Aquagreen

Abstract

This paper evaluates social behaviour of Carcharodon carcharias, white shark, by examining their evolved predatory abilities.  Carcharodon carcharias have adapted to the life history of pinnipeds (seals) which molt at coastal locations.  Adaptations of Carcharodon carcharias include electroreception, acute vision, and a countercurrent heat exchange system.  These adaptations have increased the brain size of white sharks.  A larger brain increases their ability to learn sociable behaviours.  A review of literature on Carcharodon carcharias biology and behaviour supports the view that white sharks display social behaviour.  Schooling juveniles are captured in large numbers at once.  Born to a pack of several individuals of the same age class promotes social bonding.  Carcharodon carcharias often move in and out of range simultaneously.  Coordinated movements identify patrolling within a group.  White sharks are solitary hunters that frequent the same conspecifics, individuals of the same species.  Two individual white sharks frequent each other more than other Carcharodon carcharias of a patrolling group.  Sociable behaviour occurs instead of seriously injuring each other to increase chances at acquiring more food energy.  White sharks search for prey independently but remain close enough to sense and exploit each other’s kills.

Keywords

Ampulli of Lorenzeni, Carcharodon carcharias, countercurrent heat exchange, social behaviour, vision, white shark,

Introduction

The fossil record of Carcharodon begins in the Paleocene period about 60 million years ago.  The total length (TL) of a present day adult Carcharodon carcharias is between 6 to 14 meters, females are larger than males (McConnaughey and McConnaughey 1985).  The white shark was a big game fish, but now is protected in certain areas as over-fishing has reduced its numbers.  White sharks are found worldwide in tropical and temperate seas, more commonly in the latter (Potts and Sawby 1997).  They hunt, < 37 m from the surface, 1.3 km from the shore (Klimley et al. 2001b).  Around North and South America, Carcharodon carcharias are found from Alaska to Gulf of California and from Southern Newfoundland to Brazil including the Gulf of Mexico (McConnaughey and McConnaughey 1985).  Carcharodon carcharias scavenges and preys on bony fish, sharks, sea turtles, pinnipeds, and cetaceans (whales, dolphins, and porpoises) and have even been known to attack boats off northwestern Vancouver Island (Klimley and Ainley 1996).  Carcharodon carcharias are mainly a coastal species found between the surface and depths of 1280 meters (Potts and Swaby 1997).  Carcharodon carcharias bear between 9 and 14 live young (Francis 1996).  Parturition, birth to live young, occurs in temperate waters, during the spring and summer months.

Carcharodon carcharias can detect weak electrical cues from muscle contractions of their prey.  Small pores on the white shark’s head, snout, and mouth lead to conductive jelly-filled canals that terminate in receptor cells, called Ampullae of Lorenzini.  The received electroreception information is transmitted directly to the central nervous system.  The need to integrate sensory information has increased the size of the Carcharodon carcharias cerebellum (Kardong 1998).    Carcharodon carcharias have good vision and hearing capabilities.  Surrounding the orbits of the eye, near the brain, are orbital retes (dense network of capillaries) enhancing vision ability (Helfman 1997).  A white shark’s retina is dominated with rods, which are active at night and twilight.  They allow light to strike the cells twice increasing the likelihood of detection of prey by Carcharodon carcharias.  Reduced cones in the retina, increases daytime visibility and is associated with colour ability (Helfman 1997).  Carcharodon carcharias vision is more similar to mammals than most other fishes (Helfman 1997).  Separation distance between two individual Carcharodon carcharias is less than random (Klimley et al. 2001a).  Two white sharks swim within visual range that is less than water visibility (Klimley et al. 2001a).  White sharks can also detect prey with macule, nerve cells linked to granules that vibrate in response to sound.  These predatory adaptations correlate with increased brain mass (Kardong 1998).

The blood transport of Carcharodon carcharias transports heat produced as a by-product of their exercising muscles.  Blood circulation to extremities is warm but seawater is cold.  Therefore, much heat would be lost to the environment were it not for specialized features of the circulatory system.  White sharks are endothermic, a warmer body temperature relative to their environment, because of a countercurrent heat exchange system.  Carcharodon carcharias have heat exchangers in the circulatory system.  Masses of parallel arteries and veins form the heat exchangers, the rete mirabilia (Goldman et al. 1996).  This is an elaborate intertwining network between outgoing arteries and returning veins.  Blood in a rete establishes a countercurrent pattern between outgoing arteries and these returning veins.  Before the blood reaches the integument (skin) warmth is passed through the rete.  Heat carried in the arteries is transferred almost completely to the returning blood in the veins.  By the time blood reached the extremities, little heat remains to be dissipated to the environment.  Therefore, retes function as heat blockers to prevent body heat from being lost through the extremities.  White sharks also have a thick ventricle similar to the left ventricle in humans to pump blood efficiently throughout the body (Goldman 1997).  Retes have enabled Carcharodon carcharias to develop an optimal physiological operating temperature.  Other benefits of warm blood include increased rates of neural, digestive, and muscular activity (Goldman 1997).  Increased digestion enables increased absorbency of energy from food.  Carcharodon carcharias have elevated brain temperatures enabling heightened cognition to capture swift agile prey (Goldman 1997).  Carcharodon carcharias have evolved physiological mechanisms to efficiently prey on warm bodied aquatic animals by thermoregulation.  Warm blood keeps the brain warm, enhancing quick response time from integrated sensory information.

White sharks have larger brains than ectothermic vertebrates (Demski and Northcutt 1996).  The brain-body mass ratios of Carcharodon carcharias overlap, in relative size and complexity, to brains of mammals (Demski and Northcutt 1996).  The cerebrum, forebrain, is the largest part of the vertebrate brain.  It consists of two cerebral cortex hemispheres with Grey matter surrounding each hemisphere.  Grey matter contains billions of nerve cells to integrate sensory and neural functions.  The cerebrum enlarges during vertebrate evolution (Kardong 1998).  An increase in the size of the cerebrum is due to increasingly complex behaviours and muscle control.  Reception of olfactory, visual, and other sense information is a major function of the cerebrum (Kardong 1998).  The cerebrum processes data and returns information back to the central nervous system.  Enlargement of the forebrain reflects its increasing role within the locomotion system.  A larger cerebrum offers abilities in learning and thinking (Myers 1995), enabling Carcharodon carcharias to become even more adaptive to its predatory niche.  The cerebellum, hindbrain, is smaller than the cerebrum but also consists of two hemispheres with a core of white matter and outer Grey matter.  The cerebellum’s function is to learn using short and long-term memory, and to coordinate muscles for balance (Kardong 1998).  Active aquatic organisms navigate in three dimensions so equilibrium and balance are very important.  Thus, the cerebellum is well developed.  The cerebellum helps to control ambushes by Carcharodon carcharias on seals from below the water’s surface (Demski and Northcutt 1996).  Brain development increases behavioural capabilities in white sharks (Demski and Northcutt 1996).

Increased brain mass facilitates social abilities and behaviours; tail slaps (TS), breaching (BR), and repeated aerial gaping (RAG).  I hypothesize that Carcharodon carcharias are social because this decreases their predatory energy requirements.  Staying around other white sharks promotes their chances at taking part in each other’s feedings.  If this is true then Carcharodon carcharias will spend more time with other sharks, than being alone, and will also display social behaviour to other white sharks.  This analysis will reveal how Carcharodon carcharias predatory adaptations promote intraspecific and interspecific social behaviour.

Methods and Materials

An evaluation of relevant literature on Carcharodon carcharias evolved predatory adaptations, white shark social behaviour, and ichthyology.  Search scientific databases, journal articles on biology and hunting beahviour of white sharks. Information from resources was analyzed to identify how Carcharodon carcharias exhibit sociable behaviour.

Results

Stomach temperature is a good estimate of core body temperature.  Carcharodon carcharias can keep their body temperature warm in different temperatures of water.  Two individual Carcharodon carcharias frequent each other more than time spent alone or in a group. Klimley et al. 2001b showed five sharks patrolled the same area near a seal colony at Ano Nueno Islands, California for two weeks.  The area patrolled by three sharks overlapped more than the two other sharks.  Two of these three sharks spent most of their time with each other.  Predatory adaptations can facilitate social bonding.  Electroreception, vision, and hearing adaptations increase detection of conspecifics.

Displays in Carcharodon carcharias such as TS and BR occur between white sharks (Klimley et al. 1996).  Klimley et al. (1996) found white sharks swimming repeatedly by each other at the surface, approaching within 1m of each other every time.  As they passed, one directed four splashes at the other, which splashed water and tail slapped in the direction of the first shark.  During these passes, the first shark was positioning itself between the second and a seal carcass.  The first displays were more vigorous than the second shark, resulting in the first shark winning the combat and returning to bite its prey.   Breaching consists of a white shark propelling all or two thirds of their body out of the water at a 30-90 degree angle to the sea water surface landing with a large splash.  This behaviour is witnessed during feeding battles (Klimley et al. 1996).

Repeated aerial gaping (RAG) can be displayed by Carcharodon carcharias to boat dwellers while fishing (Figs. 6 and 7).  The behaviour includes the white shark holding its head out of the water and opening and closing its mouth in a series of slow, rhythmic partial gapes while awkwardly swimming slowly along the surface.  Repeated aerial gaping can follow hindered feeding attempts when bait is withdrawn (Strong JR. 1996).  Contact with air is required to initiate RAG, as it is not witnessed underwater.

Discussion

Adaptations in white sharks increase their ability to detect prey species.  These adaptations enable interaction among Carcharodon carcharias individuals.  Adaptations such as electrochemical reception, acute vision, and warm blood all increase brain mass.  Retes have evolved to maintain highest physiological efficiency by maintaining optimal operating temperature in different water column temperatures.  Endothermy allows Carcharodon carcharias to pursue their fast moving prey in cold waters.  Warm blood increases brain activity (Kardong 1998).  Selection pressures drive brain development.  These adaptations facilitate social behaviour.  Adapting to hunt for food in murky cold water has increased the ability for social behaviour in white sharks.

Young Carcharodon carcharias grows up together in a school and form social bonds to their same age group (Francis 1996).  Klimley et al. (2001a,b) found that individual Carcharodon carcharias will spend most of their time with another familiar shark.  Highly social, nonrandom, swim modes such as rapid head on turns and staying within visual range of each other represent Carcharodon carcharias working in unison with one another.  It is disadvantageous for sharks to bite each other and thereby decrease each other’s chance for increased food abundance.  Therefore, TS and BR warn other sharks from exploiting each other’s prey.  The recipient shark perceives with electoreception, vision, hearing, and cognition.  Carcharodon carcharias’s TS and BR more in a large group than when alone.  Socializing increases feeding rate relative to foraging away from sharks (Klimley et al. 2001b).  The benefits to allowing one shark eat your prey include a trade off between sharing a carcass and gaining the chance to feast without having to expend energy to do the killing in the future.  White shark alliances could be made up of dominant and subordinate members.  By not harming the other shark, the TS signaler gains an energy advantage by increasing its chances at exploiting the other shark’s prey.  Tail slapping occurs on departure of Carcharodon carcharias kills and another white shark (Klimley et al. 2001b).  A shark fed further on a seal only if the vigor and frequency of its TS are greater than those of its opponent’s (Klimley et al. 1996).  Therefore, TS is an agonistic display to ward off competitors.  Breaching is used much less commonly but may be a higher intensity agonistic display.  These behaviours may also be used for courtship displays (Barlow 1996).

Increased brain activity promotes displayed behaviours in Carcharodon carcharias.  Strong Jr. (1996) suggested five working theories to explain repeated aerial gaping (RAG).  First, RAG may be associated with ingestion and or respiration.  As the head enters the water, after a lung, large amounts of air leave the mouth and gills.  Therefore, RAG can help eliminate air trapped in the buccal cavity and or stomach.  Second, the situation preventing the Carcharodon carcharias from completing a predatory behaviour may promote RAG.  Thirdly, RAG may decrease frustration, after RAG some sharks bite ship parts.  Similar biting of conspecifics has been observed in Carcharodon carcharias aggregations near food sources.  Larger conspecifics will stop a smaller Carcharodon carcharias from feeding and induce aggression behaviour (Strong Jr. 1996).  Fourth, RAG may be to increase probability of seizing prey at the air-water interface.  Finally, RAG may be to intimidate and confuse prey.  For example, killer whales create water disturbance at the water’s surface to confuse pinnepeds into the water (Strong Jr. 1996).

Adapting to cold, low visibility water enables white sharks to specialize an oceanic niche.  For Carcharodon carcharias to be social they would show certain behaviours to conspecifics.  Great white shark’s adaptations increase brain mass and cognitive learning abilities.  Social bonds decreases time spent to find prey, and learning to socialize creates a positive feedback by increasing efficient predation.  Tail slapping and beaching are behaviours know to be displayed during predation attempts between conspecifics.  More studies on RAG are required to identify this bazaar displays towards boaters. Further research is needed the socializing behaviour and possible hierarchy culture structure of Carcharodon carcharias.

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