Form and Function of Sharks 49
the hammer would have to account for this variation while ignoring other
variations in the shape of the head. Also, nothing says the head has to serve
only one purpose, or that the head works in the same way for all members
of the family.
Experimental evidence now shows improvement in several senses that
can be attributed to the expanded head. Sphyrnids have greater binocular
vision than pointy-nose sharks such as Lemon and Blacknose (Carcharhinus
acronotus). Binocular vision is the overlap in the fields of view of the two
eyes and is what gives us depth perception; in humans, the fields overlap
120 degrees (our entire field of view is 190 degrees, 120 of which are seen
by both eyes). The binocular overlap of Bonnetheads is 13 degrees, for
Scalloped Hammerheads (Sphyrna lewini) it is 32 degrees, and for Wing-
heads it is 48 degrees, corresponding to the differences in relative head
width. Lemon and Blacknose sharks have only about a 10-degree binocular
overlap. The overlap increases as a shark moves its eyeballs and swings its
head back and forth, improving by 37 to 39 degrees for Scalloped Ham-
merheads and Bonnetheads. Having eyes out on stalks also allows hammer-
heads to see behind them better than nonhammered species. Combined
with the forward binocular overlap, a view behind means these sharks have
a full 360-degree spherical picture of their world, front to back and top to
bottom. So a visual advantage does exist.
Although hammerheads are not more sensitive to odors than other
sharks, they may have increased stereo-olfaction. Anatomical differences
among hammerhead species show that hammerheads with broader heads
can distinguish between odors striking their different nostrils better than
more narrow-headed species. Additional stereo-olfactory ability could
The hammer, or cephalofoil, of a
Great Hammerhead Shark. Several
explanations have been offered for
the hammerheads’ expanded head.
More than one may be correct. Photo
by Gene Helfman