Tue 19 Jul 2016, 12:34
How do these creatures do what they do?
1. Shark brains aren’t really the size of a walnut
The idea that a great white’s brain is the size of a walnut is a common misconception. It’s based on measuring only the shark’s cerebrum. The complete shark brain is actually about 2 feet long from olfactory bulbs to brainstem.
The cerebrum is primarily responsible for learning and memory; the central area, for example, is thought to be connected to home ranging and social behavior in sharks.
But there’s much more to a shark’s brain. The cerebrum is attached to two huge olfactory organs. These structures detect and analyze chemical stimuli, contributing to the shark’s legendary sense of smell. Together with the hypothalamus and pituitary gland, they make up the forebrain. The hypothalamus produces hormones that regulate body processes such as heartbeat and metabolism, and the pituitary gland secretes hormones that control activities such as blood pressure and growth.
Working backward, the optic lobes of the midbrain interpret visual information. Recent research shows that the areas of the shark’s brain associated with visual input are larger than previously thought, suggesting that vision is an important and highly developed sense for sharks.
Finally, we get to the hindbrain. Here, the cerebellum coordinates body movement and some types of motor learning – for example, the fine adjustments needed for complex behaviors like attacking seals. And the brainstem is responsible for conveying sensory input from the shark's inner ear, lateral line and electrosensory systems to the rest of the brain.
2. Sharks can learn, remember, and even teach each other
A groundbreaking study in 1963 was the first to prove that sharks could learn and remember as well as some mammals. Ichthyologist Eugenie Clark trained lemon and nurse sharks to press their snouts against an underwater target and wait for a bell before getting a food reward. They learned to perform the task within a week, and remembered even after a break of several weeks.
At the Bimini Biological Field Station, young lemon sharks were also trained to perform a simple task in exchange for food. But researchers here wanted to find out more about how sharks learn, so they took their study a step further and placed the lemon sharks in different groups. Some untrained sharks were paired with “demonstrators” – sharks that had already learned the behavior that would earn them a food reward. Others were placed in groups with sharks with no experience. When the sharks were split up again, those that had observed the demonstrators were able to hit the target faster and more times than the sharks that had interacted with other untrained groups.
Social learning behavior has been documented in various forms in many different animals, including fish. But this study carried out in 2011 was the first prove that sharks are capable of learning from each other. Understanding how sharks learn and socialize could provide important lessons for preventing shark attacks and protecting the marine ecosystem.
3. White sharks have complex and fairly aggressive sex lives
It may be hard to believe, but no one has ever seen great white sharks mating. To this day, video footage of white shark sex remains one of the Holy Grails of shark science, not to mention shark filmmaking.
But even if no one has ever seen it, we do have a few clues as to what goes on when the lights go down (it’s thought that white sharks mate at great depth where there’s little sunlight). Those clues suggest that great whites have highly complex courtship rituals that involve what these sharks are best at: biting.
We know they bite because of the scars they leave behind in their throws of passion, especially on female sharks. But why do they bite?
Male sharks bite females in various places to let them know they’re interested. It’s their version of flirting – it may not work on (most) of us, but female sharks go crazy for it. And when a male shark is ready to get down to business, he’ll grasp the female’s fins and gills with his teeth in order to keep her still during the act. Though these bites can leave females scarred, they are not life-threatening, and distinctly less vicious than a shark’s normal feeding bites.
Other species of female sharks will signal their rejection or acceptance through movement of their fins and other body parts. They may also stay in shallow water during mating season in order to more easily avoid the males.
4. Sharks have killer instincts, but they can override them
Many common shark behaviors are instinctive, meaning they happen automatically in the same way every time without having to be learned. Instinctive behavior is more complex than a reflex, which is usually an action performed unconsciously in response to a particular stimulus.
One example of instinctive behavior in the great white shark is that its eye will roll back when its snout touches an object. This serves the purpose of protecting the shark’s eyes when it attacks prey.
Over time, as sharks get used to a particular object such as bait from a shark diving cage, they no longer perceive it as a threat and this eye-rolling behavior decreases. Being able to determine when an instinctive response is no longer necessary is an example of learning and intelligence.
5. Sharks are curious, have personalities and can read situations
Sharks are as curious as cats. (Or is that catsharks?) Great whites will approach and investigate just about any unfamiliar floating object, and will usually refrain from doing serious damage to it at first. Instead, they’ll bump and nip the object out of curiosity instead of trying to eat it. Sharks’ teeth and gums are far more sensitive than their skin, so exploring with their mouths is similar to us examining objects with our hands.
According to scientists, this behavior suggests that sharks are motivated by curiosity, not aggression, towards unfamiliar objects. But not all sharks are the same. Individual sharks seem to have distinct personalities, scientists say. Bold or fearless sharks will swim right up to boats and other objects to check them out. These sharks tend to have more scars than those that tend to swim away and timidly watch from a distance when they sense even a slight change in their environment.
And a recent study suggests that sharks read and react to our behavior. At an Australian dive site, sand tiger sharks became more relaxed and inquisitive when divers followed certain guidelines, like not touching or chasing the sharks. Before these rules were put in place, the same sharks reacted aggressively toward divers.
6. Some experts think sharks can play
Sharks are smarter than your average fish, but do they actually play? Scientists aren’t ready to say for sure, but there are some compelling reasons to suggest that they do.
Play is voluntary and typically associated with pleasure and enjoyment. Humans play, but so do certain animals like lions, chimps, bears and dolphins. On the surface play seems like a frivolous waste of energy. But for these species, play serves an essential role in physical and cognitive development, teaching important skills like hand-eye coordination and socialization.
Great whites seem to play. In one study, they engaged in tug-of-war with a ship’s crew, pulling on a sturdy bag of fish suspended by a floating ball while the crew pulled back on the line. The sharks would roll on their back (a sign of relaxation), keep their eyes wide open (rather than closed for protection), and repeatedly return for more tug-of-war.
At Seal Island, white sharks bite and release penguins, or beach on top of them, but almost never eat them. Many penguins here have multiple bite wounds, but rarely any tissue loss, suggesting they may be great white play toys. A great white was even seen tossing a seal repeatedly into the air and catching it in its mouth, perhaps as a sort of game.
Porbeagle sharks may also play. They’ve have been sighted off the coast of Cornwall seemingly playing with floating objects like driftwood and buoys, often in groups, for no apparent reason other than to have fun. They also seem to enjoy repeatedly wrapping and unwrapping their snouts in kelp fronds, then getting chased by other porbeagles as the long fronds stream behind them.