Structure and Function of the

White Shark Brain

The shark brain has long been impugned as being tiny, simple, and relatively unimportant. Superficial examinations of the brain of small, evolutionarily conservative sharks such as the Spiny Dogfish (Squalus acanthias) seemed to bear this out. Experiments severing the spinal cord of swimming dogfishes, so that the brain could no longer co-ordinate swimming movements, have demonstrated that these sharks can continue to swim for several hours although they no longer respond to changes in their swimming environment. Sharks were therefore dismissed as primitive, stupid automatons functioning almost exclusively on the basis of brutish instinct. By painting all sharks with the same biased brush, we have arrogantly denied their biological diversity and grossly underestimated their mental capabilities.

It turns out that the brains of at least some sharks are surprisingly large and complex. Numerous popular writers have characterized the brain of a full-grown White Shark as being about the size of a walnut. While this description creates a vivid mental image, it is inaccurate and highly misleading. In truth, this includes only one part of the White Shark brain, the cerebrum. The complete brain of an adult, 16-foot (5-metre) White Shark is Y-shaped and from scent-detecting bulbs to brainstem measures about 2 feet (60 centimetres) long. In comparison, the brain of an adult human is composed of two, wrinkly hemispheres and is about the size of half a cabbage. Of course, a full-grown White Shark is much more massive than an adult human: a 16-foot Great White weighs about 2,700 pounds (1,230 kilograms), while an 'average' adult human males and females combined weighs about 165 pounds (75 kilograms). Since large animals tend to have larger brains than small animals, a more meaningful comparison is brain weight versus body weight. In this way, the relative weight of brain tissue for each pound (or kilogram) of body weight can be readily compared among even highly dissimilar species.

A ground-breaking 1996 paper by Leo Demski and R. Glenn Northcutt examined the size and basic morphology of the White Shark brain. As their study subject, Demski and Northcutt obtained the head from a mature male White Shark that originally measured 11.75 feet (3.6 metres) in length and weighed about 950 pounds (430 kilograms). They found that its brain weighed only 1.2 ounces (35 grams), which works out to 0.008 percent of its total body weight. In comparison, the brain of a typical adult human weighs about 48 ounces (1,400 grams), or about 1.9 percent of total body weight. Thus, for each pound (or kilogram) of body weight, humans have about 238 times more brain mass than White Sharks. Yet the brains of White Sharks and humans are shaped, structured, and organized very differently from one another. Since we do not well understand how these differences affect mental capabilities, it is prudent to compare the brain weight-to-body weight ratio of the Great White to that of other sharks.

French anatomist Roland Bauchot and his co-workers have made extensive comparative studies of brain weight-to-body weight ratios in cartilaginous fishes. In an ambitious 1995 paper, Bauchot et alii survey brain size and development in 81 species of sharks, rays and chimaeras, allowing comparison with Demski and Northcutt's 1996 findings about the White Shark brain. The brain of the White Shark is relatively large compared with that of dogfishes (family Squalidae) and skates (Rajidae), but relatively small compared with that of whaler sharks (Carcharhinidae) and stingrays (Dasyatidae). Interestingly, the Manta and devil rays (subfamily Mobulinae) have the largest brains relative to their body weight of any elasmobranch studied to date. Why these gentle plankton-grazers have evolved such large brains remains a mystery. The smallest brain among elasmobranchs is found in the Angelshark (Squatina squatina), which is a bottom-dwelling, lie-in-wait piscivore with few predators. Evidently, this lifestyle does not require a huge brain. Among lamnoids the shark order which includes the Great White the toothy but phlegmatic Sandtiger Shark (Carcharias taurus) has the largest brain relative to its body size, with a brain weight-body weight ratio overlapping those of some whaler sharks. In contrast, the Basking Shark (Cetorhinus maximus) has proportionately the smallest brain among lamnoids, which may reflect its placid filter-feeding habits (it doesn't take much brainpower to outwit plankton). Therefore, compared with other actively predaceous sharks, the Great White has a brain that is roughly medium-sized relative to its body weight. In absolute terms, however, the brain of a White Shark can grow quite large because the animal itself can attain prodigious size.

Despite its sometimes impressive dimensions, the brain of the White Shark is a marvelously compact structure. Composed of countless millions of neurons (nerve cells) and supporting structures, the Great White's brain co-ordinates virtually all of its activities from protrusion of its jaws to delicately grasp a novel object to the thrashing of its tail to ward off a competitor. Unlike the brain of humans and most other mammals, that of the Great White is not rolled into a ball and most parts are easy to see. The White Shark's brain is arranged in a more-or-less linear fashion, with specialized regions strung along like pearls. These regions can be conveniently grouped into hind-, mid-, and fore-brain, each of which is dedicated to a constellation of related functions.

According to Demski and Northcutt, the overall structure of the White Shark brain is fairly generalized, similar to that of the closely-related Shortfin Mako (Isurus oxyrinchus) and Basking Sharks. As in other sharks, the spinal cord of the Great White enters the back of the skull and merges imperceptibly with the hindbrain. The hindbrain forms the base of the Y and consists of two main parts, the brainstem and cerebellum. The White Shark's brainstem bristles with the posterior cranial nerves. These include nerves responsible for conveying input from the shark's inner ear, lateral line and electrosensory systems. Although the cranial nerves are astonishingly thick (some were initially mistaken for muscles), Demski and Northcutt were unimpressed with the brain centers responsible for analyzing acoustic, vibratory, or electric stimuli. They concluded that these senses are probably relatively unimportant to the White Shark. The White Shark's hindbrain also contains the brain center (amygdaloid nucleus) that probably mediates instinctive fight-or-flight responses. Therefore, much as you or I might withdraw our hand or foot before we fully realize we have been punctured by a splinter, a Great White's 'decision' to attack or flee in self-defense apparently occurs at a very basic level, below that of learned responses in the neural hierarchy of the shark brain. The White Shark's cerebellum is perched atop the anterior part of its brainstem. This structure is quite large and well-developed in the Great White, highly convoluted and folded so that it is asymmetrical. As in humans, the White Shark's cerebellum is responsible for muscular co-ordination, especially in response to sensory input. Thus, when a White Shark flinches in response to a sudden loud noise or veers toward a novel vibration or attractive electrical signal, it is obeying signals from its cerebellum.

The top of the White Shark's midbrain features a pair of prominent swellings. These are the optic lobes, which are responsible for coordinating visual input. The central area of the White Shark's midbrain that mediates visual discrimination is relatively small. Based on this, Demski and Northcutt suggest that this species may be less adept at resolving fine details than certain other sharks. However, the 1985 study of the White Shark visual system by Samuel Gruber and Joel Cohen reported in the previous chapter suggests that vision is well-developed in this species. Since the optic nerves and eyes are actually extensions of the brain itself, image discrimination including fine detail and color in the White Shark may be taken over directly by its specialized retina. Vision is clearly very important to the predatory and social life of the Great White. It has thick optic nerves and large eyes controlled by massive muscles that not only rotate the eyeballs within their sockets but also generate significant metabolic heat. This heat, in turn, may allow faster processing of visual information and increase the efficiency of visual and other neural activity. While all these features suggest that the White Shark has excellent vision, we do not know whether this is true. Formal behavioral experiments to test what White Sharks can and cannot see have not yet been carried out. Thus, for the time being, only the Great White knows for certain what it actually sees through its large, dark eyes.

The White Shark's forebrain, however, may be the neural region of greatest popular and scientific interest. The anterior forebrain of the Great White consists of the olfactory (scent-detecting) organs and the cerebral hemispheres the part of the brain responsible for learning and memory. It had long been thought that some 70% of the shark brain is dedicated to the sense of smell. We now know that this startling figure is based on a major misinterpretation of how the shark brain is organized. In most vertebrates, the olfactory bulbs are connected to the front of the cerebral hemispheres. It was assumed that the same arrangement occurred in sharks. But elasmobranchs are unique among vertebrates in that the olfactory tracts forming the branches of the Y-shaped brain fuse to the sides of the cerebral hemispheres. This arrangement creates a false impression that the olfactory organs extend far tailward and that the cerebrum is tiny and relatively insignificant. Even today, many modern comparative anatomy and neurobiology textbooks continue to incorrectly label shark cerebral hemispheres "olfactory bulbs", perpetuating the myth that sharks are mindless eating machines led around by a phenomenal sense of smell.

The White Shark's olfactory organs are huge, with long olfactory tracts and well developed lamellae which increase their surface area and thus greatly enhance sensitivity. Of all the features of the Great White's brain, the complexity and sheer massiveness of the olfactory organs most impressed Demski and Northcutt. Based on the exceptional development of these structures, Demski and Northcutt suggest that detection of olfactory stimuli may be extremely important to the White Shark. Olfactory cues likely to be of particular importance to White Sharks are: detection and identification of potential prey; recognition of various environmental markers, such as estuaries which may serve as nursery areas or harbor concentrations of prey; and perhaps most importantly recognition of individual members of their species, including potential mates. In the visually concealing medium of the sea, the Great White may live in a perceptual universe dominated by scent.

But perception is only a single component of behavior. Once a stimulus is received and interpreted by the appropriate brain centers, the animal must decide how to act upon it: ignore, investigate, intimidate or evade. Whatever 'thinking' a White Shark does occurs primarily in its cerebral hemispheres. According to Demski and Northcutt, the Great White's cerebrum is not exceptional, being moderate in size and degree of folding. The central area of the cerebrum is thought to be responsible for home ranging and social behavior in sharks. This region is small in the White Shark compared with that of whaler sharks. In addition, like that of other sharks, the brain of the Great White is mostly hollow perforated by a series of irregular, interconnected chambers (ventricles) and filled with a complex fluid (cerebrospinal fluid) that probably helps regulate the brain chemically. These brain chambers, called ventricles, are particularly large in the White shark, resulting in a brain composed of an unusually thin 'shell' of nerve tissue. Based on these features, Demski and Northcutt suggest that the White Shark may not patrol well-defined home ranges or exhibit dynamic social hierarchies as in the more cerebral, thicker-brained carcharhinids. Yet recent research off California, South Australia, and South Africa suggest that White Sharks move through their environment in fairly predictable patterns and seem to exhibit display behaviors that serve to establish which individuals are dominant and which submissive. We know so little about how the physical structure of the brain is manifested in that elusive specter we call 'mind', it seems premature to dismiss the mental capabilities of the White Shark based on the relative size of parts of its brain.

It is also worth bearing in mind that brains do more than think. For example in addition to the olfactory organs and cerebral hemispheres the forebrain also contains two intimately related structures, the hypothalamus and pituitary gland. Slung below the optic lobes of the Great White's brain, the hypothalamus is important in regulating many activities vital to its survival. The hypothalamus produces hormones (chemical messengers) that regulate bodily processes and contains centers that control such life-sustaining activities as heartbeat, body temperature, metabolic rate, osmoregulation (internal salt and water balance), food intake, and digestion. Electrical stimulation of the hypothalamus has revealed that biting is also controlled by this multi-talented structure. Protruding from the top of the hypothalamus is the epiphysis, which is sensitive to day length and perhaps position of the sun; it may therefore may provide information useful for coordinating navigation and migration. The hypothalamus regulates the pituitary gland and as a result elegant feedback mechanisms much of its activity is, in turn, regulated by the pituitary gland. The pituitary has been called the "master endocrine gland", as it secretes the hormones that control most other ductless, hormone-secreting glands. Directly or indirectly, the pituitary controls virtually every aspect of maintaining an optimal internal milieu blood pressure, blood sugar levels, kidney activity, growth, calcium budget, metabolic rate, blood testosterone levels, ovulation, uterine contraction, and many, many others. The hypothalamus-pituitary gland system thus quietly controls many biological processes that underlie some of the Great White's most spectacular and evolutionarily vital behaviors including feeding and mating.

How the White Shark translates sensory input and its brain's internal chemistry to behavioral output largely remains a mystery. But why sharks in general have such large complex brains is far less so. A large White Shark has a brain that may measure 18 to 24 inches (45 to 60 centimetres) from the olfactory organs to the brainstem and features a relatively large, complexly folded cerebrum. Yet a scaled-up goldfish or salmon could make do with a fraction the cerebral endowment of a White Shark. All of which begs the question: what does a Great White think with all that brain?

Men's brains are roughly the same size as those of women, but on average women's bodies are smaller and lighter than their male counterparts'. As a result, the brain of a woman is typically about 2.2% of her body weight proportionally some 27% larger than a man's. Make of that what you will. [Back to Text]

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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