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Biology 103
2000 First Web Report
On Serendip

Sharks: Killing Machines?

Meghan McCabe

Hollywood has created an image of sharks that portrays them as senseless killing machines. Many people believe that sharks will attack, and even consume, a human at any chance possible. The more I researched sharks and their behavior, the more I learned that they have been falsely portrayed. Knowing this, I then wanted to learn why it is that sharks attack humans on rare (very rare) occasions. I had grown up assuming that if a shark was nearby, it would most likely attack me. However, I have learned that this is not necessarily true.

Given that there are over 375 different species of sharks, for the most part I will be discussing the White shark, also known as the Great White. With a large dorsal fin, large, triangular serrated teeth, and having gray-black white coloration, the Great White shark can reach up to five meters in length at maturity. The Great White, like all sharks, has a cartilage skeleton. While many species of shark are oviparous (11), the Great White is live bearing (1). Great Whites are predatory animals and generally feed on animals with high quantities of blubber such as sea elephants, sea lions, seals. Often times Great Whites will scavenge on floating carcasses.

Sharks go back through the fossil record about 400 million years, and Great Whites alone have remained unchanged for the last 60 million years (2). Some early species of sharks even predate trees (11). Thus, some physiological characteristics of sharks seem highly evolved, among these being their senses. Sharks' eyesight has been a topic of controversy in the scientific community. It was once believed that sharks have poor eyesight, but studies have shown that sharks' eyes are much more sensitive to light than human eyes, making hunting at night easy. Great Whites have a pair of nostrils, used only for smelling, rather than for breathing. The Great White has an incredible sense of smell that it uses to sniff out prey. Another interesting sense of the Great White is the ampullae of Lorenzini, tiny pores in the skin that are highly sensitive to electrical discharges. These pores can sense charges as little as 0.005 microvolts. This "sixth sense" is useful in tracking prey's movement from long distances (3). In fact, they can feel disturbances in the water from up to a mile away (2).

If Great White sharks have such developed senses, and could eat anything they like, particularly fatty animals why do they attack humans that are relatively low in fat? Little is known about shark behavior, but there are many theories as to why sharks attack humans. Of about 375 species of sharks, only 32 have been documented to have attacked humans, and the three species that have been identified repeatedly in attacks are the great white, tiger, and bull sharks (4).

Sharks, like any animal, will defend themselves if they feel threatened or if they're territory is invaded (5). How a shark will react when threatened ultimately depends on the kind of shark. Many will simply swim away, or some will attack very quickly. The great white tends to attack quickly, catching the victim off guard.

One theory of attack on humans is mistaken identity. Often times divers, surfers, and even kayakers appear like seals or sea lions swimming at the surface of the water. Studies have been done where researches have equipped a surfboard with an underwater camera (6). The camera shows that the great white swims along the bottom of the sea at first, so as not to be seen, then launches a quick, vertical attack. Some scientists believe that great whites do not use their keen sense of sight when hunting prey, but rather, senses a familiar image swimming at the water's surface and assumes it is some kind of prey.

Another theory is that sharks attack as purely inquisitive testing. Researchers say that 96% of all shark attacks are single strikes, or "hit and run" attacks (5).In this type of attack, the shark might bump or slightly graze the victim with the jaws, often times without biting. Often times in the hit and run type attack, the attacked never sees the attacker, and the shark swims away and never returns. Sharks like to explore with their mouths, and have been known to gently mouth objects floating in the sea. If a shark wanted to eat a human, it would bite and rip tissue from the victim, rather than grazing or nipping. This suggests that shark attacks have little to do with hunger and feeding, but rather are a shark's way of exploring its environment.

One controversial theory is that sharks attack humans for sexual reasons. Shark experts have noticed heavy scarring on female sharks' fins and tails from what many scientists believe to be bites from males in sexual circumstances. The bites, which most likely resulted when a male "affectionately rakes its teeth across the female's tail" (2),/a> ,match similar rakings on humans attack victims. However, most shark experts reject this idea, accounting the "rakings" on humans to be due to investigation, or curiosity encounters.

No species of shark has shown that humans are a regular part of their diet (5) ,nor do they show that they like the taste of humans. Even great whites, which are great predators and known to consume sea animals, don't show much interest in eating humans. Studies show that 75% of great white attacks on humans are non-fatal and allow the human to escape, showing that sharks probably don't like the taste of humans, perhaps because they are low in fat and blubber (4) .

There are approximately 75 shark attacks each year, 10 of which are instances in which the victim has died (7). Actually, more people are killed each year in the United States by dogs than have been killed by great white sharks in the last 100 years (9) . However, humans kill over 100,000 sharks a year (8) . Since sharks have long lives, mature slowly, and have a slow rate of reproduction turn-over, it is hard for them to overcome the effects of hunters and fishers. Sharks are an important part of the oceanic food chain, and mass killing of sharks could cause detrimental effects in underwater ecosystems. This is very unfortunate because the study of sharks could offer scientists new information about different areas of science. For example, sharks have survived "all the mass upheavals of the last 400+ million years of natural selective chaos" (10). Their highly developed senses of smell, sight, hearing, and electroactivity sensing can offer scientists new insights to evolution. Also, sharks appear to be nearly unaffected by cancer and circulatory diseases, and have also heal very rapidly from major injuries. The extreme and senseless fear of sharks that has been instilled in people is resulting in the slaughtering of fascinating creatures that have much to offer the sea as well as the rest of the world. It is important to learn more about them so we can understand why they attack and end the fear that drives us to destroy them.

WWW Sources

1) 1) South African White Shark Research Institute

2) 2) MetroActive Features

3) Great

4) The Center for Shark Research

5) ReefQuest Expeditions

6) ReefQuest Expeditions


8)University of Oregon

9)University of California, Berkeley

10)The Pelagic Shark Research Foundation

11)Mote Marine Laboratory

Visitor's Comments

2 December 2001 The above manuscript provided some interesting interpretations of shark attack motivation.  However, not all researchers believe the new terminology of 'hit and run,' 'bump and bite,' and 'sneak attack' provide useful categories or definitions for a high percentage of the authenticated unprovoked shark attacks on humans that are reported.  Unfortunately, these are the terms that are most frequently mentioned by media and uninformed shark/human interaction researchers.  Your web site is very impressive for which you and your colleagues should be commended.
Ralph S. Collier
Shark Research Committee
P. O. Box 3483
Van Nuys, CA  91407

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