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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.

Not only is the “myth” true, it’s even tru­er than the pun­dits knew, the re­search­ers found.

A scalloped hammer­head (Sphyr­na le­wi­ni) at Co­cos Is­land, Cos­ta Ri­ca. (Cour­tesy Ter­ry Goss/Ma­r­ine Pho­to­bank)


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.