It's hard to imagine sharks as scavengers, but at least one species doesn't mind becoming one when the chance presents itself.
That was the observation of researchers from the United States, England and Australia, who have documented a kind of scavenger mode tiger sharks adopt during green turtle nesting season in waters off the Great Barrier Reef's Raine Island.
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The island's beaches comprise the biggest green turtle egg-laying site in the world, hosting more than 10,000 females at nesting season. This draws tiger sharks to the scene, the biggest predatory sharks that haunt the Great Barrier Reef. They'll hunt down a wide variety of animals, with green turtles being fair (or fare) game.
However, the scientists say, attacks on live turtles in the island's waters during nesting season are rarely seen. With multiple thousands of turtles swimming in and out of the waters leading to the beach, why are so many of them left unharmed?
It turns out that the sharks in the area, despite their hunter's instincts, take an easier path to their next meal.
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The researchers examined Raine Island satellite tracking data on the movements of tiger sharks and sea turtles that had been collected during an earlier study.
They found that during green turtle nesting season, the sharks spent the better part of their time cruising in waters closer to the shore instead of hunting further out in deeper seas.
This preference for patrolling the shoreline, alongside the lack of attacks on live turtles, told the researchers that the sharks were feasting on turtle carcasses, which are plentiful at that time, as many turtle moms die of exhaustion or exposure during nesting season.
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The live turtles, for their part, seemed to know they were not being hunted. During nesting season, their movement data did not include typical predator-avoidance strategies they'd usually employ, such as spending less time at surface waters where they can be easily seen, and attacked from below, by hungry sharks.
Here's one of the study's authors, Richard Fitzpatrick, of James Cook University, discussing earlier endeavors to track the Raine Island tiger sharks and making note of them feeding off turtle carcasses: