Shark-Eat-Shark: Are Great White Sharks Cannibals?
A "colossal cannibal great white shark" made headlines this week after it supposedly gulped down a 9-foot great white shark as though it were a steak dinner, but the jury is still out and many experts believe that great whites are pickier eaters than most people might think.
The colossal cannibal theory is posited in the forthcoming documentary film "Hunt for the Super Predator," which airs in the United States on June 25. In the film, researchers find a tracking device that had previously been planted on the 9-foot-long great white. The device washed up on an Australian beach, and recorded that it had undergone a 30-degree spike in temperature before plunging down 1,900 feet in the ocean.
The heat could have resulted from the shark (and tracking device) going into another animal's digestive system before the hunter nose-dived into the watery depths, filmmaker Dave Riggs explained, adding that large great white sharks were later spotted where the possible fatal plunge took place.
The great white eats great white shark tale has since gone viral.
"I too have heard portions of the 'white shark cannibalism' story in the popular press and over the radio," John McCosker, chair of aquatic biology at the California Academy of Sciences, told Discovery News.
"I don't hold much stock in anything that I have heard, and suspect that there is a more reasonable explanation," he added.
Thus far, the only direct evidence appears to be the missing shark, the rogue tag, its data, and the presence of other sharks in the area. If this incident involved people, the accused could not be convicted. Even the fate of the 9-footer is unclear. If the shark did become another animal's supper, orcas also live in Australian waters and have been filmed feasting on great whites. In other words, killer whales can kill killer sharks.
It's true that great white sharks have been documented practicing cannibalism, but this occurs primarily before birth.
"Great whites come out fighting; they are born well developed and must fend for themselves from day one," Stephen Wroe, a senior research fellow in the University of New South Wales' Evolution & Ecology Research Center, told Discovery News.
Wroe added that "intrauterine (pre-birth) cannibalism is practiced by white shark pups, and it is likely that any white shark that makes it into the big wide world has already eaten one or more of its siblings. It's a hard life!"
A great white shark feasting on a whale.Daniel Mietchen, Wikimedia Commons
As teenagers, he said, great whites primarily feast on fish and go through an awkward hunter stage when they cannot hunt large prey very effectively.
"It seems paradoxical that the iconic jaws of great white sharks, made infamous by the classic Steven Spielberg movie "Jaws," are actually rather vulnerable when these sharks are young," Wroe's colleague Toni Ferrara said. "Great white sharks are not born super-predators; they take years to become formidable hunters."
It might seem that, as adults, great whites would dine often and well during their ocean migrations. Prior research, however, has found that the sharks fuel up -- especially with elephant seal flesh -- before moving offshore.
Great white sharks store energy in the form of oil in their massive livers. The oil also helps with buoyancy, Barbara Block and her team from Stanford University found. Too much girth would weigh the sharks down, so there is not much incentive for an adult great white to overeat, or to practice cannibalism, if it is not starving to death and if other desirable prey is available.
In fact, when great whites scavenge on whale carcasses, they are rather picky. Researchers from Apex Expeditions, in collaboration with University of Miami Scientists, determined that adult great whites preferentially feed on the whale's fluke, leaving the lower quality parts for smaller, younger sharks.
McCosker indicated that adult great whites are capable of eating each other, and mentioned that other shark species, such as sevengill cow sharks, regularly feed on other sharks.
"However, I don't know if they are cannibalistic," he said. "I am not aware of adult white sharks cannibalizing other adults."
If the "colossal cannibal great white shark" theory does prove true over time, it's likely to have been similar to the Donner Party tragedy: rare and out of desperation.