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Shark Info 3 / 99   (10-15-1999)



Circling around alleged victims

Shark Info

  Main article:

Shark Behavior: Circling around alleged victims

Dr. E. K. Ritter

  Article 1:

What do you know about shark brains and noses?

Dr. J. F Morrisey

  Article 2:

Film Review: Deep Blue Sea

Shark Info

  Article 3:

Annual Shark Congress in Pennsylvania

Shark Info

  Article 4:

Weak shark protection

Shark Info

  Fact Sheet:

Bull Sharks

Dr. E. K. Ritter

What do you know about shark brains and noses?

By Dr. John F. Morrissey

The world is full of shark enthusiasts with a seemingly insatiable appetite for any type of information they can obtain on sharks. Fortunately, a steady stream of books, articles, documentaries and movies are produced and available to satisfy this appetite. Unfortunately, a fairly large portion of these sources deliver inaccurate or false information, some of which is the creation of only one single author. However, most mistakes are so often repeated or copied from source to source that in time many of us unquestionably accept such information as fact. Inevitably, such data will eventually develop a life of its own. Once established, such information is all the more frequently quoted or recopied and thus molds already existing dogma on sharks more and more. The objective of this article is to uncover some of these "false truths" which surface at alarmingly regular intervals in the press or other media.

Statement No. 1: "Sharks smell blood many kilometers away."

The target of this meaningless statement which can be found in many different versions in the media, is clear. The author wishes to express the notion that sharks have a well-developed sense of smell. This in effect is correct, sharks are capable of recognizing even very small concentrations of scents in their immediate environment. However, the fact that sharks may smell blood at long distances says virtually nothing about their sense of smell, the reason being that the author sets the shark's sense of smell in relation to the distance from its source. Nothing is mentioned about the concentration of molecules necessary for the shark to notice the smell. But this is exactly the decisive point. The actual distance to the source is irrelevant. For example, humans have a significantly worse sense of smell than sharks, and yet we may still identify the smell of blood from significant distances if the amount of blood is large enough for the molecules to reach our mucous membranes (the nose) in adequate concentrations. Personally, I at least once smelled the smoke of a forest fire burning about 100 km away! But not because my sense of smell is as phantastic as that of sharks, but rather because the source of the smell (the fire) was big enough to allow a sufficient concentration of smoke to reach my nose from a distance of 100 km.

The significance lies in the concentration of molecules and the decisive question can be posed accordingly: Does the available concentration of odorous substances exceed the perception limit of a shark (or a human)? This is the only way to properly discuss the perception limits of a species. All other statements on distance have nothing to do with the question and are irrelevant. So what are the facts? How effective are shark noses in reality? Sharks can recognize most amino acids, the basic component of proteins, even in such small concentrations of 10-10 molar (a "one molar" solution is equivalent to the molecular weight of a substance in grams dissolved in one liter of water). Permit me to express this astronishing sensory perception in a more understandable manner. First, it means that sharks smell about 10,000 times better than humans. Second, to reach this concentration about 30 mg (measuring half a pinch, editor's comment) of an amino acid would have to be dissolved in a large swimming pool filled with saltwater! Third, such a swimming pool would have to have a volume of 2,600 cubic meters and this amount of saltwater would contain 3.5% of cooking salt and other ions, in other words, an approximate total of about 92 tons of salt. It is very hard to imagine that a shark can still perceive these few milligrams of amino acid when mixed with tons of salt. Fourth, sharks can even sense the presence of certain amino acids better than others. For example, they smell the amino acid Serin in concentrations of only 10-14 molar! Fifth and finally, it must also be said that although such sensory perception levels may be quite astronishing to humans, for other animals they are fairly run-of-the-mill. Ordinary animals such as lobsters or catfish have a comparable if not better sense of smell than sharks.

However, all comparisons with chemoreception performance such as the sense of smell must be made in relation to the concentration of the odorous substance and not the distance to their source.

Statement 2: "The brains of sharks and other primitive animals (such as dinosaurs) are not bigger than walnuts, golf balls or similar, rather small objects."

This statement is even less relevant than the first. The first statement at least has the advantage that in principle it is correct. It only becomes meaningless when based on the erroneous unit of distance rather than the correct unit of concentration. But the second statement tries to indicate that sharks are not very intelligent and this is substantiated by an unsuitable comparison.

Let us analyze this second statement and try to determine its effective relevance.

First, sharks, like all predatory vertebrates, are relatively intelligent. This is necessary since they have to catch their prey. As a rule their prey are herbivore (plant eaters) which usually are not very intelligent since they only have to "catch" grass. Second, statements on brain size are usually not very informative. Is an otter less intelligent than a dog simply because his brain is smaller? Or is a dog less intelligent than an elk just because the elk's brain is larger? The answer must be a categorical NO.

Brain / body weight

The relationship of brain to body weight according to Glen Northcutt. Sharks have relatively large brains when compared with other classes of animals.

© Shark Info

To quote absolute brain sizes ("the shark's brain is the size of a golf ball") makes no sense at all, comparing relative brain sizes may, however, be useful. Comparing species on the basis of relative brain sizes (expressed in a percentage relationship to body weight) yields reasonable values for their relative intelligence. In 1978 Glenn Northcutt was able to show that sharks have a bigger brain in comparison to their body size than skeletal fish, most birds and numerous mammals. This was to be expected since most sharks consume meat and being the hunter they need bigger or more intelligent brains than their prey.

Last but not least. Regardless of the intelligence of sharks compared to the rest of the animal kingdom, statement 2 makes no sense whatsoever. Visualize a shark with the brain the size of a golf ball. To which shark species does he belong? And how old is he? Is the shark an adult pigmy shark (Squaliolus laticaudus), a species whose maximum size reaches approximately 15 cm? If yes, this species may look like a lollipop (and would probably be VERY intelligent!). In reality the brain of a squaliolus is much smaller than a golf ball, and this is okay when one considers his body size. Or, is the shark an adult whale shark (Rhincodon typus), a species which may grow to a length of 12 meters? That could make him the world's dumbest "fish", when in reality the whale shark has a brain of about 1 meter in length (in line with his body size). Or is the shark in question a lemon shark (Negaprion brevirostris) with a length of about 45 cm. A golf-ball sized brain would be just right for such a young shark. But what happens when the shark becomes an adult and 15 years later is about three meters long? His golf-ball sized brain would then measure about one half of one of his eyes and be practically useless. Obviously the brain of a young shark grows along with the rest of his body. This is why general testimony on absolute brain sizes is of absolutely no value. Comparing brain size in relation to body size is, however, relevant (once again measured in relation to shark species and age).

I hope this article has slightly changed your perceptions on this subject and will thus motivate you to read certain publications more critically.

* Dr. John F. Morrissey is Assistant Professor at Hofstra University, New York, and a leading shark researcher in his field.

May be published only by indicating the source: Shark Info / Dr. John F. Morrissey


last change: 12-06-2009 12:52 / go