The olive ridley sea turtle, Lepidochelys olivacea, is widely distributed in all oceans except the Mediterranean Sea, and is currently listed as Vulnerable by IUCN due to declining numbers in the past several decades (< http://www. redlist. org>). Some populations of olive ridleys, along with Kemp’s ridleys, exhibit a particular nesting strategy called arribadas, which consist of the simultaneous nesting of hundreds and even thousands females on a relatively small portion of beach (Hughes & Richard 1974). Ridleys also exhibit solitary nesting, the common reproductive behavior of other chelonians. The occurrence of these two contrasting nesting strategies, unique for ridley sea turtles, may demonstrate a capacity to maximize offspring survival in a complex ecological environment (Bernardo & Plotkin 2007).

In the Eastern Pacific (EP), the olive ridley is the most abundant sea turtle species and ranges from US to central Chile; however, it is most commonly observed in waters off Mexico and Central America (Eguchi et al. 2007; Olson et al. 2001a, 2001b). Currently, arribada beaches occur in Panama, Costa Rica, Nicaragua and Mexico (Abreu-Grobois & Plotkin 2007; NMFS & USFWS 1998). Non-arribada (ie solitary) beaches are located mainly in Mexico but nesting is reported from Mexico to Colombia and rarely from Ecuador and Peru (Hays-Brown & Brown 1982; NMFS & USFWS 1998). All the females that nest in the different rookeries in the EP are believed to belong to the same subpopulation (Abreu-Grobois & Plotkin 2007); however, it seems that some demographic independence does exist among beaches (Abreu-Grobois & Plotkin 2007; Lopez-Castro & Rocha-Olivares 2005).