Why the strange head?

The peculiar shape of hammerhead sharks is a biological conundrum that has long puzzled scientists.

Now American scientists have uncovered some of the secrets of these odd hammer-shaped heads, showing that having eyes spaced far apart on their wide heads lets hammerhead sharks see better than more conventionally-shaped sharks. In particular hammerheads may have enhanced depth perception, a crucial skill for predators to judge how far away prey is before swooping in for the kill.

Various ideas have been put forward over the years to explain the hammerhead sharks' extraordinary compressed and laterally expanded head, known as a cephalofoil. Possibilities include greater sensitivity to smells or the weak electric signals given off by prey animals, greater lift and manoeuvrability in the water or maybe it helps in catching and manipulating prey. But until now, few studies have tested out any of these ideas.

The research team led by Michelle McComb from Atlantic Florida University set out to test another idea which has sparked a lot of controversy among shark biologists, namely, do hammerhead sharks see better than normal sharks.

To do this the researchers mapped out the 3D visual fields of various shark species, including three hammerheads (the bonnethead, scalloped and winghead hammerhead sharks), by anaesthetising each one and using an electrode they worked out if an electrical signal is generated in the retina when a narrow beam of light is shone into the eye from a range of angles.

Of all the sharks, the hammerheads had the most overlap of vision from their two eyes in front of them - and sharks with wider heads had even more visual overlap. The team also discovered that all the shark species tested had extraordinary 3600 vision in the vertical plane - they can see all the way above and below them.

One thing hammerheads seem to be bad at is seeing what's going on behind them. There are reports of prey fish hanging behind hammerheads, perhaps hoping not to be spied.

McComb and colleagues analysed videos of sharks swimming and found that to make up for having compromised vision behind them, hammerhead sharks swing their heads from side to side to see what's going all around them.

Of course, having better vision might not be the only thing that having a hammer-shaped head might be good for. But this study unveils one of the secrets behind the evolution of these amazing animals.