Study sheds light on hammerhead shark

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Research published in a recent issue of the Journal of Experimental Biology may have solved the mystery of why hammerhead sharks have such strange-shaped heads.

One hypothesis contends that the widely set eyes give the sharks better binocular vision, an issue that has sharply divided scientific opinion for decades.  

Michelle McComb, Timothy Tricas and Stephen Kajiura tested this hypothesis by comparing the visual fields of three hammerhead species: the bonnethead shark, (Sphyrna tiburo), scalloped hammerhead shark (Sphyrna lewini) and the winghead shark (Eusphyra blochii), with that of two normal shark species: the lemon shark (Negaprion brevirostris) and the blacknose shark (Carcharhinus acronotus).

The authors placed sensors on the shark's skin to measure its brain activity when beams of light were shone from different locations around the tank; they also measured eye rotation and head yaw to determine if the sharks compensate for large blind areas in front of the head.

The results indicate that the field of binocular vision was up to four times higher in hammerhead sharks than in normal sharks: the anterior binocular overlap of the lemon and blacknose sharks were 10 and 11 degrees respectively, while those of the bonnethead, scalloped hammerhead and winghead sharks were 13, 34 and 48 degrees respectively.  

These results indicate that the greater the sideways expansion of the head, the larger the binocular field of vision (bonnethead sharks have the smallest sideways expansion of the head and winghead sharks the largest).

For more information, see the paper: McComb, DM, TC Tricas and SM Kajiura (2009) Enhanced visual fields in hammerhead sharks. Journal of Experimental Biology 212, pp. 4010–4018.