Gruber: They make hydrodynamic sounds when swerving and accelerating, and they
make crunching sounds with their teeth and jaws. But they don't have special
organs for sound production and apparently don't communicate with
NOVA: Sharks have an endless supply of teeth?
Gruber: Yes, and they are unique in this way, too. They produce teeth
continuously. Their teeth are not set in sockets like those of other
vertebrates but rather loosely and flexibly attached by tendons to their jaws.
If they didn't replace teeth continuously—up to 50,000 in a lifetime—their teeth would fall out. That is why shark teeth are the most abundant
vertebrate fossil material on the Earth and in the sea.
Scalloped hammerhead on the reef.
NOVA: Do sharks have to keep swimming to breathe?
Gruber: Several species do, including hammerheads and mackerel sharks.
Typically, pelagic sharks that never encounter the bottom are adapted to swim
all their lives. But the vast majority of sharks have a buccal (mouth) pump and
are not so-called "obligate ram ventilators." (Ram ventilators like hammerheads
and great whites must swim to pass water over their gills.) So the answer is
no, emphatically. There are even some sharks with spiracles, holes on the top
of their head that allow water to enter their gill chambers when the mouth is
on the bottom.
Mouth of a hammer.
NOVA: Do sharks have to roll over to bite?
Gruber: No way, Jose! Sharks have a unique, loose jaw suspension that allows
them to raise their snout and loosen their jaw from the braincase, thrusting it
forward so that, in effect, the shark has a terminal mouth right out there at
the end. But in reality, sharks rarely bite pieces out of a prey. Usually, they
engulf them with their amazing buccal or mouth pump, which acts like a slurp
Great white shark
NOVA: Are sharks warm-blooded?
Gruber: Mackerel sharks, including great whites, makos, threshers, and
porbeagles, have a heat-conserving mechanism known as the rete mirabile
("wonderful net"), a structure for warming blood passing over the gills. Whites
can keep their core temperature up to 15°F above ambient
Cocos whitetip and lobster.
NOVA: Are sharks insensitive to pain?
Gruber: Hard to say, because we don't know what pain means to an animal and can
hardly describe it ourselves. Some people are almost insensitive to pain,
others are highly sensitive. I have done minor surgery on sharks, and they seem
not to feel it. On the other hand, using what is clearly aversive conditioning
I was able to train sharks, showing that they do respond to pain, which can be
used as a conditioned stimulus—and a powerful one. So yes, sharks feel
something we would call pain.