What drives shark attacks?

[SkepticLance]

As a mad keen scuba diver, I have always been fascinated by sharks. One aspect of the human/shark encounter that has been very apparent is the extreme rarity of shark attacks.

 

In spite of literally millions of humans swimming in many places in different parts of the world every day, and the fact that sharks must swim nearby and perceive these human swimmers with their very acute senses, only about 4 fatal shark attacks happen each year globally. Of course, there are many more attacks than that - most are non lethal. Almost never is the attack followed up by the shark actually eating the human victim.

 

I have always thought that the real puzzle is not why sharks attack (they are top predators after all) but why the attacks are so rare. This is especially puzzling in comparison with land predators. A lion or tiger will readily attack, kill and eat a human.

 

Not only that, but the pattern appears to be the same with other large marine predators. Near where I live is a woman scientist who has devoted her life to studying Orca. Along with sperm whales, they are the top marine predators. The woman I mentioned has found that each population of Orca have a very limited diet. Here in NZ, it is mostly sharks and rays, with the occasional seal and dolphin. She swims with them regularly and has never experienced the least interest from them of a gastronomic nature!

 

I have formulated a theory, which I would invite comment on. The theory is based on the fact that the sea (as opposed to the land) contains a large range of organisms, and especially fish, that are toxic. Marine predators must quickly learn not to eat just anything, as it may result in seriously nasty symptoms. They settle for a limited range of target organisms. It is as though they swim around with a shopping list in their brains, and eat just what is on the list.

 

So why are people attacked at all? For unprovoked attacks, I suggest that it is for two possible reasons.

1. A shark uses its mouth as people use hands - to investigate the environment. If a curious big shark mouths a human, it is likely to leave nasty - even fatal - wounds.

2. Mistaken identity. A swimmer's hands or feet may flash in the water like a target fish species, and the shark snaps on it by reflex. Sometimes hard enough to amputate.

 

Yet documented cases of people actually being eaten are almost unknown. Any comments?

[Cap'n Refsmmat]

Perhaps sharks, while carnivorous, aren't always hungry. They may not always want a taste of that tempting scuba diver, no matter how much of an easy target he is.

[too_bored]

Since sharks have been found swimming close to shore, your theories about sharks exploring the environment through their mouths and possibly mistakening a diver for a fish are interesting. Even though people swim and dive in the ocean all the time, maybe sharks don't have an acquired taste for our flesh.

[DrDNA]

Sharks are a very diverse group with a wide range of diets and feeding habits. Some are even herbivores.

So I doubt that there is any one answer to this question. It would certainly depend to a great extent on the particular species in question.

 

BTW: A surfer with his arms and legs hanging off his surf board waiting to hook up a wave looks a lot like a seal or sea lion from underneath.

[SkepticLance]

To the capn.

 

Nice theory, but I doubt it. I have been with sharks that are being fed by dive tourist operators. Whenever these guys go down with a bucket of fish scraps, the sharks are VERY hungry.

 

I counted the species of sharks I have encountered on scuba dives, and got to 18 from memory. I have encountered many of each species. Unless they are being fed, they tend to keep their distance. Some swim in to within about 10 metres of so, apparently out of curiosity, but most like to keep well away from a scuba diver.

 

Out of all the hundreds of sharks I have seen, only twice have I seen aggressive behaviour. Both times it was from an Oceanic White Tip Shark, in very deep water. Even the one time I saw a White Pointer (Jaws) shark, it showed only curiosity. I have dived with Bull Sharks - sometimes said to be the deadliest - and cannot get close enough to take a photo. I always end up chasing them away.

[Skye]

Great whites apparently need to learn how to hunt particular species, they spend a bit of time making unsuccessful attacks. There was interesting BBC (I think) documentary a while ago showing a young great white spending a day trying to catch a sea bird resting on the surface. To start with it was just sort swimming up to the bird on the surface, and the bow wave of the shark pushed the bird out of the way. It eventually attacked from below at speed, and this was seen as possible evidence that great whites learn how to hunt. So, the fairly clumsy behaviour of great whites when attacking people, mauling but not killing and eating, is possibly just learning behaviour.

[Mr Skeptic]

I have always thought that the real puzzle is not why sharks attack (they are top predators after all) but why the attacks are so rare. This is especially puzzling in comparison with land predators. A lion or tiger will readily attack, kill and eat a human.

 

As I understand it, lions and tigers aren't much for hunting humans either. I think "human" is an acquired taste. I've heard that most tigers that attack humans are maimed or sick and unable to catch other pray -- inevitably a tiger hunt ensues. Once a tiger eats a person, they are likely to be repeat offenders.

 

It could be that sharks don't attack people because one of their main senses is smell. They can smell blood from a great distance, but maybe they don't know how to sniff out people, but can smell seals?

[CaptainPanic]

I'm no expert, but here's my theory:

Any animal that attacked humans in the past has been hunted down. This not-so-natural selection has caused animals to watch out for humans.

 

However, this theory seems more applicable to land animals...

 

Ok... (I'm warming up)... here's another theory.

Sharks have a 6th sense that can pick up the electric field of fish (prey). The hammerhead shark even has evolved a weird shape head to fit a very large version of this sensor. Perhaps surfers don't have such a strong electric field so that we look like a swimming rock to a shark? Or perhaps we have such a strong electric field that we look like a whale to a shark? Both cases we don't look like food.

 

Just my thoughts... speculation...

[SkepticLance]

To Mr Skeptic

 

There have been a number of occasions where a shark has bit a human, releasing vast amounts of blood, sometimes enough to kill the person. Yet they do not press home the attack. I doubt their senses are insufficient to sense the blood.

 

Much more likely in my opinion that they identify from the smell that the animal they bit is not part of their normal list of prey, and hence might be toxic.

 

Of course, when you read the accounts of people who survive such attacks, you get a very different account. "The shark stopped attacking because I hit it hard." "I gouged its eyes." etc. etc. etc."

 

Yeah Riiiiight! Shark attack survivors delude themselves that their survival has something to do with their own actions. Where it all happens is in the sea, and the shark is master. if they allow a human to survive, it is because they choose not to press home an attack. Why?

[DrDNA]

Try telling someone that survived the sinking of the USS Indianapolis that sharks don't like to eat people.

If you are unfamiliar with the story, here is a pretty good link...

http://www.ussindianapolis.org/story.htm

[SkepticLance]

To DrDNA

 

We get a lot of stories of the sort you mentioned. It is often difficult to separate truth from fiction, since the survivors of such disasters are frequently in shock, and have their stories unclear. It is very common for drownings to be ascribed to shark attack.

 

On the other hand, the shark that comes to such sites is the Oceanic White Tip. It is the only shark that has ever acted aggressively towards me. I have twice been charged by this species - both times when I have been in deep tropical water. Both times the charge was ended by the shark stopping abruptly, turning and fleeing for the horizon.

 

Thus, it is possible that some human deaths have occurred due to attack by this shark. Trying to quantify it is pretty much impossible, since truth and exaggeration are impossible to disentangle.

 

If these sharks were truly serious about those attacks, there would have been no survivors at all from the Indianapolis. In the deep ocean, the shark is the master, and humans are helpless.

[foodchain]

To DrDNA

 

We get a lot of stories of the sort you mentioned. It is often difficult to separate truth from fiction, since the survivors of such disasters are frequently in shock, and have their stories unclear. It is very common for drownings to be ascribed to shark attack.

 

On the other hand, the shark that comes to such sites is the Oceanic White Tip. It is the only shark that has ever acted aggressively towards me. I have twice been charged by this species - both times when I have been in deep tropical water. Both times the charge was ended by the shark stopping abruptly, turning and fleeing for the horizon.

 

Thus, it is possible that some human deaths have occurred due to attack by this shark. Trying to quantify it is pretty much impossible, since truth and exaggeration are impossible to disentangle.

 

If these sharks were truly serious about those attacks, there would have been no survivors at all from the Indianapolis. In the deep ocean, the shark is the master, and humans are helpless.

 

I think this is a common fallacy that people live with. When you go out into an area in which predatory animals live you run a risk with danger. Its rather simple I would think, I mean that’s kind of how they live. Shark attacks on humans can occur in various situations that I know of, they can also be highly coordinated efforts between multiple individuals.

 

In fact a pride of lions in Africa was found to do things during an attack to confuse people. They simple strategy was to spread out, cause one to panic making others rush from there homes to try and help.

 

As far as I know the statistics for such are overwhelmingly small. Most predatory animals also tend to try not to get hurt, there is no hospital in the wild. A troubled man lived in Alaska making a sort of documentary about things in which he placed himself as a guarding of grizzly bears. Now these are not tame creatures and he had no protection. Eventually he got turned into a food but it was a very long time before this occurred and even then only with the most feral bears in the population when food was very scarce.

 

Most the time the fear of an attack causes widespread damage to various populations of species you might find in such movies like jaws.

 

http://en.wikipedia.org/wiki/Timothy_Treadwell

[DrDNA]

To DrDNA

 

We get a lot of stories of the sort you mentioned. It is often difficult to separate truth from fiction, since the survivors of such disasters are frequently in shock, and have their stories unclear. It is very common for drownings to be ascribed to shark attack.

 

On the other hand, the shark that comes to such sites is the Oceanic White Tip. It is the only shark that has ever acted aggressively towards me. I have twice been charged by this species - both times when I have been in deep tropical water. Both times the charge was ended by the shark stopping abruptly, turning and fleeing for the horizon.

 

Thus, it is possible that some human deaths have occurred due to attack by this shark. Trying to quantify it is pretty much impossible, since truth and exaggeration are impossible to disentangle.

 

If these sharks were truly serious about those attacks, there would have been no survivors at all from the Indianapolis. In the deep ocean, the shark is the master, and humans are helpless.

 

I'll bet the guys getting munched on thought that the sharks were serious!

 

What an experience that must have been, sharks bobbing in a barrel for humans. Listening to the sound of your buddies getting eaten for hours on end....knowing that it was probably a matter time before they started gnawing on you.

 

The USS Indianapolis incident has been well documented and I seriously doubt that the 317 survivors had the same bad trip.

 

The sun came out and that's when the first sharks showed up and started to sniff around like hungry scavengers. The doomed sailors started to panic as they saw these monster Tiger Sharks lurking around and felt them poking their noses on their feet. These 30-foot carnivores started to smell blood and that's when they proceeded to rip big chunks of flesh from the survivors. The bloody message eventually got out to other sharks and an estimated two to three hundred reported sharks were seen at the killing field, in hours. The terrorized survivors held hands and formed large circles to try to defend themselves but the sharks were on the outer layers just picking them off, one by one. There were limbs and body parts floating all over the place. Many sailors could not take it anymore and would just commit suicide by taking their jackets off and peacefully submerging into the salty water, never to be seen again. All this horror continued for days. The shark attacks would slow down for 3 or 4 hours, but would return again without announcement and with a relentless fury. By the 3rd day many men had died from malnutrition and dehydration, the others were slowly being slaughtered by the frenzied sharks. From the time the sharks first arrived, to the time the survivors were discovered, an estimated 350 to 400 men were devoured by these merciless eating-machines. I must say, to see a man being eaten alive by a powerful shark like this, must be the most brutal and most shocking experience that anyone can go through. A total of 317 men managed to survive the 5-day massacre.

http://cubanology.com/Articles/The_Fate_of_the_USS_Indianapolis.htm

[Mr Skeptic]

Well, when sharks go on a feeding frenzy, they're much less picky about what they chew on.

[SkepticLance]

To DrDNA

 

I have no doubt that some of those men were killed by sharks. What I was talking about was the inevitable exaggerations that follow such incidents. For a start your account talks about it being Tiger Sharks. Sorry guys. Tigers prowl the fringes of coral reefs, and are rare in deep oceanic water. The shark was almost certainly the Oceanic White Tip Shark. This would be the mere beginning of the errors.

 

Those seamen wore kapok life jackets, which are nowhere near as good as modern life jackets. If you are afloat too long, some of the jackets get water logged and can actually drag you down.

 

Your account talks of 5 days drifting - quite long enough for many life jackets to turn lethal. It also talks of the state of mind of dehydrated and exhausted survivors. Their perceptions would be utterly screwed. A person who simply sank would be seen as being a shark attack victim. As for the description of limbs and gore everywhere .... Take that with a VERY BIG pinch of salt.

 

A very well documented aspect of human psychology is the tendency to report dramatic experiences in a more and more exaggerated form as time goes by. The sharks become more numerous, larger, and more fearsome.

 

I can accept that some degree of shark attack would occur, but if you believe that superstitious seamen in that environment would not allow their imaginations to take over and ascribe every drowning to shark attack, then you are naive.

 

If you saw the Blue Planet series (BBC) on TV, you would have seen the results of a serious attack. That was on a bait ball of fish. By the end, no smal fish survived. That was an attack by different kinds of predators, but a massed shark attack on seamen would be no different if it was a genuine major attack. Oceanic White Tips can sense prey across a wide area, and an attack of the sort you describe would attract every shark within several hundred kilometres. I am talking hundreds of sharks. With 5 days before rescue, there would be no survivors.

[Sayonara]

Yet documented cases of people actually being eaten are almost unknown. Any comments?

 

Bear in mind that when someone is eaten or mortally wounded by a shark, it is not necessarily very likely that anyone will ever know that is what happened to them. Take under-reporting into account; there are probably good figures available for this.

[CDarwin]

Bear in mind that when someone is eaten or mortally wounded by a shark, it is not necessarily very likely that anyone will ever know that is what happened to them. Take under-reporting into account; there are probably good figures available for this.

 

I'm not so sure about that. In what context would someone be completely alone and eaten by a shark? I'm sure those circumstances occur, but they can't be very common.

 

 

Personally, I've always wondered why people aren't attacked by predators more too. We're pretty defenseless animals.

 

There's a book, Man the Hunted that talks extensively about the role of predation in human evolution and if anyone wants a non-sensationalized sort of text on this subject that would be a good one, even if it more from the side of human evolution than predator evolution. It actually calls on anthropologists to study predators better.

[Paralith]

Hi Lance, sorry I'm late to this very interesting discussion.

 

I mostly agree with your original theory, that most shark attacks are probably investigatory, or that some aspect of the human and/or its behavior at the time elicits a predatory reaction. Along these lines, they say that the worst thing you can do when faced with a big cat is to turn and run from it - running is prey behavior, and even though they may not have been particularly interested in you to begin with, you can elicit the predator's instincts by acting like the food they eat. (Anyone with a pet cat knows how much more interested they become in a given toy once it starts moving.) I suspect that this happens a lot with shark attacks on humans as well.

 

I think you're underestimating the effect of "fighting off" a shark, though. You don't have to represent a significant threat to the shark to make it go away - you just have to cost more energy in the struggle than the shark will get from eating you. I remember a study done on lizard-eating snakes - if the lizard struggled for a certain amount of time, the snake would let it go - the cost of the meal was going beyond what it was worth. These lizards posed no actual threat to the snakes other than inefficiency. Even if a shark has gone beyond the initial reaction/investigation and does decide it wants to eat this human it has in its mouth, if it seems like the human will be too hard to subdue, the shark may then decide it's not worth the effort.

 

This may help to explain the USS Indianopolis incident somewhat. Many of the sailors in the water were already injured, and all were tired and weak from the ordeal of the ship's sinking, and only became more so as the days went on. I agree that the role the sharks played was probably exaggerated to some degree, but I wouldn't be surprised if some sharks did take advantage of their weakened state.

[hermanntrude]

Lance, anyone who has watched a documentary on sharks will have heard your theories on what provokes a shark attack. they are not new.

 

sorry to rain on your parade.

[Sayonara]

I'm not so sure about that. In what context would someone be completely alone and eaten by a shark? I'm sure those circumstances occur, but they can't be very common.

Argument from incredulity. Also note that "completely alone" has been added by yourself.

[CDarwin]

Argument from incredulity.[/Quote]

Argument from low likelihood. I'm not saying that it's incredulous that someone be eaten by a shark and no one know about it. I'm saying that the circumstances in which such a happenstance would occur are relatively rare.

 

Also note that "completely alone" has been added by yourself.

 

How else would someone eaten by a shark not be registered?

[iNow]

How else would someone eaten by a shark not be registered?

 

The person with them got eaten too?

The person with them had severe memory problems?

The person with them didn't know their real name?

The person with them died of a heart attack on their way to report it?

The person with them drowned on their way back to shore?

The person with them was abducted by aliens?

 

I dunno, how I'm doing?

[CDarwin]

The person with them got eaten too?

The person with them had severe memory problems?

The person with them didn't know their real name?

The person with them died of a heart attack on their way to report it?

The person with them drowned on their way back to shore?

The person with them was abducted by aliens?

 

I dunno, how I'm doing?

 

Ok, put it another way. How many people of groups in any number find themselves in the ocean where sharks are without being near a boat or populated beach? Maybe its more that I think.

 

I doubt that it would be so many, though, as to make the statement that "shark attacks are rare" be untrue considering all of the people who we know come into contact with sharks and are heard from again. Perhaps I'm wrong there, too.

[SkepticLance]

To Paralith

Welcome to the discussion. Your input is welcome.

 

In relation to land predators, I remember talking to a guy who was an African safari guide for some years. He stated unambiguously that a lion would kill and eat a person, without hesitation if given a chance. He said that his bus would often stop close to sleeping lions so that the tourists could take photos. If he opened the door, the lions would get up and start walking towards the bus. Apparently, on one occasion, a stupid tourist actually got out of the bus to take a better photo before the guide could stop her. A lion promptly took her down and killed her. The lion was in the process of dragging away the corpse, presumably to eat it, when shot.

 

It is the lack of such behaviour I note in sharks. As I said, I have personal experience with lots of sharks. I do not know exactly how many sharks I have encountered underwater at various times, but it must be many hundreds. I counted, from memory, encounters with 18 different species.

 

I have only twice experienced aggression. The Oceanic White Tip lowers its very long pectoral fins, and arches its back as a sign of aggression. It then swims at the target of its aggression. The two times I saw this, the shark abruptly stopped, turned very rapidly and swam off into invisibility.

 

For most the other hundreds of sharks, I experienced the frustration of wanting to photograph them, and being totally unable to get close enough. They just would not approach close enough.

 

My first Bull Shark encounter was actually quite humorous. This shark (known as the Zambesi Shark in Africa) is considered by many to be the most dangerous of the 'man eaters'. I encountered one in Fiji. I was snorkelling along a channel in the reef, and saw this large shape beneath me, at least my size (I am six foot tall). It was ascending, and our two paths, if continued, would have ensured coincidence in space-time! I was more than a wee bit nervous. I remembered in the old Hans Hass books, the author said you could scare sharks away with an underwater shout. So I took my snorkel out of my mouth and screamed - not too difficult under the circumstances. The shark had clearly not read the same books! It did not even flinch, but slowly started to rotate. I saw a big eye moving up the side of the rotating head, till it encountered my gaze. After a moment that took a million years, it shot off at enormous acceleration and disappeared. It also left a cloud of white stuff in the water. So scared that it s^@t itself!

 

To Sayonara

 

I have to say I agree with CDarwin. Most people do not enter the water alone, whether swimming or snorkelling, surfing or scuba diving. People like company, and do these things in company. If, occasionally, a solo person gets killed and eaten, then we would expect much larger numbers of people with company to get killed and eaten, and that does not happen.

[Paralith]

To Paralith

Welcome to the discussion. Your input is welcome.

 

In relation to land predators, I remember talking to a guy who was an African safari guide for some years. He stated unambiguously that a lion would kill and eat a person, without hesitation if given a chance. He said that his bus would often stop close to sleeping lions so that the tourists could take photos. If he opened the door, the lions would get up and start walking towards the bus. Apparently, on one occasion, a stupid tourist actually got out of the bus to take a better photo before the guide could stop her. A lion promptly took her down and killed her. The lion was in the process of dragging away the corpse, presumably to eat it, when shot.

 

It is the lack of such behaviour I note in sharks.

 

I have several thoughts on this. First of all, if we follow your terminology that lions will kill a person "without hesitation if given a chance," then I think my own personal experience has disproved that. I've been on safaris myself, and gotten very close to several lions in a very open truck that was not particularly high off the ground. If humans really were a prime menu option for lions, then I think my truck and many others like it or even more open than it would be highly unsafe; a determined lion would have no trouble getting to a person inside one.

 

Secondly, simply by being on the ground, a human enters a much more prey-like position relative to a lion. And I don't know the details of the story you related, but I'd imagine that once the woman saw a lion stand up and move towards her, her first instinct was probably to run back to the bus.

 

Thirdly, there are several other reasons why a lion might kill another animal that are not related to their desire to eat it. A threat to females and cubs can easily arouse the aggression of a pride male. Lions are also known to kill cheetahs and hyenas, adults and juveniles alike, and not eat them. They kill them because these species are the lions' primary competitors. In fact, lions in a park will learn that tourist trucks relentlessly mob the few, poor cheetahs in the park whenever they are sighted (the trucks literally form a circle around the cheetahs, following them wherever they move, and it only gets worse when other drivers see the commotion, and zoom in ASAP to get their load of tourists a photo opportunity - again, this is from personal experience). The lions will follow this commotion to the cheetahs, and kill them. I think behaviors of this kind are less common in sharks, and I think in general they are less likely to view a human as a threat to them that needs to be attacked rather than escaped from.

 

Fourthly, lions in a park are exposed to hundreds of tourists on a nearly daily basis. Many sharks, especially the larger ocean-going ones, spend a vast majority of their time out in open ocean with nary an mammal to be seen, let alone people. In short, land predators like lions have much more opportunity to develop abnormal behaviors in relation to humans than sharks do. And to connect back to my third thought, they also have much more opportunity to learn to recognize humans as threats of some kind. A lion has a limited territory where he hunts and where his females live; a shark has giant swathes of ocean it can retreat to should should it encounter an unpleasant human.

 

If, occasionally, a solo person gets killed and eaten, then we would expect much larger numbers of people with company to get killed and eaten, and that does not happen.

 

Unless a solo person presents a much less threatening situation than a mob of thrashing humans, making it more likely that the shark may approach and investigate the possibility of an easy meal.