24 Sharks: The Animal Answer Guide
Are sharks cold-blooded or warm-blooded?
The great majority of sharks, rays, and chimaeras are cold-blooded,
meaning their body temperature is the same as the surrounding water.
However, despite the classic description of sharks as “cold-blooded kill-
ers,” some sharks—including the highly predatory lamnid sharks (White,
makos, Porbeagle, Salmon) and the closely-related thresher sharks (Alopi-
idae)—are actually warm-blooded: they maintain a body temperature that
is warmer than the water around them.
Muscle activity warms the blood deep in the body of any shark. In most
sharks, this blood is carried in veins to the gills to pick up oxygen. Because
the gills are in direct contact with the outside water, heat is lost, and blood
is quickly cooled to outside water temperature. It is then pumped by the
heart and into the arteries and back into the body. Blood returns to the gills
via veins close to surface of the shark, again losing heat to the outside. A
warming and cooling cycle continues, with the net result being that blood
is maintained at about the same temperature as that of the surrounding wa-
ter. This is also the situation in most bony fishes except tunas and billfishes.
But lamnids have a circulatory system different from most other sharks.
Both arteries and veins lie deep in the body and in close contact with one
another. This allows heat to be transferred from the veins to the arteries,
warming the blood coming from the heart before it reaches the muscles.
This arrangement is called a counter-current exchange system, because
blood vessels carrying blood flowing in opposite directions exchange heat
owing to their closeness. Hence, cool arterial blood is continually warmed
by warm venous blood. As a result, the body and stomach temperature of a
lamnid shark can be 7°C to 10°C warmer than the outside water tempera-
ture.
How does being warm-blooded help these sharks? The answer is a mat-
ter of speculation, but a popular explanation invokes the relationship of
body temperature to activity level. Lamnid and thresher sharks are among
the largest active sharks in cooler oceans, where they feed on active prey.
(Other large, high-latitude sharks include the somniosid sleeper sharks,
whose activity levels are described by their name, and the cetorhinid Bask-
ing Shark, which feeds on slow-moving zooplankton.) White Sharks are
more common in cool-temperate than tropical regions, as are the marine
mammals (seals, sea lions, whales) that they prefer. Salmon Sharks (Lamna
ditropis) are common in North Pacific waters, where they feed on salmon;
Porbeagles (Lamna nasus) are active predators in the northern North Atlan-
tic; and makos are worldwide predators of tunas. Warm muscles are more
powerful, and higher body temperatures may allow for the faster move-
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