140 Sharks: The Animal Answer Guide
fishes. Surprisingly (to us), sharks can tell where those squeaks and thumps
are coming from, up to 200 m (660 ft) away (see “How well can sharks
hear?” in chapter 2). We cannot locate the source of a sound underwater.
Sharks are often called “swimming noses” because of their incredible
sensitivity to olfactory cues (smells). A shark’s nose is connected to the very
large olfactory lobes of its brain, a sure sign that olfaction is important to
sharks. A Lemon Shark can detect fish extracts at levels as low as one part
per 25 million parts seawater; Blacktip and Gray Reef sharks react to one
part per 10 billion—which is the equivalent of about one drop of fish goop
dispersed in an Olympic-sized swimming pool. This sensitivity is greatest
to proteins, amino acids, and amines (see “How good is a shark’s sense of
smell?” in chapter 2).
As with sounds, smells can be detected far from the source, but smells
are rapidly diluted and weakened as distance increases. Directional cues
can be provided by current flow, with sharks often swimming upcurrent to
an odor. An increasing amount of an odor, its concentration gradient, can
also provide a directional cue. Differential arrival times at the shark’s two
nostrils also provide directional information. Hammerhead sharks might
be better able to localize an odor source because the wider spacing between
their nostrils would increase the time lag between an odor hitting the left
and right nostrils as the shark swam at an angle through the odor trail. And
the food source doesn’t necessarily have to be directly submerged in water.
White Sharks have shown up off southern California beaches where gov-
ernment agencies had buried a dead blue whale, much to the vexation of
local surfers.
As a shark approaches something it has heard or smelled, vision comes
into play, although acuity is obviously affected by water clarity. Sharks have
well-developed eyes and large optic lobes in their brains, indicating some
reliance on vision. Their eyes are best adapted for viewing objects more
than 23 cm (9 in) away and in dim light conditions. Color doesn’t matter,
at least not to selachian sharks; rays and chimaeras have better color vision
(see “How well can sharks see?” and “Can sharks see color?” in chapter
2). Again, the hammer of hammerhead sharks improves their vision by in-
creasing the visual field around them greatly over that of pointy-headed
sharks; the wider the hammer, the greater the visual field (see “Why do
hammerhead sharks have such strange heads?” in chapter 2).
The final stages of finding food rely on touch, taste, and especially bio-
electricity. Sharks often bump nearby objects as a means of testing their
edibility. We use touch when we handle food to assess consistency (such as
hard/unripe versus soft/ripe versus mushy/rotten fruit). Exactly what in-
formation a shark gains isn’t clear, and it is possible that the shark is sens-
ing electrical fields as well as consistency (see next paragraph). But sharks
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