This is what a tiger shark's back-off warning looks like (video)

Tiger sharks have a reputation as the more aggressive species among their finned kin, but mindless killing machines they certainly are not. In fact, as this close encounter filmed off the coast of Western Australia shows, these predators can display very nuanced behaviour.

During a snorkelling trip, Coral Bay resident Tim Van Beelen noticed a tiger shark swimming erratically near his vessel. The animal was hovering just above the seabed, moving in tight circles as if preoccupied with something located beneath it. Van Beelen decided to jump in for a closer look, and a quick peek under the surface revealed that the shark was attempting to feed on a sea turtle.

It should go without saying that interrupting a predator during mealtime is never a good idea – and Van Beelen acknowledges the risk he took by approaching the animal. He made several dives towards the turtle-tiger shark pair without trouble, but things escalated when the shark suddenly turned and headed in his direction, mouth agape.

"This is what happens when you get in the way of a tiger shark and his dinner!" he later wrote on Facebook. "I was aware of the situation I had put myself in, and knew I had to keep my distance to avoid interfering with nature. What happened next I definitely did not expect and will never forget!"

University of Miami shark researcher Dr Neil Hammerschlag, who has done extensive work with tiger sharks in the Bahamas and elsewhere, suspects that this individual mistook Van Beelen for a potential competitor. "Sharks and other ocean-scavenging predators are often drawn to carcasses and will compete for them, with one winning out, often not the one that even made the initial kill," he explains.

Sea turtles can be challenging prey, even for tiger sharks. The crafty reptiles have learned to dodge their finned assailants by swimming in tight circles (check out that "cold shoulder" evasion technique here), and the tactic can be difficult to outmanoeuvre – especially for juvenile sharks like the one in this video.

It's very possible that this young predator had been chasing the turtle for some time, so it's not surprising that it was so eager to guard its spoils. "I believe the shark likely thought the diver was another predator that had been attracted to the scene by the smell of the carcass and sound of it being torn apart. With that in mind, it's possible the tiger thought the snorkeller, as another predator, wanted to compete for the turtle carcass."

This explanation fits both the gradual change in the shark's behaviour, and the nature of those changes. For example, the tiger shark remained tolerant of Van Beelen's presence for several minutes. It wasn't until the diver repeatedly swam towards the carcass that the shark turned its attention on the "interloper".

And despite what a few dramatic news headlines would have you believe, Van Beelen was not afraid for his life during the encounter. "Tiger sharks are not the sharks the media makes them out to be," he said. "They are very placid and generally avoid any contact with us. This was not an attack, it was simply a warning to get away. You can see how slowly the shark swims up and doesn't even attempt to bite me. If he wanted me, he would've had me, plain and simple."

The "warning" Van Beelen received mirrors actual shark-on-shark interactions of this nature: their rivalries rarely turn violent (though it does sometimes happen) given the high-risk nature of such events. Healthy individuals tend to settle their clashes with warning signs and bravado, since the outcome of an all-out battle can be detrimental – even deadly – for both sides.

"In this case, the tiger shark showed off some typical behavioural posturing that is seen among sharks competing for a carcass," explains Hammerschlag. The mouth-gaping behaviour seen in the video is a known agonistic display – a way for sharks to tell other sharks to back off. Exaggerated tail movements, lowered pectoral fins and swimming directly towards the competition also sends a similar message. Van Beelen's hasty retreat at the shark's approach may well have signalled to the animal that its "competitor" had backed down from the turtle snack.

"I came away from this experience with a lot of thoughts," says Van Beelen, "but I have not grown any fear. If anything, a ton more respect for these amazing animals."