They don't go hunting sharks as an afternoon activity... they are able to kill sharks when something is wrong with the situation. Say, that one of the dolphins are injured and a shark is ready to attack. They have been known to save people by killing sharks too. The dolphins ram the sharks and several blows to the shark can kill it. There doesn't necessarily have to be a pack of dolphins, just enough attempts at ramming the shark.

Might I add, that they ram at a velocity to break bones and do very bad internal damage. And since dolphins are better at evading than sharks, its alot easier to dodge. (when you're at the top of the food chain for being offensive, you lose alot of knowledge about being defensive)

The idea that dolphins will seek out sharks for undersea battle is largely incorrect. It is the unfortunate perception stemming from the 60's "Flipper" generation, when scientists like John Lily were respected for preaching about the psychic abilities of dolphins and their 'cosmic connection' to humans. What we know now as the result of legitimate research is exactly the opposite - dolphins will AVOID natural predators whenever possible. Sharks can often swim two to three times faster than dolphins and have a much more ferocious disposition. Fortunately, in many cases, dolphins are able to elude attack by detecting them with the use of biological sonar (i.e. echolocation). In addition to sonar, dolphins rely on each other for defense. They travel in numbers for protection (females with offspring, adult males, and sub-adult males each comprising their own social groups). Those who fall prey to attack are usually either alone, ill, injured, very young or very old. Healthy, alert, social animals are not often at risk. In the case of a shark attack, dolphins will defend themselves. What they lack in speed they make up for in maneuverability. Between the whip of their tail and the ram of their rostrum, dolphins can be a force to reckon with. An open mouth full of sharp, conical teeth can also prove a deadly weapon. Source:http://www.dolphinencounters.com/shark.htm Also here is a video of these people swimming in the Bahamas. There is a tiger shark lurking underneath these ladies but fortunately there is a dolphin who scares off the tiger shark.

Well, with a lot of speed, dolphins ram sharks with their melon area [the name for their head] and the velocity DOES do internal damage, but, to answer davoomac's question in the comment to walrus's answer, it cant break the sharks bones because sharks are made of cartilidge, not bones, and the cartiledge is damaged easier than the dolphins bones.

That youtube video is nonsense. Sharks are just as curious as dolphins. The dolphins just happened to be there. Nothing special. How many stories tell you about the times dolphins actually drag people out to sea or hold them underwater? I love dolphins but c'mon, occurances where dolphins have 'saved' humans are extremely rare and we only ever hear one side of the story. But yes, dolphins can kill sharks.

Only when the are pissed off. Don't mess with Dolphins, all sharks know this.

I LOVE THIS MOVIE

Like any creature, Dolphins will defend themselves when they feel threatened, just as humans do. They most certainly don't go out looking for a kill, but if trouble comes their way, they are more than capable of protecting themselves and their young. Jamesiree