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August 03, 2010
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“Hammer” heads give sharks super vision: study
Dec. 7, 2009
Courtesy The Company of Biologists
and World Science staff
Three scientists got a big surprise after they set out to disprove a “myth” repeated on many television shows about sharks. A scalloped hammerhead
(Sphyrna lewini) at Cocos Island, Costa Rica. (Courtesy
Terry Goss/
Not only is the “myth” true, it’s even truer than the pundits knew, the researchers found.
The story has to do with hammerhead sharks. Supposedly, they have unusually good vision because of their wide head shape. And it’s really true, the scientists now say: the
odd shape gives the fish excellent depth perception and a front-and-back view of the world.
“Everyone wants to understand why they have this strange head shape,” said Michelle McComb
of Florida Atlantic University.
There have been two competing ideas on how that shape would affect vision, she added. One is that the wide-out eye placement would make it impossible for sharks to have binocular vision—the use of both eyes to see the same objects, necessary for good depth perception. But in 1984 it was proposed that the wide separation would actually give the sharks better depth perception, by allowing objects to be seen from two very different angles at once.
“In fact one of the things they say on TV shows is that hammerheads have better vision than other sharks,” said McComb, “but no one had ever tested this.” She and two colleagues,
adherents of the first theory, decided to conduct such tests with a
view to disproving what they thought was a TV show “myth.”
Hammerheads come in many forms, so the team opted to work with species with heads ranging from the narrowest to the widest. Fishing for juvenile scalloped hammerheads off Hawaii and bonnethead sharks in the waters around Florida, the team caught the fish and took them back to local labs to test their eyesight.
The researchers tested the field of view in each shark’s eyes by sweeping a weak light around each eye and recording the eye’s electrical activity. Comparing the hammerheads with pointy-nosed species, the team found that the scalloped hammerheads had the largest single-eye visual field, at 182 degrees. That angle ranged down to 159 degrees for lemon sharks.
The team then plotted the visual fields of both eyes on a chart of each fish’s head to see whether they overlapped. To the researchers’ surprise, they did. The overlap ranged from about 10 degrees up to a huge 48 degrees for the widest hammerhead, the winghead shark. Wide heads clearly improved binocular vision and depth perception, the scientists said.
Finally, the team factored in the sharks’ eye and head movements and found that the forward binocular overlaps rocketed to an impressive 69 degrees for the scalloped hammerheads and 52 degrees for the bonnetheads. Even more surprisingly, the team found that the bonnethead and scalloped hammerheads must have an excellent stereo rear-view: they have a full 360 degree view of the world.
The findings are published in the Nov. 27 issue of the Journal of Experimental Biology.
“When we first started the project we didn’t think that the hammerhead would have binocular vision at all. We thought no way; we were out there to dispel the myth,” said McComb. But despite their preconceptions, the team have found that the sharks not only have outstanding forward stereovision and depth perception, but a respectable stereo rear view too, which is even better than the TV shows would have us believe.
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Three scientists got a big surprise after they set out to disprove a “myth” repeated on many television shows about sharks. Not only is the “myth” true, it’s even truer than the pundits knew, the researchers have found. The story has to do with hammerhead sharks. Supposedly, they have unusually good vision because of their wide head shape. And it’s really true, the scientists now say: the odd, widened head shape gives the fish excellent depth perception and a front-and-back view of the world. “Everyone wants to understand why they have this strange head shape,” said Michelle McComb from Florida Atlantic University. There have been two competing ideas on how that shape would affect vision, she added. One is that the wide-out eye placement would make it impossible for sharks to have binocular vision—the use of both eyes to see the same objects, necessary for good depth perception. But in 1984 it was proposed that the wide separation would actually give the sharks better depth perception, by allowing objects to be seen from two very different angles at once. “In fact one of the things they say on TV shows is that hammerheads have better vision than other sharks,” said McComb, “but no one had ever tested this.” She and two colleagues decided to conduct such tests with a view—they thought—to disproving the TV show “myth.” Hammerheads come in many forms, so the team opted to work with species with heads ranging from the narrowest to the widest. Fishing for juvenile scalloped hammerheads off Hawaii and bonnethead sharks in the waters around Florida, the team caught the fish and took them back to local labs to test their eyesight. The researchers tested the field of view in each shark’s eyes by sweeping a weak light around each eye and recording the eye’s electrical activity. Comparing the hammerheads with pointy-nosed species, the team found that the scalloped hammerheads had the largest single-eye visual field, at 182 degrees. That angle ranged down to 159 degrees for lemon sharks. The team then plotted the visual fields of both eyes on a chart of each fish’s head to see whether they overlapped. To the researchers’ surprise, they did. The overlap ranged from about 10 degrees up to a huge 48 degrees for the widest hammerhead, the winghead shark. Wide heads clearly improved binocular vision and depth perception, the scientists said. Finally, the team factored in the sharks’ eye and head movements and found that the forward binocular overlaps rocketed to an impressive 69 degrees for the scalloped hammerheads and 52 degrees for the bonnetheads. Even more surprisingly, the team found that the bonnethead and scalloped hammerheads must have an excellent stereo rear-view: they have a full 360 degree view of the world. The findings are published in the Nov. 27 issue of the Journal of Experimental Biology. “When we first started the project we didn’t think that the hammerhead would have binocular vision at all. We thought no way; we were out there to dispel the myth,” said McComb. But despite their preconceptions, the team have found that the sharks not only have outstanding forward stereovision and depth perception, but a respectable stereo rear view too, which is even better than the TV shows would have us believe.