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Why do hammerhead sharks have a hammer-shaped head? – Landon, 10 years old
Hammerhead sharks are the weird looking ones. It looks like someone grabbed his skull by the sockets and turned his head sideways, while the rest of his body looks like a normal shark.
You might be wondering: What are the benefits of having a hammer head? And how did hammerhead sharks get that way in the first place?
I am a scientist who has been studying sharks for almost 30 years. The answers to some of these questions have surprised even me.
Hammer benefits
Scientists think hammerhead sharks have three main advantages.
The first has to do with sight. If your eyes were pointing in two opposite directions, say over your ears, it would give you a much wider field of view. Each eye would see a different part of the world, so you’d have a better idea of what’s around you. But it would be difficult to say how far things are.
To compensate for this, hammerhead sharks have special sensory organs, called ampullae of Lorenzini, scattered along the underside of their hammerhead. These pore-like organs can detect electricity.
If you look closely at this large hammerhead shark (S. mokarran) you can see the sensory pores on the underside of its hammerhead. Alexis Rosenfeld/Getty Images News via Getty Images
The pores basically act like a metal detector, detecting and locating prey buried beneath the sand on the ocean floor. Normal sharks also have these sensory organs, but hammerheads have more of them. The further these sensory organs are located in the elongated head of a hammerhead, the more accurate they are in identifying the location of food.
And finally, scientists think hammerheads help sharks make faster turns while swimming. If you’ve ever walked in a strong wind with an umbrella or flown in an airplane, you know how powerful large moving surfaces can be. If you are a hammerhead shark and your intended dinner swims quickly, you can turn faster to catch it than other fish.
The hammer family tree
It would be nice if scientists like me could look at fossils and trace the development of hammerhead sharks over time. Unfortunately, hammerhead shark fossils are almost entirely of their teeth. This is because sharks’ bodies do not have bones. Instead, they are made of cartilage, which is what the ears and nose are made of. Cartilage breaks down much faster than teeth or bones, so it rarely fossilizes. And fossil teeth tell us nothing about the evolution of hammerhead skulls.
Nine different types of hammerhead sharks swim in the oceans today. They vary both in size and in the shape of their head. Some have very wide heads in relation to their bodies. These include the thresher shark (E. blochii), the great hammerhead (S. mokarran), the smooth hammerhead (S. zygaena), the scalloped hammerhead (S. lewini) and the Carolina hammerhead (S. gilberti) .
The narrowest hammerhead belongs to the head shark (S. tiburo). D. Ross Robertson/Smithsonian Tropical Research Institute
Others have smaller hammerheads relative to their bodies, including the bonnethead (S. tiburo), the tip shark (S. media), the small-eyed hammerhead (S. tudes), and the scalloped bonnet head (S. corona).
Scientists long assumed that the first hammerhead sharks didn’t have much of a hammerhead, but over time, some larger hammerheads slowly evolved. We thought that the different hammerhead sharks living today were snapshots from different periods of the evolutionary process: the little hammerheads were the oldest species in the family tree, and the huge hammerheads are the newest on the scene.
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 how a living thing will look and function. It can also be used to see how living things are related.
We took DNA from eight of the nine hammerhead species and used it to analyze the relationships between them. The results were not at all what we expected. Older species had proportionally larger hammers and younger species had smaller hammers.
Deformities as an asset
When scientists think about evolution, we usually assume that living things change a little at a time, slowly adjusting to make better use of their environment. This process is called natural selection. But that’s not always how it works, as the evolution of the hammer shows.
On the left is the expected evolution of the hammers, assuming a gradual change in head shape. On the right is the observed pattern of evolutionary change based on DNA sequence data. Gavin Naylor, CC BY-ND
Sometimes an animal can be born with a genetic defect that turns out to be really useful for its survival. As long as the abnormality can survive and the animal is able to mate, this trait can be passed on. We believe this is exactly what happened to hammerhead sharks.
The hammerhead species that branched off the earliest is the tip shark (E. blochii), which has one of the widest heads. Over time, natural selection has reduced the size of the hammer. It turns out that the most recent hammerhead species is the basking shark (S. tiburo), which has the smallest hammerhead of all.
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