Great Hammerhead Shark's Eye

Why do hammerhead sharks have a hammer-shaped head?

The eyes of a great hammerhead shark can be 3 feet apart on opposite sides of its skull. Image Credit: Sail Far Dive Deep/

conversationHammerhead sharks are strange looking sharks. They looked like someone had grabbed their skulls, grabbed their eye sockets, and stuck their heads to the side, while the rest of their bodies looked like a normal shark.

You might be wondering – what are the benefits of a hammer head? How does a hammerhead shark do this?

I am a scientist who has studied sharks for nearly 30 years. The answers to some of these questions even surprised me.

The benefits of a hammer

Scientists believe that sharks with hammer-shaped heads have three main advantages.

The first has to do with vision. If your eyes point in two opposite directions, say your ears, it will give you a wider field of view. Each eye sees a different part of the world, so you can better understand what’s around you. But it’s hard to tell how far things are going.

To compensate for this trade-off, hammerhead sharks have special sensory organs, called ampullae of Lorenzini, scattered around the base of their hammers. These pore-like organs detect electrical current.

Hammerhead shark feeling pores

If you look closely at this giant hammerhead shark (S. mokarran), you will see the sensory hole under its hammer. Image credit: Martin Prochazkacz/

These pores are basically like a metal detector that senses and locates prey buried under the sand on the seafloor. Common sharks also have these sensory organs, but hammerhead sharks have more. The farther these sensory organs are on the hammerhead’s outstretched head, the more accurate they are at pinpointing the location of the food.

Finally, scientists believe the hammer could help sharks turn faster while swimming. If you’ve ever walked with an umbrella in high winds or flew in an airplane, you know how powerful a large surface can be in motion. If you’re a hammerhead, the dinner you want swam by in no time, and you can turn around to catch it faster than other fish.

hammerhead family tree

It would be great if scientists like me could look at the fossils and track the development of hammerhead sharks over time. Unfortunately, the fossils of hammerhead sharks are almost entirely teeth. That’s because sharks have no bones in their bodies. Instead, they’re made of cartilage, the building blocks of your ears and nose. Cartilage breaks down much faster than teeth or bones, so it rarely becomes fossilized. Fossil teeth don’t tell us anything about the evolution of hammerhead skulls.

There are nine different species of hammerhead sharks swimming in the ocean today. They vary in size and head shape. Some people have a very wide head relative to their body. These include winghead sharks (E. blochii), Hammerhead (S. mokarran), smooth hammer head (S. zygaena), fan-shaped hammer (S. lewini) and the Carolina Hammerhead (S. Gilberty).

Roundhead Shark (S. tiburo)

Others have hammers that are smaller relative to their bodies, including the hood (S. tiburo), mako (S. Media), Small Eye Hammer (S. tudes) and scalloped hood (S. Corona).

Scientists have long thought that the first hammerhead sharks didn’t have many hammers, but over time, some hammerheads slowly evolved larger hammers. We think of the different hammerhead sharks living today as snapshots of different times in the evolutionary process – the small hammerhead is the oldest species on the family tree, while the giant hammerhead is the newest.

Since we don’t have fossils to look at, scientists like me have explored this idea using DNA. DNA is the genetic material found in cells that carries information about an organism’s appearance and function. It can also be used to view relationships between organisms.

We extracted DNA from eight of the nine hammerhead species and used it to study their relationships. The result was totally unexpected. Older species have larger hammers, while younger species have smaller hammers.

Deformity as an asset

When scientists think about evolution, we usually assume that organisms change a little at a time, slowly adjusting themselves to better use their environment. This process is called natural selection. But that’s not always how it works, as Hammerhead Evolution shows.

Hammerhead shark evolution
On the left is the expected evolution of the hammerhead, assuming a gradual change in head shape. On the right is the observed evolutionary change pattern based on DNA sequence data. Image credit: Gavin Naylor, CC BY-ND

Sometimes an animal may be born with a genetic defect that is very useful for its survival. As long as the anomaly survives and the animal is able to mate, the trait can be passed on. We think that’s exactly what happened with the hammerhead.

The earliest branch of the hammerhead shark was the winghead shark (E. blochii), which has one of the widest heads. Over time, natural selection has actually reduced the size of the hammer. It turns out that the newest hammerhead shark species is the flathead shark (S. tiburo), which has the smallest hammer.

Gavin Naylor, Director of the Florida Shark Research Program, University of Florida

This article is republished from The Conversation under a Creative Commons license. Read the original text.

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