Late last year, while on a tour of California’s Año Neuvo State Park, I saw a shark attack victim lying on the beach. She was a Northern elephant seal, and looked quite placid despite the gaping, crescent-shaped hole in her neck. She bore the traumatic hallmark of the great white shark.
Years of watching Discovery’s Shark Week taught me that seals and sea lions are the preferred prey of Carcharodon carcharias. Nothing like blubber to fuel the body of a constantly-swimming predator with a physiology that runs hotter than that of the average shark. I remember one researcher likened baby elephant seals, in particular, to hot dogs – the bread of the snack corresponds to the fat content of the young pinnipeds, making the weener seals easy-to-catch and energy-rich mouthfuls for the sharks.
When the sensational documentaries weren’t showing awful re-enactments of great white shark attacks on humans, they brought their cameras in close to seal kills. The programs took a philosophy similar to the fictional marine biologist Matt Hooper in JAWS – all great white sharks do is swim and eat. (Yes, yes, “and make little baby sharks”, but I have yet to see that on basic cable.) If they aren’t chomping people, then they strip the fat from seals. We think of them in the typological way that we approach many species. Great white sharks eat seals and sea lions. That’s all that there is to it.
But great white sharks don’t live on a strict diet of marine mammals. Study sites situated near pinniped colonies, as well as nature films, have restricted our view of what great white sharks feed on. In actuality, great white sharks consume different prey based upon age, size, and location. When they are just pups, for example, the leviathans-to-be seek out a wide variety of smaller fare before graduating to more difficult menu options. And, as a new paper indicates, many sharks retain their cosmopolitan tastes as they age.
In a PLoS One study published this week, University of Wyoming researcher Sora Kim and colleagues used chemical clues in great white shark vertebrae to track feeding preferences among fifteen individuals collected between 1957 and 2000. The logic behind their technique is simple, and has been used on a variety of other creatures – living and extinct – to outline diet. As an animal feeds, chemical tracers in the form of carbon and nitrogen isotopes become incorporated into their teeth and bones. There is a correspondence between certain carbon isotope ratios and particular food sources. Match the chemical signature in the consumer in with the isotopic profile of what’s being consumed, and you can reconstruct an animal’s diet.
Even better, shark vertebrae contain long-running records of these isotopes. As great white sharks grow, their vertebrae accrete new rings on a yearly schedule. Each ring, therefore, holds a chemical snapshot from a year in the shark’s life. By comparing the isotope ratios in different vertebral bands, Kim and collaborators were able to follow how the diets of individual sharks shifted during their lives.
Contrary to assertions that pinnipeds are a great white shark staple, the fish sampled in the study were highly variable. Both age and individual variation were at play in their diets. For example, five sharks in the sample showed the expected shift from a diet of fish and small prey to marine mammals and other more substantial fare at about age four. But this wasn’t true of all sharks. Five other sharks in the same sample showed no difference between juvenile and adult diet. These sharks may have scavenged pinniped carcasses or fed on large squid while young, giving them an “adult” feeding profile at a young age. There are some possible confounding factors with this hypothesis – such as young sharks inheriting an adult isotope signal from their mothers – but the researchers appear to favor the idea that some sharks were more precocious in their prey choices than others of their kind. Not all great white sharks follow the same life history.
While Kim and colleagues point out that some sharks followed the expected dietary switch, the change was not the dominant signal in their results. Many of the Pacific great white sharks they sampled were generalists who took different prey in varying locations. Some sharks were nearshore marine mammal specialists, but others had more flexible foraging approaches. And even though the isotopic data are not refined enough to tell us exactly what species the sharks were eating, the cataloged chemical traces are enough to detect distinct dietary patterns.
The study raises new questions about great white shark biology. For one thing, why did the sharks have such individualistic diets? Competition may be the key, Kim and co-authors hypothesize. Imagine if all adult great white sharks were seal specialists who congregated at the same beaches. There may not be enough food for all, and swimming in the same waters as bigger, more experienced sharks would be risky for smaller novices who could wind up as meals themselves. By being flexible – able to tackle elephant seals as well as squid, tuna, and other food sources – great white sharks may lessen competition with their own kind.
As the researchers behind the new study state, further isotopic studies and satellite tracking programs may help marine biologists better understand the ecology of their prodigious fish. For now, though, one thing is clear. The sharks weren’t all cruising near shore, looking up for seal silhouettes. Great white sharks have much more varied tastes than blood-spattered basic cable shows would have you believe.
Kim SL, Tinker MT, Estes JA, Koch PL (2012). Ontogenetic and Among-Individual Variation in Foraging Strategies of Northeast Pacific White Sharks Based on Stable Isotope Analysis PLoS One DOI: 10.1371/journal.pone.0045068