Why have hammerhead sharks evolved their distinctive heads?
We're still some way off from hitting the nail on the head when it comes to these unusually-shaped sharks.
Asked by: Anonymous
These sorts of ‘why’ questions are tricky. We can get ideas from how the various hammerhead species use their bizarre ‘cephalofoil’, but those aren’t necessarily why it originally evolved. Nor why other sharks manage fine without it. Basically, the head provides extra lift and manoeuvrability, wider eye separation and greater area for sense organs such as smell and electric-field detection. In some species, it’s used to pin down struggling prey.
However, unless something’s evolved separately several times over, it can be difficult to state with confidence what selection pressures were involved. Fortunately, hammerhead species with very different head shapes and sizes exist. Very recent DNA-based family trees of these eight species reveal something unexpected: species with astonishingly wide heads, like the winghead shark (Eusphyra blochii), branched off first. That implies the cephalofoil started huge before evolving into the much smaller heads seen in, say, bonnethead sharks (Sphyrna tiburo). It’s been suggested that this indicates a role for extreme sensory advantage – increased binocular vision or olfaction – rather than hydrodynamics.