Abstract
Group formation is a common behaviour among prey species. In egg-laying animals, despite the various factors that promote intra-clutch variation leading to asynchronous hatching and emergence from nests, synchronous hatching and emergence occurs in many taxa. This synchrony may be adaptive by reducing predation risk, but few data are available in any natural system, even for iconic examples of the anti-predator function of group formation. Here, we show for the first time that increased group size (number of hatchlings emerging together from a nest) reduces green turtle (Chelonia mydas) hatchling predation. This effect was only observed earlier in the night when predation pressure was greatest, indicated by the greatest predator abundance and a small proportion of predators preoccupied with consuming captured prey. Further analysis revealed that the effect of time of day was due to the number of hatchlings already killed in an evening; this, along with the apparent lack of other anti-predatory mechanisms for grouping, suggests that synchronous emergence from a nest appears to swamp predators, resulting in an attack abatement effect. Using a system with relatively pristine conditions for turtle hatchlings and their predators provides a more realistic environmental context within which intra-nest synchronous emergence has evolved.
Highlights
Individuals aggregating in temporary or permanent groups is a common behaviour among many species
We used a generalized linear mixed model (GLMM) with a binomial error distribution to test the effects of group size, time of day, date and the distance from the nest to the high tide line, with two-way interactions included between group size and each of the other variables
Positively related to the time of day as expected, the two variables were not collinear (Spearman’s rank: rs 1⁄4 0.43, p 1⁄4 0.0014). When this variable and its interaction with group size was included in the model explaining predation risk, the previously significant interaction between group size and time became non-significant (GLMM: F1,12 1⁄4 0.47, p 1⁄4 0.51), while the interaction between group size and number of hatchlings already depredated was significant (F1,15 1⁄4 6.20, p 1⁄4 0.025; all other effects p . 0.1)
Summary
Individuals aggregating in temporary or permanent groups is a common behaviour among many species. They did not present the relationship between per capita risk and group size, leaving it unclear whether dilution counterbalanced the suggested increased encounter rate with predators [20] Neither of these previous studies demonstrates that synchrony in emerging from a sea turtle nest has an anti-predator role, and it remains unknown whether the net effect of aggregation is to decrease per capita predation risk in natural systems [20]. We used a GLMM with a binomial error distribution (glmmPQL was used as the data were overdispersed) to test the effects of group size, time of day, date and the distance from the nest to the high tide line, with two-way interactions included between group size and each of the other variables (non-significant interactions were removed as above). All analyses were performed in R v. 2.15.1 [37]
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