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I've heard numerous anecdotal claims of people being rescued from shark attacks by dolphins. One source mentions that such rescues happen "surprisingly often", and cites three unreferenced incidents, including one where a pod of dolphins supposedly fended off a 10-foot great white shark.

Yet this site references a program in the 1960's by the U.S. Navy that trained Bottlenose Dolphins to incapacitate large sharks. The program appears to have been a qualified success: the dolphins would readily attack sharks not known to attack dolphins in the wild, but they refused to approach Bull Sharks (which are known to attack dolphins in the wild). That site summarizes with this:

In the wild, similar-sized dolphins and sharks pretty much leave each other alone. Some 75% of wild dolphins show some degree of shark scarring — and we usually see only the ones that got away. I have pulled dolphin remains from the stomachs of many sharks over the years, particularly from Tiger Sharks (Galeocerdo cuvier). Thus, it seems that in most battles between dolphins and sharks in the wild, dolphins get the worst of the encounter.

This seemingly contradictory information makes me question whether dolphins really risk confronting large, aggressive, dolphin-eating sharks to protect random humans, anecdotal data aside.

Is there more concrete information about dolphin-human-shark interaction?

Edit: In light of a comment in ChrisW's answer, I would also be interested in any studies confirming that dolphins attack large, aggressive, dolphin-eating sharks in the absence of distressed humans.

  • Maybe all the anecdotal rescues were by the dolphins released after the U.S. Navy training. – Alain Jun 27 '11 at 19:41
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    @Alain except the U.S. Navy couldn't convince the dolphins to go anywhere near sharks that were actually dangerous (proving, to me at least, that dolphins really are intelligent!). – Beofett Jun 27 '11 at 19:48
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    Another take on this issue would be that perhaps dolphins are just playing with humans and we get to hear only about the fortunate cases where humans happened to be rescued thanks to this meddling. On the other hand, we wouldn't hear about persons who were unfortunate in that dolphin intervention ended up in not preventing a shark attack. – Andy Jun 28 '11 at 13:47
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    @Andy Reminds me of the "Dolphin pushing people to shore" stories. Maybe those Dolphins liked you. Or, maybe they push people randomly and we just never hear from the ones that are pushed out to deep sea... :-) – Brian Knoblauch Jul 1 '11 at 17:16
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    The MythBusters tested if sharks are afraid of dolphins. They used an animatronic dolphin and found out that it's presence deterred sharks from attacking prey. – Oliver_C Jul 1 '11 at 17:32
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In the presence of a shark, dolphin anti-predator behavior varies with the circumstances. Some simply swim away from the shark, others ram or bite it, and yet others launch coordinated group attacks to drive the predators away. -- Smithsonian National Zoo

If dolphins interact with sharks in defense of members of their pod (e.g., protecting young), it might explain the (occasional) defense of humans.

Update: "There are a lot of publications on sharks attacking / predating dolphins, but I've been unable to find any in the other direction." Jim Carretta SWFSC, NMFS, NOAA [personal communication]

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    Those allegations by the Smithsonian aren't verifiable. I'm not disputing them (in fact they concur with other stories I've read about dolphins) but they're not verifiable. – ChrisW Jun 29 '11 at 1:47

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