No creature has a reputation more fearsome than the great white shark. Despite all we’ve learned about them, including how they really don’t have much interest at in all eating us, movies and basic cable documentaries still show them as “machines” that do little more than “swim and eat and make little sharks.” And that’s not to mention the various video games where your goal as a great white is to chomp everything in sight in as little time as possible.
But what do great white sharks really do all day? It’s easy for the mythology of these predators to overshadow their real biology because it’s difficult to spend an extended amount of time following and observing animals that live beneath the waves and can cross entire oceans. We mostly see these burly sharks when they’re near the surface, and, while ingenious, strategies like Crittercam have literally been limited in scope and what can be recorded. That’s why shark researcher Gregory Skomal and colleagues turned to a different technology to see what the great fish are up to.
Thanks to documentaries and celebrity sharks like “Deep Blue“, Guadalupe Island off the coast of Mexico has become known as a great white hotspot. Yet, despite the abundance of sharks and observers – including cage divers – in the area, no one has seen how these sharks go about getting their meals. At locales off California and South Africa great white sharks make the most of their natural countershading to hide their outline against the bottom while looking up to the surface for seals to surprise. Some propel themselves with such power that they actually launch themselves out of the water in the process. But no one has seen behavior like this at Guadalupe Island.
The great white sharks of Guadalupe Island feed on the fur seals, elephant seals, and sea lions that loll about in the shallows there. Sharks have been seen feasting on the blubbery mammals at the surface. But the initial strikes have never been seen. Given the waters around Guadalupe Island rapidly drop off from the shoreline, Skomal and coauthors write, some researchers expect that the sharks are attacking their prey at depth and follow the carcass up the water column as it bobs to the surface.
To find out, the ichthyologists turned to an autonomous underwater vehicle or AUV. It looks like a shark-seeking torpedo, except with cameras and scientific equipment rather than explosives. So after tagging a shark with a device that would allow the researchers to follow the location of the fish remotely, the researchers launched the AUV to follow the shark and hopefully catch one in pursuit of mammalian prey.
Unfortunately, none of the four sharks actively tracked in the study felt particularly peckish. Or, at least, the AUV couldn’t see it. Some of the predators dove much deeper than the limits of the AUV, sometimes spending the bulk of their tracked time down below. But Skomal and colleagues propose that they were able to catch a different sort of predatory behavior on camera.
Even though the team actively tracked four sharks over six sessions, the cameras picked up at least eight other great white sharks in the area. Some of these sharks apparently didn’t take too kindly to the AUV. Altogether, great white sharks approached the AUV 17 times, bumped it four times, and bit it nine times. These interactions sometimes went beyond gentle, exploratory mouthings. During one mission, the tracked shark was down near the bottom while another swam up from below and behind and bit the AUV for 11 seconds, returning to bite it four more times over the next eight minutes. Then a different shark came by a half an hour later and chomped the device so hard that she breached the AUV’s hull.
Being that none of us know the mind of a shark, it’s difficult to say for sure what these fish were doing. Were they truly considering the AUV as prey, or were they frustrated with the buzzing oddity making a ruckus on their turf? There may not be a conclusive answer, but Skomal and coauthors argue that at least some of these sharks were biting to kill. When great whites fight they tend to bite each other around the pectoral fins and heads, but they tend to target the rear of creatures they aim to eat – biting off the rear flippers is one way to immobilize a seal. The fact that most of the sharkbite damage to the AUV was to the rear suggests that the sharks were trying to use the same strategies they employ to kill seals, regardless of what their motivation was.
But it’s not as if all the sharks in the area immediately descended on the AUV to snap it in half. For the most part, the observed sharks just… swam around. They followed the coast, dove deep for a while, swam by other sharks, and just generally propelled their bulk through the water with swishes of their crescent-moon tails. Some were even curious about the AUV, popping up and down from deeper water to investigate the weird object. In a mammal, we’d probably have no qualms about calling this curiosity. So even though the researchers didn’t exactly get what they were hoping for, they picked up something different – the mostly-peaceful and inquisitive ramblings of some of the greatest predators in the sea.
Skomal, G., Hoyos-Padilla, E., Kukulya, A., Stokey. R. 2015. Subsurface observations of white shark Carcharodon carcharias predatory behavior using an autonomous underwater vehicle. Journal of Fish Biology. doi: 10.1111/jfb.12828