the wider the head the better the shark's binocular vision, and hence its perception of distance
Several factors that influence the evolution of the unusual head morphology of hammerhead sharks (family Sphyrnidae) are proposed but few are empirically tested.
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This study is the first to test the predictions of the ‘enhanced binocular field’ hypothesis
In their 400 million year evolutionary history, elasmobranch fishes (sharks, skates and rays) have evolved a rich morphological diversity (Karatajute-Talimaa, 1992; Capetta et al., 1993). One of the most unique features is the dorso-ventrally compressed and laterally expanded cephalofoil of the hammerhead sharks (Elasmobranchii, Carcharhiniformes, Sphyrnidae). This head expansion ranges from modest in the bonnethead, Sphyrna tiburo, to extreme in the winghead shark, Eusphyra blochii, in which the wing-like head has a width equal to nearly 50% of the total body length (Compagno, 1984). The head width of all of the other hammerhead species are between these two extremes (Fig. 1). The uniqueness and peculiarity of the expanded cephalofoil has generated much speculation about its function, and several hypotheses to explain its evolution. The cephalofoil may confer advantages that include: greater lift and maneuverability (Thompson and Simanek, 1977; Compagno, 1984; Nakaya, 1995; Kajiura, 2001; Kajiura et al., 2003), enhanced prey acquisition and manipulation (Strong et al., 1990; Chapman and Gruber, 2002), greater electrosensory capability (Compagno, 1984; Kajiura, 2001) and superior olfactory gradient resolution (Compagno, 1984; Johnsen and Teeter, 1985). Of particular relevance is how the lateral displacement of the eyes on the distal tips of the cephalofoil constrain or enhance the visual capabilities of the hammerheads in contrast to their sister shark taxa which possess a more conventional head shape. Walls stated that in hammerheads, each eye field is independent with no possible overlap thus precluding anterior binocular vision (Walls, 1942). By contrast, Compagno argued that the widely spaced eyes on the tips of the head enhance binocular vision anteriorly and provide an increased stereoscopic visual effect (Compagno, 1984). Thus, a comparative test of binocular overlap among related species is needed (Schwab and McComb, 2007).
