Behavioral evidence suggests facultative scavenging by a marine apex predator during a food pulse
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The ability of predators to switch between hunting and scavenging (facultative scavenging) carries both short-term survival and long-term fitness advantages. However, the mechanistic basis for facultative scavenging remains poorly understood. The co-occurrence of tiger sharks (Galeocerdo cuvier) and green turtles (Chelonia mydas) at Raine Island (Australia), provides an opportunity to examine a top marine predator’s feeding mode in response to seasonal pulses in nesting turtles that offer both hunting and scavenging opportunities. Using satellite telemetry, we evaluated home range overlap between sharks and turtles and quantified their surfacing behavior around Raine Island during the turtle nesting season. We found core home range overlap to be highest during the nesting season. Both sharks and turtles spent significantly more time at the surface in areas of greatest range overlap closest to shore, where turtle density was highest. Both sharks and turtles showed decreased surfacing with increasing distance from Raine Island. Combined with published data on turtle demography at Raine Island, we propose the following: (1) sharks patrol the surface to increase scavenging opportunities on turtle carcasses and intercept weakened individuals after nesting; (2) healthy turtles may not perceive sharks as a major threat and/or other biological factors override anti-predatory responses; and (3) sharks during the nesting season may primarily scavenge on dead turtles individuals rather than actively hunt. Our study results and approach may be applicable to other situations in which direct observations of predator-prey interactions are limited.
Every animal encounters dead or dying resources, yet the role of facultative scavenging has been difficult to study, and thus largely overlooked in marine behavioral ecological research. Movement analyses of tiger shark and green turtle movement and surfacing behavior at Raine Island (Australia) suggest that facultative scavenging may be a prevalent, yet underappreciated, feeding strategy in tiger sharks. Our integration of behavioral ecology theory with multi-species electronic tagging provided a valuable approach for investigating predator-prey interactions in situations where direct observations are limited or not possible.
KeywordsKeystone species Predation risk Scavenging Shark Turtle Telemetry
We thank J. Rumney from Eye to Eye Marine Encounters and the crew of Undersea Explorer for help with data acquisition. Thanks also to Emily Nelson, Julia Whidden, and Rachel Skubel for helping with formatting and proof editing the manuscript. Thanks to the editor and reviewers, whose comments helped significantly strengthen this paper.
Compliance with ethical standards
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. Research was approved and conducted under Australian Fisheries Management Authority Scientific Permit #901193 and Great Barrier Reef Marine Park Authority G11/33231.1. Funding was supplied by Digital Dimensions, Australia, through the production of documentaries related to shark research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. All authors confirm no conflict of interest or competing interests with respect to this study.
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