50 Sharks: The Animal Answer Guide
come from the lag in time when odors strike the right versus the left nos-
tril. Smooth Dogfish (Mustelus canis) respond to this time lag, turning to-
ward the side where the odor first strikes. Having the nostrils out on the
ends of the hammer means this time lag is greater and allows a hammer-
head to sense the direction from which an odor is coming over a greater
encounter angle than narrow-headed species.
The jury is still out on the possibility of improved electrical sensitivity.
Hammerheads, especially juveniles, have more electrosensitive pores (Am-
pullae of Lorenzini) on the undersides of their heads than other sharks, an
increase made possible by the greater surface area of an expanded head. But
hammerheads are not necessarily more sensitive to electrical stimuli than
other sharks. Still, they might be able to “sweep” a larger area electrically
than a pointy-headed shark.
Tests of improved hydrodynamic efficiency have proved surprising. The
idea that the head could serve as a front-end rudder that allowed greater
maneuverability provided results opposite to expectation. Hammerheads
are more maneuverable, meaning that they can make faster turns than car-
charhinid sharks. But the head isn’t used as a rudder but more as a stabi-
lizer; a hammerhead’s body doesn’t roll as much in a sharp turn, and the
head may help it keep level. It’s just such unanticipated results that make
research exciting for shark scientists.
Having a broad head does allow hammerheads to catch their favorite
prey, stingrays. Divers have seen hammerheads pinning stingrays to the
bottom, which would be harder to do with a pointy head. And some go
even further, using their heads literally as a hammer. Great Hammerhead
Sharks have been observed using the flat underside of their head to deliver
powerful downward blows that knock a stingray to the bottom before pin-
ning it down, pivoting around, and biting pieces off the ray’s wings.
Using the expanded head to hammer prey is a good example of the
way evolution capitalizes on innovation. The expanded hammer probably
evolved first because it improved vision or olfaction, and only later was
it put to use to strike and pin prey. Often, when a species evolves a new
structure that works well for certain purposes, the same structure gets put
to use in novel ways that probably had little to do with its initial evolution.
For example, the saw of a sawfish probably evolved as a means of striking
baitfish up in the water column, disabling them, and making them easier to
capture (also a major use of the sword of Swordfish, sailfish, and marlins).
By chance, the same structure in sawfishes is also useful for digging in the
mud and dislodging organisms there, a very different use of the saw.
But another “rule” of evolution is that every adaptation carries some
liability or trade-off. Such is the case in hammerheads. As with all carcha-
rhiniform sharks, hammerheads are live-bearers, with the young emerging
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