The anti-predator role of within-nest emergence synchrony in sea turtle hatchlings

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.

Keywords: anti-predator behaviour; attack abatement; dilution effect; predation risk; sea turtles; synchronous hatching.

Figure 1.

Temporal distribution of emerged green turtle hatchlings (columns, data from the nest emergences); mean (±s.e.) density of yellow crabs at night (solid line) and mean (±s.e.) relative number of crabs that have captured a green turtle hatchling (dashed line, data from crab surveys). (Online version in colour.)

Figure 2.

Number of emergence events per nest for the 33 green turtle nests from Trindade Island, Brazil. (Online version in colour.)

Figure 3.

Determinants of predation risk in green turtle hatchlings. Per capita predation risk is represented by bubble area; groups without any mortality (i.e. zero risk) are represented by diamonds. Risk is plotted against group size and (a,b) time of day or (c,d) number of prey already killed that evening. (a) and (c) show the observed risk per group, while (b) and (d) show the fitted (i.e. predicted) risk from GLMMs with the two axes as interacting covariates and nest as a random factor. (Online version in colour.)