Science

Sea Turtles Escape Tiger Sharks By… Not Moving

Have you ever wanted to experience the ocean through the eyes of one of its top predators? Well, thanks to crittercams, scientists can! Safely worn by wildlife, this technology captures video, sound, and other data that is giving researchers rare views into the private lives of animals.

Sure, it makes for a cool TikTok video to see from a shark’s point of view, but the footage captured also gives us a glimpse into how these predators hunts. “The animal borne cameras are now commonplace in the field of ecology, but few researchers have taken the next step to really consider the videos they provide in terms of what the subject animals can actually see. This is the next frontier in this form of tagging,” says co-author Dr. Mark Meekan from the Australian Institute of Marine Science. New research published in the Journal of Experimental Marine Biology and Ecology does just that, analyzing the hunting behavior of tiger sharks (Galeocerdo cuvier) thanks to the combined knowledge we already have on the structure of their eyes with images of prey and habitat from small cameras deployed on these animals.

A common shark in tropical and sub-tropical waters throughout the world, it is easily recognized by the tiger-like stripes on its body that fade with age. A scavenger, tiger sharks aren’t picky with what goes in their stomach, eating a wide range of prey and even some indigestible objects like tires and license plates! In Australia, they are observed from south-western Western Australia around the tropical north and south to the southern coast of New South Wales coast. It was within this range in Ningaloo Reef where an international team of scientists (from Macquarie University, the Australian Institute of Marine Science, University of Western Australia, Murdoch University, Stanford University and Oregon State University) attached small video camera tags to them. Ningaloo is home to the world’s largest fringing reef and is known for various wildlife like one of the tiger shark’s favorite prey animals: sea turtles.

The battle between sea turtle and tiger shark has been captured before (see this encounter at Ningaloo earlier this year) but never from the shark’s point of view. A new virtual visual system for the sharks was used to process the videos from the crittercam tags to understand how these predators visually experience interactions with turtles. The tags were equipped with special movement sensors to track the fine-scale movements of the shark so scientists could really focus on their minute reactions to the world around them.

“When watching the raw camera footage of tiger sharks approaching sea turtles it seemed strange that often tiger sharks swam directly over a turtle sitting on the reef, a potentially easy meal,” says lead author Dr. Laura Ryan from the School of Natural Sciences at Macquarie University. “However, when we look at the visual cues through the visual system of the tiger shark it is in fact extremely difficult to detect the turtle, and particularly when they remain motionless, blending into the background can allow them to camouflage themselves from attack.”

Yup. Sea turtles play the deadliest game of “hide-and-seek” and stay completely still to avoid detection by prowling tiger sharks.

“Tiger sharks have much lower visual acuity than humans and the video cameras. This means the sharks must rely on any form of movement from the sea turtles to be able to identify them. For sea turtles, their best form of defence from attack may be to simply remain still in the presence of the predator,” says Dr. Ryan.

For turtles who visually stood out, the crittercams showed a change in the tiger shark’s behavior and movement highlighting that although it may not be their strongest sense, vision is still a key sensory system for these predators. Once a sea turtle was seen by a shark, they slowed down and began to turn a lot, suggesting were on “search mode” for their prey.

“The picture that emerges through the eyes of the shark is one of almost slow-motion pursuit of a slow-moving prey, rather than a high velocity ambush that we tend to think of when we see other big predators in action, such as white sharks,” says co-author Dr. Samantha Andrzejaczek from the Hopkins Marine Station at Stanford University. “This probably reflects the fact that these sharks inhabit environments that are generally nutrient poor, and these predators have to be careful not to expend too much energy in chasing prey to make a meal.”

The new research is fascinating, especially since the Australian Marine Conservation Society (AMCS) and Humane Society International (HSI), called for the species to be listed as endangered back in 2019 after a reported a 71 percent decline of tiger sharks in only three decades. Another study indicates that the oceanic continent wiped out a genetically distinct population of tiger sharks before it was even known that they existed! AMCS shark expert Dr. Leonardo Guida told Yahoo! News that the new study’s results should be taken as a ‘canary in the coal mine’: “Tiger sharks are caught incidentally in the commercial fisheries of New South Wales and Queensland and are kept for their meat and fins. In Australia, their numbers are declining and fishing rules specific to this species are sorely needed.”

The team hopes they can now apply this approach to other shark species.


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