Wide heads give hammerheads exceptional stereo view

November 27, 2009,
Hammerhead shark. Image: Wikipedia

Hammerhead sharks are some of the Ocean's most distinctive residents. 'Everyone wants to understand why they have this strange head shape,' says Michelle McComb from Florida Atlantic University. One possible reason is the shark's vision. 'Perhaps their visual field has been enhanced by their weird head shape,' says McComb, giving the sharks excellent stereovision and depth perception. However, according to McComb, there were two schools of thought on this theory. In 1942, G. Walls speculated that the sharks couldn't possibly have binocular vision because their eyes were stuck out on the sides of their heads.

However, in 1984, Leonard Campagno suggested that the sharks would have excellent depth perception because their eyes are so widely separated. 'In fact one of the things they say on TV shows is that hammerheads have better vision than other sharks,' says McComb, 'but no one had ever tested this'. Teaming up with Stephen Kajiura and Timothy Tricas, the trio decided to find out how wide a hammerhead's field of view is and whether they could have binocular vision and publish their results on November 27 2009 in the .

Hammerheads come in all shapes and sizes so McComb and Kajiura, 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 successfully landed the fish and quickly transported them back to local labs to test the fish's eyesight.

The team tested the field of view in each shark's eyes by sweeping a weak light in horizontal and vertical arcs around each eye and recorded the eye's electrical activity. Comparing the hammerheads with pointy nosed species, the team found that the scalloped hammerheads had the largest monocular , at an amazing 182 deg., and the bonnethead had a 176 deg. visual field, which was bigger than that of the pointy nosed blacknose and lemon sharks, at 172 deg. and 159 deg., respectively.

Having collected the animals' monocular visual fields, the team plotted the visual fields of both eyes on a chart of each fish's head to see whether they overlapped. Amazingly, they did. The scalloped hammerhead had a massive binocular overlap of 32 deg. in front of their heads (three times the overlap in the pointy nosed species) while the bonnet head had a respectable 13 deg. overlap. And when the team measured the binocular overlap of the shark with the widest hammerhead, the winghead shark, it was a colossal 48 deg. The hammerheads' wide heads certainly improved their binocular vision and depth perception.

Finally, the team factored in the sharks' eye and head movements and found that the forward binocular overlaps rocketed to an impressive 69 deg. for the scalloped hammerheads and 52 deg. for the bonnetheads. Even more surprisingly, the team realised that the bonnethead and scalloped hammerheads have an excellent stereo rear-view: they have a full 360 deg. view of the world.

'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,' says McComb. But despite their preconceptions, the team have shown 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.

More information: McComb, D. M., Tricas, T. C. and Kajiura, S. M. (2009). Enhanced visual fields in hammerhead . J. Exp. Biol. 212, 4010-4018. http://jeb.biologists.org

Source: The Company of Biologists (news : web)

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4.5 / 5 (2) Nov 27, 2009
Interesting indeed.
If I get it right, the researchers have measured the overlapped sight for both eyes of a stationary shark head.
But with their swimming motion sharks are able to virtually widen this area. What if their weird look comes with weird wiring in the head and they somehow use this overlapping in time to make up for the [somewhat] narrow stationary overlap.

One more thing - it does not seem that better vision is the primary evolutionary gain of heaving this flat head.
As much as I can remember these creatures feed primarily on pray hidden beneath the sand, and they use the flat head fitted with receptors that detect the electrical impulses of the nervous system of that hidden pray - large area -> better perception.
1 / 5 (1) Nov 27, 2009
the simple truth is the brain of the shark is the sharks vision, the eyes reflect changes in the brain===the part that makes meaning of the photon delivery device.

how and why hammer heads see better will probably never be quite understood.
5 / 5 (1) Nov 27, 2009
full 360 front to rear. I'm curious if they see a full 360 top to bottom as well. Do Hammerheads have a vision bubble around thier body? That would be cool.
4 / 5 (1) Nov 27, 2009
Last I read the hammerhead shape was due to an increase in ampullae and the higher electropore density over a wider area giving them an increased electroreceptive capability.

Hammerhead sharks scour the ground for prey that has dug in and detect them by their nerve signals. Binocular vision is completely useless for this.
The wide head allows them to search an area in half the time that they'd need if they had 'regularly shaped' heads (like using a large metal detector instead of a small one).

It's nice that these researchers do this kind of research - but when they ignore what the sharks feed on and how they hunt it leads to such garbage results like "the hammerhead is the shape it is because of binocular vision"

Having the eyes on the edge of the head is an advantage but not the reason for the shape (let's face it - any other placement of the eyes would have the head partially obstruct the field of vision, which would have never passed evolutionary pressure).

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