Tuesday, March 5, 2013

The Biology of JAWS: We're gonna need a bigger boat!

by Ellen Mintz


 And what did you say the name of this shark is?”-Mayor Vaughn
It's a Carcharodon carcharias. It's a Great White!”-Hooper

The Great White Shark
Everyone has a story about watching Jaws (Steven Spielberg, 1975) and the terror, or lack thereof, it evoked.  Whether you were unable to enter the water for a period of time, increasingly wary of boats, or amused by the poor quality (by today’s standards) of the special effects, most of us have experienced the classic movie that resulted in a lingering curiosity and unanswered questions about one of the greatest ocean predators.  In reality, great white sharks are fascinating and not the canonical terrors of the deep they are thought to be.  The movie Jaws is full of inaccuracies about great whites.  Let’s take this opportunity to do some movie mythbusting and learn some real facts about sharks!

 The Hunting Strategy and Territoriality of Jaws
“When he comes at ya, doesn't seem to be living... until he bites ya, and those black eyes roll over white and then... ah then you hear that terrible high-pitched screamin'. The ocean turns red, and despite all the poundin' and the hollerin', they all come in and they... rip you to pieces.” –Quint

           When they do attack, sharks make quite a scene!  Of all the exaggerations in Jaws, the depiction of what a shark attack looks like straddles the line of truth.  Sharks do most of their hunting in areas where their prey enter or exit the water, where there is a greater chance of ambush.  This usually means they are close to shore; in fact, most attacks happen 25-450 meters from land (not 10 feet, as Hooper states in the movie).  Attacks begin either underwater, resulting in a huge pool of blood in the vicinity, or with an explosive splash as the shark hits its target.  This can be accomplished with the shark remaining underwater.  After this, the rest of the attack can usually be observed from the surface of the water.  Great white sharks do swim back and forth at high speeds carrying dying prey, like the first traumatic (yet slightly overdone) attack seen in the movie.  However, contrary to the movie, the prey cannot usually be seen from the surface because they are held in the jaws of the animal as it swims.


Attack patterns of the great white shark
            Great white sharks are mainly solitary creatures.  They will approach each other when mating and after a kill, but in the case of the latter are often warned away by a violent tail slap against the water.  If this does not work, they will fight.  When no prey is present in the water, they are for the most part ambivalent towards one another.  They also do most of their hunting during the day, indicated by observational studies and the fact that the retina of a great white has a high density of cones and a reduced number of rods.  Cones and rods are photoreceptors that allow us to see light.  Cones are used for daytime, color vision, and rods pick up shades of grey, black, and white and are often very concentrated in organisms that are active at night.

            There is little evidence of territoriality in sharks, at least in the sense of them being protective over their own area and keeping other great whites out.  They do exhibit Hooper’s version of territoriality (see quote below), moving with food availability that corresponds to water temperature and breeding seasons of the prey (juveniles are easier to catch).

 
Rogue, yeah. Now this guy, he... he keeps swimmin' around in a place where the feeding is good until the food supply is gone, right?”-Brody
“It's called "territoriality". It's just a theory that I happen to... agree with.”-Hooper

 
How Does a Great White Bite?
“Well, this is not a boat accident! And it wasn't any propeller; and it wasn't any coral reef; and it wasn't Jack the Ripper! It was a shark.”- Hooper

               Great white sharks have characteristic bite patterns and jaws that are perfectly formed for the catching their prey and killing them in the method that they use.  The bite sequence of a great white has five components: elevation of head, lowering of bottom jaw, protrusion of upper jaw, closing of lower jaw, and head depression.  The bite of a great white can generate a maximum of 18,216 N, or approximately 4,095 lbs of force!  After the initial bite, great white sharks have been known to shake they captured prey from side to side, resulting in extensive soft tissue damage.

Great white sharks kill their prey but exsanguination, or extreme blood loss.  When and if the prey surfaces after being attacked, it is no longer bleeding as most of the blood is lost underwater.

Feeding Ecology of a Great White
“What we are dealing with here is a perfect engine, an eating machine. It's really a miracle of evolution. All this machine does is swim and eat and make little sharks, and that's all.” –Hooper

          In the movie, we all watch as the shark eats person after person, regardless of size, age, or body composition.  Is this realistic or merely terrifying fiction?  As you may have guessed, sharks are much more discerning and selective in real life about what they choose to chase and ingest, something we humans could probably learn a thing or two about (brownies, anyone?)!

            As juveniles, great white sharks mainly feed on fish.  As they grow older, they hunt bigger prey like seals and sea lions.  Besides learning this from looking at stomach contents of recently captured animals, we can also see this in their migratory patterns.  On the coast of California, great whites given birth off the coast of Southern California in the Spring, and then move north to the seal and sea lion breeding grounds of Ano Nuevo and the Farallon Islands, near San Francisco.  This prey preference based on age can also be studied by looking at the teeth of great whites.  Juveniles have narrow, sharply pointed teeth without serrated edges that are better for grasping fish.  Adults have broader and serrated teeth that can more easily bite through thick skin.  Humans are not preferred prey for sharks.  In the case of most attacks, the victim (often with a surfboard) is mistaken for a seal or sea lion and released once the shark realizes its mistake.  People are too bony with not enough fat to be a good meal for a great white, a fact that is only comforting from dry land.

 “I'm only trying to say that Amity is a summer town. We need summer dollars. Now, if the people can't swim here, they'll be glad to swim at the beaches of Cape Cod, the Hamptons, Long Island...” –Mayor Vaughn
That doesn't mean we have to serve them up as smörgåsbord!” –Chief Brody

 Temperature and Digestive Speed of a Great White

“probably not the shark that killed the little boy…which I wanted to prove today, by cutting the shark open.”-Hooper

       In the movie, Hooper and Brody want to know for sure that the tiger shark caught in the first half of the movie was the killer.  Hooper explains that sharks digest their food slowly, so if they cut it open they would be able to tell if it had recently eaten any humans.  Is it reasonable to expect to find human remains in a shark the day after it has eaten?

            As it turns out, the metabolism of great white sharks is very similar to that of birds and mammals.  There are several indicators of metabolic speed that were used to compare these different groups.  The hemoglobin (molecule that carries oxygen) count and hematocrit (proportion of red blood cells) can be measured as a determination of the oxygen carrying capacity of blood.  A higher metabolic rate means higher oxygen consumption, and that the blood needs to be able to carry more oxygen.  Also, the heart weight to body weight ratio and the size and output volume of the ventricles of the heart of great white sharks is very close to that of humans. This indicates that the work the heart must do to pump blood through the body is similar.  This high metabolic rate allows sharks to be very active and catch fast swimming prey, as well as enables them to live in many different water temperatures.

            It takes about 24 hours or possibly more for humans to digest a meal.  It is believed that this would be the same for sharks, however because of their different diet, it usually takes them much longer than this for them to digest a meal.  One seal can feed a shark for up to six weeks.  This is because the fat content of seals, the usual prey organism for great whites on the West Coast, is extremely high.  Seals and sea lions have a lot of blubber to keep them warm in the cold waters around us.  Blubber is extremely rich in fat, which contains a lot of energy for the shark to consume.  It also means that it is very slow to digest.  However, because great white sharks have a high metabolism they are able to digest fat faster than other species of sharks; this is beneficial because the faster a shark can digest food, the less energy it needs to expend and more energy can be put away for storage.

            Would Hooper and Brody have found the body of Alex Kintner had they caught the correct shark? Probably, if they were there fast enough.  But they would have had to rely on bones at that point; the low fat content of a young boy means that he would have been digested rapidly!

Great white sharks are endothermic, which means that they produce their own heat (due to their metabolism) to maintain body temperature.  Along with the digestive speed that we just discussed, this allows great white sharks to live in cold water and still be able to move fast enough to catch rapidly moving prey.  They have a body temperature that can be kept higher than the 55 degrees Fahrenheit off the California coast.  In particular, their brains, eyes, stomachs, and swimming muscles are much warmer than the water, even compared to the rest of their body.  How does this happen?  The same mechanism can be seen in animals that walk on ice or snow, like polar bears.  It is due to the presence of vascular counter current exchangers, which allow organisms to retain some heat rather than losing more to the environment.  In sharks, this happens when blood passes through the gills, picking up oxygen but losing heat.

            Counter current exchangers are sets of parallel arteries and veins, or other vessels that transport blood in opposite directions.  Warm blood from the body of the shark travels through the blood vessels of the animal.  As it passes through the thin tissue of the gills, it becomes extremely cold because it is closer to the external environment.  Luckily, vessels carrying blood the opposite direction run very close to these warm vessels and are already cold due to having passed through the gills first.  Because of the proximity of these sets of arteries and veins, the warm blood transfers heat to the cold blood through the vessel walls, so by the time it actually gets to the gills, there is not as much heat to lose and more of it is retained by the animal, saving a lot of energy and preventing tissue damage caused by cold!  In great white sharks, this allows them to swim faster, see better, and keep digesting!

 
Killing the Shark
“Smile, you son of a [you know…].”-Brody

               In the movie, the shark is killed in the end by Brody when he shoots an air tank clenched in the jaws of the shark, causing a huge explosion and ending the beast’s reign of terror.  While no one has ever tested the feasibility of this on a real shark (Mythbusters Jaws special episode aside), mechanoreceptors located in the jaw of the shark make it sensitive to contact with a hard object, and electroreceptors sensitive to galvanic fields emitted by metal objects underwater that can also be found on the head of the great white make it unlikely that this could happen.

            Admittedly, it took me four tries to watch the entire movie, and the fourth and final time was largely attributed to being distracted at opportune (read: scary or bloody) parts and to what I hope is increased maturity and an ability to think scientifically.  While Jaws is admittedly enjoyable if you like that sort of thing, its loosely factual basis has left behind a legacy of exaggerated fear and misunderstanding.  Great white sharks are commonly seen on both coasts, and are extensively studied off the coast of Northern California near San Francisco.  Most surfers have seen more than a few, and accept them as part of the scene.  Scientists (aided by the Discovery Channel and Shark Week, among many) have been doing their best to dispel the irrational fear people have regarding sharks, and while they are incredible predators and should be respected, great whites are not the monsters seen in Jaws.

 
“Its all psychological…You yell shark, we’ve got a panic on our hands.”-Mayor Vaughn

References
Estrada, J.A., A.N. Rice, L.J. Natanson, and G.B. Skomal. 2006. Use of isotopic analysis of vertebrae in reconstructing ontogenetic feeding ecology in white sharks.  Ecology 87(4):829-834

 Goldman, K.J. 1997. Regulation of body temperature in the white shark, Carcharodon carcharias. Journal of Comparative Physiology and Biochemisty 167:423-429.

 "Jaws (1975)." IMDb. N.p.. Web. 5 Mar 2013. <http://www.imdb.com/title/tt0073195/>.

 Klimley, A..P. 1994. The predatory behavior of the white shark. American Scientist 82(2):122-133.

 Klimley, A.P., B.J. Le Boeuf, K.M. Cantara, J.E. Richert, S.F. Davis, S. Van Sommeran, and J.T. Kelly. 2001. The hunting strategy of white sharks (Carcharodon carcharias) near a seal colony. Marine Biology 138:617-636.

 Roesch, Ben. "White Shark Physiology: Warm Bodied and Ready to Go.". Web. 4 Mar 2013. <http://web.ncf.ca/bz050/wsphysio.html>.

 Wroe, S., D.R. Huber, M. Lowry, C. McHenry, K. Moreno, P. Clausen, T.L. Ferrara, E. Cunningham, M.N. Dean, and A.P. Summers. 2008. Three-dimensional computer analysis of white shark jaw mechanics: how hard can a great white bite? Journal of Zoology  1-7.

Image References
http://oceanlink.island.net/biodiversity/shark/shark.html

 http://www.arkive.org/great-white-shark/carcharodon-carcharias/image-G2550.html

 Klimley, A..P. 1994. The predatory behavior of the white shark. American Scientist 82(2):122-133.

http://www.huffingtonpost.com/2012/10/31/scott-stevens-eureka-shark-attack-surfer-california-humboldt-video_n_2049345.html

http://tolweb.org/treehouses/?treehouse_id=4847

http://www.releasedonkey.com/dvd/jaws-dvd-release-date-1533/still-3176771584

 
 

3 comments:

  1. Hello , you say in your comments that the body of the boy alex kintner would have been digested rapidly by the shark , what about the red swim trunks he was wearing, what will happen to them. thanks

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