1. Shark repellent
2. How dangerous?
3. Shark lovers
4. A social killer?
In one test, lemon sharks proved smarter
than cats. Courtesy Grant Johnson, Bimini
Biological Field Station.
the social hierarchy of a lemon shark, bigger is better. Courtesy
Grant Johnson, Bimini
Biological Field Station.
Not a good sight if you're planning on snorkeling,
but these black-tipped sharks in the Bahamas were described as "quite
Courtesy Shark Defense
When you think about social animals, you
are more likely to think about ants or wildebeest than sharks.
But some sharks do have a social life, says shark expert Samuel
Gruber of the University of Miami. He says he became enamored
of lemon sharks while studying their vision years ago. "I was
amazed by the fact that sharks were thought to be blind swimming
noses, yet they had these highly sophisticated eyes, retinas
and mobile pupils. They had all kinds of incredible adaptations
... and it got me thinking that there was more to these fish
than meets the eye."
Sharks also seemed smart. "Right away, I was impressed by their ability to learn," Gruber says. One study showed Pavlovian conditioning, where an animal is taught to respond to a stimulus that originally has no meaning. Russian physiologist Ivan Pavlov noticed that if he rang a bell every time he fed a dog, the mutt would eventually drool at the sound of the bell, whether grub was around or not.
Gruber noticed that lemon sharks wink when given an electric shock, so he started flashing a light with the jolt. "After only 100 pairings, it would wink at the light alone. It takes a cat 800 trials to make that connection."
Now, any animal smarter than a fur person catches that Why File eye.
More recently, Gruber has examined the social structure of lemon sharks penned at the Bimini Biological Field Station. When two sharks approach, one tends to give way. "It's like when you walk down the street and somebody moves aside," says Gruber. "We reckoned the submissive shark would give way and the dominant one would continue on."
With lemon sharks, dominance was all a matter of size. But sex played a role in bonnet-head sharks, he adds. "Smaller males could dominate bigger females in some cases. We thought that had to do with their propensity to bite the female to induce mating." Presumably, if females were not in the mood, they could glide aside to avoid a love bite.
So why didn't this behavior appear in the lemon sharks? Perhaps because they were immature, and didn't have sex on the brain.
These results help overturn the image of the shark as a dumb, efficient predator. "People often think about sharks as lone rovers of the open ocean," says Gruber, "and don't consider them to have a social life, other than perhaps when they mate. That's not true for lemon sharks, they hang out together, display social behavior, are attracted to one another, and are dominant or submissive in a group."
One caution: Gruber's students saw the social behavior in penned sharks. Whether it appears in the open ocean remains to be seen.
Is the hammerhead shark a freak of nature?