Can leftovers from predators be reliably used to monitor marine turtle hatchling sex-ratios? The implications of prey selection by ghost crabs

Abstract

In marine turtles, the sex of an individual is determined by temperatures experienced during embryonic development. Gonad histological observation is still the only reliable way to determine hatchling sex, hampering the study of reproduction and of the demographic consequences of context-dependent sex-ratios, a subject of interest in a warming planet. We investigated whether hatchling remains from predation by Ocypode cursor can be used to estimate sex-ratio trends in a green turtle rookery at Poilão, Guinea-Bissau (10°52′N, 15°43′W). Sex could be readily determined in 77 and 79% of the predated hatchlings in 2008 and 2009, respectively. By comparing hatchlings killed by crabs, hatchlings accidentally dying on the reefs, and live hatchlings, we show that ghost crabs select the smaller prey, but do not select according to hatchling sex, which is explained by the lack of hatchling size dimorphism in this population. The proportion of male hatchlings was 0.45 ± 0.06 and 0.15 ± 0.06 for early and late-season clutches, respectively, these differences most likely being explained by rainfall. Using leftovers from predation by crabs may be a good solution to non-invasively monitor broad trends in sex-ratios of sea turtles.

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Acknowledgments

Our work on sea turtle sex-ratios on Poilão is supported by Project 09/74 of MAVA—Fondation pour la Nature, by Project 77340-000 of IUCN—International Union for Nature Conservation and by FCT—Fundação para a Ciência e a Tecnologia (Portugal) through Programa Plurianual (UI&D 331/94). We also thank the support by the “Fondation Internationale du Banc d’Arguin” (FIBA). Permits were issued by IBAP—Instituto para a Biodiversidade e Áreas Protegidas of Guiné-Bissau. Authorization for euthanization of a sample of hatchlings in 2009 was obtained from the IBAP and the João Vieira and Poilão Marine National Park Direction. P. Segurado draw Fig. 1a. We thank the comments of two reviewers and are grateful to Thierry Renaud and Mathieu Ducroq for support and encouragement, for the support of the National Park staff João Pereira, Santinho, Rofico, António Baca, César Banca, and especially of the IBAP Director, Alfredo Simão da Silva.

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Correspondence to Rui Rebelo.

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Communicated by R. Lewison.

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Rebelo, R., Barbosa, C., Granadeiro, J.P. et al. Can leftovers from predators be reliably used to monitor marine turtle hatchling sex-ratios? The implications of prey selection by ghost crabs. Mar Biol 159, 613–620 (2012). https://doi.org/10.1007/s00227-011-1839-8

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Keywords

  • Carapace Length
  • Green Turtle
  • Carapace Width
  • Nest Temperature
  • Nest Beach