June 4th 2011, 60 nesting and 25 false crawl events were intercepted. Our results support significant differences in nesting densities with twice as many turtles observed nesting in HDNA as opposed to the LDNA, supporting 8 years of anecdotal evidence of nest preference at Pacuare. While we report no significant differences in many of the physical parameters measured (e.g., sand color, moisture content, nesting distance from mean high tide water line), HDNA nests were significantly farther away from vegetation than nests in the LDNA (11.49 m vs. 7.6m, p=.02). Most metrics typically reported of nesting turtles (e.g., curved carapace length and width, clutch size) were invariant relative to nesting density, though nest depth was significantly deeper in low density regions (78.56 cm vs. 71cm p=.0007) . Initial bathymetry analysis suggests that gently sloping regions of the surf zone and shallow offshore are correlated to higher nesting regions. Our research suggests that the critically endangered Leatherback sea turtles of Pacuare Nature Reserve preferentially and differentially nest in specific beach regions that are related to physical and biological parameters starting offshore and continuing until nest selection. This study provides an initial step into evaluating the potential for survival differences of nests placed in such a differential manner