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Thread: Why don't marine predators eat people?

  1. #1 Why don't marine predators eat people? 
    Forum Radioactive Isotope skeptic's Avatar
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    I was stimulated into making this post by a New Zealand Herald item about Orca swimming around divers, exhibiting curiosity, but not aggression. Dr. Ingrid Visser, who has researched Orca for many years, says that is typical. They may be curious about people, but there has never been a single record of one ever attacking a human, much less eating someone.
    http://www.teara.govt.nz/en/orcas/2/6

    I am a very keen scuba diver, and I love sharks. In spite of being in the water with sharks literally hundreds of times, including those with the greatest reputation for aggression, I have never been attacked. In fact, even Great White Sharks, Tiger sharks, Bull sharks and the like have only ever swum around me looking at me, and then swum off. If we look at records of shark attack, we find that they tend to fall into the category of mistakes, and are not followed up, and it is almost unheard of for a shark to attack, kill and eat someone.

    I have a personal theory on this, to which I invite comment. The ocean is full of animals that are poisonous. All members of the box-fish family, moray eels, puffer fish, and even pelagics which sometimes eat fish that have eaten toxic plankton.

    I think that marine predators have a kind of 'shopping list' in their heads, of prey species that are safe to eat. They will not normally taste anything not on that list, due to the risk of poisoning. This means that attacks on humans by marine predators are rare, and incredibly rare for a marine predator to actually eat someone.

    Contrast this to terrestrial predators, where toxic prey are very unusual. Lion, tigers, leopards, hyenas etc will all attack, kill, and eat a human. No hesitation.


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  3. #2 Re: Why don't marine predators eat people? 
    WYSIWYG Moderator marnixR's Avatar
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    Quote Originally Posted by skeptic
    They may be curious about people, but there has never been a single record of one ever attacking a human, much less eating someone.
    correction: i know of at least one darwin award account where a man went into a killer whale enclosure and did not live to tell the tale - although, being a darwin award, his death may be more due to his own stupidity than the killer whale's aggression

    http://www.darwinawards.com/darwin/darwin1999-18.html


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  4. #3  
    Forum Radioactive Isotope skeptic's Avatar
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    marnix

    Correction noted. It might be moot to suggest the Orca carried out an attack. It certainly did not eat a human. It appears from your description more likely that it was playing with the human intruder, and the drowning was purely accidental. In fact, the description suggests death by loonycide.

    It does not change my central point, about marine predators rarely attacking and almost never eating a human. Do you think my explanation makes sense?
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  5. #4  
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    They look alike, but there are really two kinds of orca. One kind eats fish. The other kind - in my area called "transients" - eats mammals.

    The transients of British Columbia eat seals, sea lions, whales, deer, and moose. A crazy coastline and thousands of tightly packed islands ensure that land mammals frequently swim from shore to shore, and that's when the transients nab them. I think that submerged land mammals give off a powerful mammal-concentrate scent, which probably appeals to mammal-eating transients like highly seasoned junk food. From the orca's perspective land mammals must all appear as thrashing limbs and bubbles, so they can't discriminate on sight. Humans very rarely swim without wetsuit in these cold waters.

    Your insights, Skeptic, sound good to me. I wanted to point out that some orcas do prey on land animals and probably aren't too picky about it, for as you suggest nothing they've known to bob and thrash on the surface ever gave them tummy aches.

    I don't hear much support for this opinion, but I believe humans look freaky weird to most animals, and our survival owes a lot to that. Moreover I hypothesize that we evolved this way, like the skunk or crested hissing lizard, in part to warn predators away. We stand up on two legs, wave our arms and yell. Freaky weird, and unmistakably that bad news species.

    I understand sharks won't hesitate to bite odd shapes of flesh. And they are famous for eating trash. So I think that rather than "shopping list" they just don't eat certain things. Maybe we smell like the mammals that bully them? And here's a thought: maybe in the cases where sharks bit once and swam away (that's the norm isn't it?), they recoiled at the dreaded taste of mammal blood?
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    If you have ever done any fly fishing for trout, you know that the normal fishing strategy is to "match the hatch." That is, you try to mimic the food the fish normally eats, and especially what they are eating on that particular day. You could float a nice. fat, juicy looking fly past them, and they will ignore it if it doesn't resemble something they normally eat.

    As far as terrestrial predators are concerned, there are a lot of them that do not usually bother people. Black bears hardly ever, grizzlies uncommonly, and mountain lions hardly ever, even though they are about the size of the much more dangerous leopard. Even in Africa, where it seems like everything bites, most lions keep away from humans. Occasionally, a rogue lion will develop an appetite for humans, and will have to be destroyed.
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    Forum Cosmic Wizard paralith's Avatar
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    Skeptic - your hypothesis is testable, in theory, though I think it would be a little difficult to categorize the diets of ALL marine carnivores and see if they are in fact more constricted on average than the diets of ALL terrestrial carnivores. That's quite a tall order and I think you will find that, in both realms, some animals will have a more constricted diet than others. And not all of that will be based on what they can eat but avoid; a lot of that will be based on what they are simply incapable of hunting, largely because they did not evolve to specialize on that type of prey.

    Though it's easy to forget sometimes, humans are very large animals compared to most others. I think for the most part we are a lot bigger than any animal a shark would consider going for, at least while its still alive and kicking, except perhaps for sharks who are big enough to eat sea lions and I believe (though I could be wrong so feel free to double check this) that these are some of the most common shark attacks. And of course there are lot of predatory fish and other animals in the ocean, but for the most part they are far smaller than we are. Cone snails are predatory and they sting their prey to death. They tend to focus on a particular type of prey but this is largely because they use neurotoxins specific to that prey - another example of a constricted diet because that's what you evolved to target, not because you're avoiding poisonous things. But there have been cases of cone snails killing humans - usually when we pick one up or step on it, and it defends itself.

    Now, I'm not even sure terrestrial predators eat humans more than marine ones do, though I bet you could actually research that. However you'd also have to account for the fact that the encounter rate would be much higher on land because, of course, we're terrestrial animals too. However, if terrestrial predators are more of a risk that would make more sense because lions and hyenas are pack hunters that target animals of very large body sizes, even larger than themselves, in which case a human falls well within the size range of things that could count as food.

    Orcas are an interesting case because I know they do target sea lions - even big bull males, since they too are pack hunters. Why they would not on occasion at least sample a human is an interesting question. This is of course excluding "attacks" that are actually just orcas being overly friendly. (In fact all dolphin "attacks" are actually dolphins attempting to engage in mating behavior with a human, except in the water with a frolicking 300 - 400 dolphin, the human is at a slight disadvantage.) I would wonder about the encounter rate with humans and orcas and if that has anything to do with it.
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  8. #7  
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    Paralith

    Thank you for the intelligent and analytical reply.

    For your information, the shark species that most commonly attacks humans is the Bull shark. This seems to be because it lives in estuarine regions, although it swims far up rivers and far out to sea at times. Interesting ability to cope with massive salinity changes. The higher rate of attacks is probably partly because it lives in areas where it contacts humans more, compared to other large shark species, and partly due to the turbidity, which makes mistakes more likely. However, they rarely, if ever, actually eat the people they attack. And most encounters with Bull sharks do not result in attack, even by mistake. I have scuba dived with Bull sharks on several occasions, and they have been reluctant to get too close to me.

    As far as Orca are concerned, we have here in New Zealand, a good record of encounters with swimmers, divers, kayackers etc., because we have a full time researcher working on Orca. She swims and dives with them very frequently. Of all marine predators, it may be that Orca have the most restricted diet of all. The NZ ones live mainly on sting ray and shark. In South America, it is mainly seals. In Papua New Guinea, they seem to target spinner dolphins . In Norway, they prey on herring. In each case, I suspect it is because they have learned that a certain food is OK to eat, and stick to it. They are often curious or playful, but never attack humans.
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  9. #8  
    WYSIWYG Moderator marnixR's Avatar
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    i once heard a safari tour guide comment on how people seem to be safe from lions if they stay inside their (even open-topped) vehicle, as if the car (even with passengers) doesn't trigger the prey switch

    however, he said, as soon as you step out of and away from the vehicle, that mental picture changes completely, and you DO become potential prey
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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