34 Sharks: The Animal Answer Guide
“chumming,” which involves mashing fish parts with beef blood in a bucket
of sea water and ladling it overboard. Sharks approach from downstream,
often zigzagging across the chum slick as they move upcurrent. The closer
to the source, the stronger the odor, so by swimming up the gradient of
intensity, the shark knows it is getting closer. Sharks also swing their heads
back and forth while swimming, which allows them to “sample” a wider
swath of ocean and gives some directional cues as their head moves in and
out of the odor field. Some sharks can also locate an odor by detecting a
slight difference in odor strength in the right versus left nostril, or even by
the time difference the odor molecules arrive at one versus the other nos-
tril. Either would help tell a shark the direction from which an odor was
coming.
The sensitivity of sharks to chemical substances is legendary. Compari-
sons are difficult to express, but statements in the scientific literature talk
about responses in sharks and skates to odor concentrations as low as one
part odor substance in one thousand or even ten billion parts water.
Sensing chemicals includes not just smell but also taste, and the two
senses are closely related. Sharks have taste buds inside their mouths and
throats, especially just behind their teeth, which would be the first place
a meal would land after a bite. Some sharks, such as Nurse Sharks, have
a pair of external whiskers called barbels on the underside of their snout
ahead of their mouth that also detect chemical cues, as do many catfishes.
Saw sharks (Pristiophoridae) have barbels projecting out from their saws,
in between some of the saw teeth.
Research on olfaction in sharks has led to some intriguing, and still puz-
zling, findings. White Sharks possess a cranial nerve in their brains that ap-
pears to be specialized for sensing chemical signals put out by other White
Sharks. Such intraspecies chemical signals are called pheromones. A pher-
omone detector could help a male locate a female or determine her readi-
ness to mate, either across large distances or up close, assuming a female
released a pheromone when she was receptive.
Another study showed that Oceanic Whitetip Sharks (Carcharhinus
longimanus) are able to trap and detect surface bubbles that could carry
airborne odors. By swimming near the surface, a Whitetip could there-
fore detect scents carried on the wind, a faster and wider means of odor
dispersal than water currents. Killer whale researchers use dogs perched
on the bow of a boat that can smell and react to whale poop, telling them
that killer whales have been in the area recently (the researchers collect the
poop to see what the whales have been eating). Perhaps some sharks also
learn of prey whereabouts similarly via airborne odors. And this may be a
(partial) explanation for why some sharks swim at the surface in deep water,
with their dorsal fins sticking out. Swimming near the surface would also
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