A great hammerhead shark’s two eyes can be 3 feet apart on opposite sides of its skull.
Why do hammerhead sharks have hammer-shaped heads?
Hammerhead sharks are the strange-looking ones. They look like someone grabbed their skull by the eye sockets and stretched their heads out sideways, while the rest of their bodies look like those of a normal shark.
You might wonder – what are the advantages of having a hammer-shaped head? And how did hammerhead sharks get that way in the first place?
I’m a scientist who has been studying sharks for almost 30 years. The answers to some of these questions have surprised even me.
Hammerhead sharks boast an extraordinary field of vision. Their uniquely placed eyes, set on the wide edges of their hammer-shaped heads, allow for nearly 360-degree vision. This positioning enables them to see above and below them with ease, providing a significant advantage in detecting prey and predators.
Benefits of the Hammer
Scientists think sharks with hammer-shaped heads have three main advantages.
The first has to do with eyesight. If your eyes were pointing in two opposite directions, say, by your ears, it would give you a much wider field of vision. Each eye would see a different part of the world, so you’d have a better sense of what was around you. But it would be hard to tell how far away things are.
To make up for that trade-off, hammerhead sharks have special sense organs, called ampullae of Lorenzini, scattered on the underside of their hammer. These porelike organs can detect electricity.
If you look closely at this great hammerhead shark (S. mokarran) you can see the sensory pores on the underside of its hammer.
The pores basically act like a metal detector, sensing and locating prey buried under sand on the ocean floor. Regular sharks have these sensory organs too, but hammerheads have more. The farther apart these sensory organs are on a hammerhead’s stretched-out head, the more accurate they are at pinpointing the location of food.
And finally, scientists think hammers help sharks make quicker turns while swimming. If you’ve ever walked in gusty wind with an umbrella or flown on an airplane, you know how powerful large surfaces can be in motion. If you’re a hammerhead shark, and your intended dinner swims by quickly, you can turn more rapidly to catch it than other fish can.
The Hammerhead Family Tree
It would be nice if scientists like me could look at fossils and trace the development of hammerhead sharks over time. Unfortunately, fossils of hammerhead sharks are almost entirely of their teeth. That’s because the bodies of sharks do not have bones. Instead they’re made of cartilage, which is what your ears and nose are made of. Cartilage breaks down much more quickly than teeth or bones do, so it rarely gets fossilized. And tooth fossils don’t tell us anything about the evolution of hammerhead skulls.
Nine different kinds of hammerhead sharks swim in the oceans today. They vary both in size and in the shapes of their heads. Some have very wide heads relative to their bodies. These include the winghead 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 hammer belongs to the bonnethead shark (S. tiburo).
Others have smaller hammers relative to their bodies, including the bonnethead (S. tiburo), scoophead shark (S. media), small-eye hammerhead (S. tudes) and scalloped bonnethead (S. corona).
Scientists long assumed the first hammerhead sharks did not have much of a hammer but, over time, some slowly evolved bigger hammers. We thought the different hammerhead sharks living today were snapshots from different periods in the evolutionary process – with the small hammerheads being the oldest species on the family tree and the huge hammerheads being the newest ones 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.
Hammerhead sharks are excellent navigators. Researchers believe that the shape of their head could help in navigating the oceans by enhancing their ability to detect the earth’s magnetic field. This sense of direction is crucial for their long migratory journeys in search of food and breeding grounds.
We took DNA from eight of the nine hammerhead species and used it to look at the relationships among them. The results were not what we expected at all. The older species had the proportionally bigger hammers and the younger species had the smaller hammers.
Deformities As Assets
When scientists think about evolution, we usually assume that living things change a little bit at a time, slowly fine-tuning themselves to take better advantage of their environment. This process is called natural selection. But that’s not always the way it works, as hammerhead evolution shows.
On the left is the expected evolution of hammerheads, assuming a gradual change of head shape. On the right is the observed pattern of evolutionary change based on DNA sequence data. Credit: 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. So long as the abnormality is survivable and the animal is able to mate, that trait can be passed down. We think that’s exactly what happened with hammerhead sharks.
The hammerhead species that branched off the earliest is the winghead shark (E. blochii), which has one of the widest heads. Over time natural selection has actually shrunk the size of the hammer. It turns out the most recent hammerhead species is the bonnethead shark (S. tiburo), which has the smallest hammer of all.
Written by Gavin Naylor, Director of Florida Program for Shark Research, University of Florida.
This article was first published in The Conversation.
evolution is a farce and a hoax. These wannabee so-called scientist weave these fantastic tales of how things developed over millions of years, with absolutely no concrete evidence of any kind. Modern science has totally refuted this philosophical, religion for the sham it is. They spew this nonsense under the guise of intellect and the tyranny of authority. It flew for a bit in the 19th century because of it’s lack of science compared to today. But modern science, genetics and DNA has totally decimated this lie that has been force down our children’s throats for decades. Most credible scientist have totally distanced themselves from this embarrassing pseudo-science.