A shark's bite may kill prey, but it's the teeth covering its body that make the fish such a good hunter, new research suggests.
Sharks are covered in flexible scales—nearly invisible to the human eye—that are made of the same material as teeth. The scaly hide serves as both a suit of armor and a means of streamlining movement, according to Amy Lang, an aerospace engineer at the University of Alabama.
For instance, previous research had suggested that a shark can "bristle" or otherwise manipulate its scales to change its direction mid-sprint—agility that's crucial for capturing fast-moving prey such as tuna.
But a recent experiment revealed that sharks don't actively move their scales, which are loosely embedded in the skin via rubber band-like tendons, Lang said.
Instead, the structures bristle when water flowing around the shark "detaches" from the fish's aerodynamic body.
The way the scales turn helps reduce the water's drag on the speeding shark. (Solve a shark jigsaw puzzle.)
Lang compared the phenomenon to dimples on a golf ball, which enable the ball to travel farther in the air.
"Imagine a stream of flow going over the ball," she said. "You get a low-velocity wake behind the body, but the dimples help to decrease the size of the wake—that's what we think the scales are helping to do with the shark."
Mako Shark's Scales Built for Speed
Lang partnered with a team of biologists to study the shortfin mako, a relative of the great white shark, in the lab.
Combining lab observations of the shark's scales with computer models, the team discovered that a shortfin mako's scales differ in size and flexibility over its body.
For instance, the most tapered—and thus most movable—scales were found behind the gills and on the sides of the body. Scales in these areas can bristle up to angles of 60 degrees or more from the skin, according to Lang.
These are also same areas where water flow would separate from the shark and create drag.
Overall, sharks' 400 million years of evolution for strength and speed may someday inspire better designs for machines that are prone to drag, such as aircraft, Lang noted.
Research presented November 23 at the American Physical Society's Division of Fluid Dynamics meeting in Long Beach, California.