雙髻鮫的頭，為什麼形狀這麼特別?

雙髻鮫的頭，為什麼形狀這麼特別?

研究顯示，這種頭型使雙髻鮫擁有360度全方位的視野，而當兩眼的視野重疊時，就會出現立體視覺(Binocular vision)，能幫助鯊魚精確感知深度和距離，提高捕獲獵物的機會。

雙髻鮫靠左右搖擺動來看到正前方，判斷獵物與它們之間的距離。頭幅越寬，兩眼視覺重疊的範圍越廣。正常頭型的鯊魚，如檸檬鯊，重疊的視野只有10度。路氏雙髻鯊(Sphyrna lewini)的頭部相對較寬，視覺交疊達到32度。布氏真雙髻鯊Eusphyra blochii)則達到48度。

雙髻鮫奇特的頭形給牠帶來更多好處。牠們只要稍稍扭動腦袋，就能看到身後的情況。更能看到垂直面360度的動向，也就是說，雙髻鮫隨時都能觀察到四面八方的情況。

Widely set eyes give hammerhead sharks exceptional binocular vision

The hammerhead shark’s head is one of the strangest in the animal world. The flattened hammer, known as a ‘cephalofoil’, looks plain bizarre on the face of an otherwise streamlined fish, and its purpose is still the subject of debate. Is it an organic metal detector that allows the shark to sweep large swathes of ocean floor with its electricity-detecting ability? Is it a spoiler that provides the shark with extra lift as it swims? All of these hypotheses might be true, but Michelle McComb from Florida Atlantic University has confirmed at least one other -the hammer gives the shark excellent binocular vision. The T-shaped hammerhead configuration also allows the sharks to see 360 degrees, with respectable stereo rear view.

原文下載Download the full research paper here：
McComb, D. M., Tricas, T. C. and Kajiura, S. M. (2009). Enhanced visual fields in hammerhead sharks. J. Exp. Biol. 212, 4010-4018.
http://www.science.fau.edu/sharklab/pdfs/mtk09.pdf