This little bit of pseudoscience is a perfect example of how popular culture can distort our view of how nature works.  Most people are familiar with Flipper, whether it’s the old TV show from the 60’s, the short-lived 90’s reboot featuring Jessica Alba, or the movie starring Elijah Wood and featuring a robot hammerhead.  Even if they’re not huge Flipper fans, most people seem to be under the impression that pods of dolphins regularly beat on sharks, which are reduced to a Wile E. Coyote-esque pursuit of these cute, intelligent sea mammals.  What’s more, dolphins are in the regular practice of protecting people from sharks, because cute, intelligent animals have to stick together, right?  So how does this pan out in real life?

First, let’s take a look at the Flipper model of dolphin/shark interactions.

So here we have Sandy being menaced by a (rather diminutive when you consider Sandy is like 10 years old) tiger shark while snorkeling in the Florida Keys.  Heroic bottlenose dolphin Flipper sees this going down, steps in and easily beats down the mean old shark, and everyone lives happily ever after (except of course the shark).  Meanwhile the shark appears to be either dead or heavily sedated and can’t seem to decide whether it’s a tiger shark or a Caribbean reef shark.  Snarky nitpicking aside, ask most people what interactions between sharks and dolphins are like in the wild and they’ll probably describe something akin to this, often complete with human-in-distress.

But really, it’s not the “dolphins saving humans” aspect that bothers me and I’m sure someone else could cover that better than I could.  It’s the idea that sharks are reduced to hapless cartoon villains when they come up against dolphins.  Since this is an Ocean of Pseudoscience post, you know it’s time for some bubble-bursting.

Fortunately, those of us with a pro-shark bias have Heithaus (2001), a very thorough meta-analysis of known scientific data pertaining to dolphin/shark interactions.  With one noteworthy exception, I can happily report that large sharks regard most dolphins as either an equal competitor or prey (sadly, spiny dogfish appear to be on the menu for some of the larger dolphins and porpoises).  Based on a compilation of diet data, field observations, and the occasional anecdote, Heithaus (2001) breaks some of our favorite sharks into categories based on how they interact ecologically with odontocetes.

“Regular predators” include heavyweights like the great white, tiger, and bull sharks, along with a couple surprises in the form of sixgill and sevengill sharks (in a nifty role-reversal, sevengills apparently bring down marine mammals by hunting cooperatively).  These are the sharks that can make mincemeat of Flipper and, according to Heithaus, “are almost certainly a predation threat” to any dolphin sharing the same waters.  Dusky and oceanic whitetip sharks are considered “occasional predators,” since these species are primarily fish-eaters but when fully grown are known to munch a dolphin or two.  Interestingly, the shortfin mako is considered only a “suspected predator” based on its size, power, and appetite (there isn’t much evidence that sea mammals make up much of its diet), and shares this category with Greenland and sleeper sharks, which are suspected of ambushing sea mammals in the darkness of the deep sea (fresh seal and dolphin meat is regularly found in their stomachs, but it’s unclear how these massive, sluggish sharks catch such fast prey).

However, dolphin/shark interactions are more interesting than just sharks eating dolphins.  Sharks and dolphins occupy a similar position in the oceanic food web, which makes the large sharks mentioned above intraguild predators of dolphins.  Intraguild predation is a term explained by Polis and Myers (1989) right in the title of their paper “The Ecology and Evolution of Intraguild Predation: Potential Competitors that Eat Each Other.”  In these cases the “winning” predator gains a double advantage: not only is it getting the energetic benefits of consuming prey, it’s also removing a source of competition.  This creates some very strong ecological interactions.  In Shark Bay in Australia Heithaus and Dill (2002) observed that bottlenose dolphins will actually give up more productive feeding grounds when tiger sharks are in the area.  The dolphins and sharks are both feeding on the abundant fish in the shallow seagrass beds, but because tiger sharks can also eat dolphins the cetaceans decide that discretion is the better part of valor and forage in less-productive but safer areas.  This is actually a more significant interaction in terms of energetic costs and benefits than the few dolphins that the sharks bother eating.

So what about interactions between dolphins and large fish-eating sharks that don’t even try to eat them?  According to Acevedo-Gutierrez (2002), it comes down to a numbers game.  While studying competitive interactions between silky sharks and bottlenose dolphins feeding on the same schools of fish, he found that when sharks had the numerical advantage the dolphin pods would hang back in the periphery of the bait ball while the sharks went to town on the fish.  It was only after the numbers of sharks dropped off as more of them became satiated and left that the dolphins began to move in.  Dolphins would only act aggressively towards the sharks if the mammals had a clear numerical advantage.

So in conclusion, Flipper is a damn dirty liar, and in the real ocean would run screaming from that tiger shark in the video.  However, to prove that I’m not totally anti-dolphin, here is a video of some dolphins doing something admittedly awesome that no shark could ever do (for some reason whoever posted the video disabled embedding, so you’ll have to hit the link).

Sources:

Acevedo-Gutiérrez, A. (2002). Interactions between marine predators: dolphin food intake is related to number of sharks Marine Ecology Progress Series, 240, 267-271 DOI: 10.3354/meps240267

Heithaus, M. (2001). Predator–prey and competitive interactions between sharks (order Selachii) and dolphins (suborder Odontoceti): a review Journal of Zoology, 253 (1), 53-68 DOI: 10.1017/S0952836901000061

Heithaus, M., & Dill, L. (2002). FOOD AVAILABILITY AND TIGER SHARK PREDATION RISK INFLUENCE BOTTLENOSE DOLPHIN HABITAT USE Ecology, 83 (2), 480-491 DOI: 10.2307/2680029

Polis, G., Myers, C., & Holt, R. (1989). The Ecology and Evolution of Intraguild Predation: Potential Competitors That Eat Each Other Annual Review of Ecology and Systematics, 20 (1), 297-330 DOI: 10.1146/annurev.es.20.110189.001501