48 Sharks: The Animal Answer Guide
air, a response that presumably prevents their eyes from drying. Epaulette
Sharks, a shark that feeds at night over shallow tidal flats, can live for at
least an hour without oxygen at water temperatures as high as 30°C (86°F)
(see “How do sharks survive droughts?” in chapter 5). Researchers working
with live sharks in tag-and-release programs try to get sharks back into the
water in less than four minutes; they also keep sea water from a hose run-
ning across a shark’s gills during the tagging operation.
Why do hammerhead sharks have such strange heads?
Over the years, a number of hypotheses have been put forward to ex-
plain the function of the “expanded rostral cartilage,” or “cephalofoil,” of
hammerhead sharks. The head cartilage of hammerheads is greatly ex-
panded from side to side, giving it the hammer or mallet shape from which
the group gets its common name. But people often forget that this widen-
ing is also accompanied by flattening of the entire structure. When viewed
from the front, a hammerhead’s head looks like someone smashed it with a
hammer, with the upper surface rounded and the lower surface flat. From
the front, the head looks like a wing, or airfoil, which is where the “cepha-
lofoil” (“head wing”) name comes from.
Ideas about the function(s) of this greatly modified (for a shark) shape
include better binocular vision, a wider path to detect odors and locate
them (“stereo-olfaction”), improved electrical sensitivity because the re-
ceiving poles are wider apart, and better hearing because the lateral line
receptors are spread across the head. The broader head could also have a
hydrodynamic function, providing additional lift to the front end to coun-
teract downward pivoting caused by the asymmetric tail, or better maneu-
verability when the head serves as a forward-end rudder. Or it could some-
how improve prey catching and handling.
An important fact to remember in this search for adaptations is that the
family of hammerheads, the Sphyrnidae, contains eight species that vary
substantially in the shape and “spread” of the hammer. Bonnetheads have
a slightly broadened head, only about 1.5 times wider than long. (Head
length is measured from the tip of the snout to the back of the mouth.) At
the other extreme is the Winghead Shark (Eusphyra blochii), in which the
head is short but exceedingly wide, almost 5 times wider than long. (Don’t
think that evolution has produced progressively wider and wider heads; the
Winghead is evolutionarily the most primitive member of the family and
the Bonnethead the most recently evolved.) Most of the other hammer-
head species have heads about 2 to 2.5 times wider than long; and “normal”
carcharhinid sharks, the group from which the sphyrnids evolved, have
heads about as wide as they are long. So any presumed, adaptive function of
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