A study in the Journal of Experimental Biology made big news late last year by beginning to answer a fundamental, fable-like question: why are hammerhead sharks shaped the way they are? The answer, as it turns out? The better to see their prey with, my dear.
The researchers found some surprising results about hammerhead vision. One of those researchers, Michelle "Mikki" McComb of Florida Atlantic University, happens to also be an enthusiastic Oceana supporter. Mikki was kind enough to answer some questions about the research:
Can you summarize the conclusions you and your colleagues reached about hammerhead vision?
The popular literature is filled with claims that hammerheads have better vision, but this was never tested. Our goal was to quantify the extent of the visual fields of hammerhead sharks in contrast to more typical head shaped sharks, in order to determine if they possessed binocular vision.
The visual field is the entire expanse of space visible around the head and can be described by one eyes field of view (monocular field), two eyes field (cyclopean) and the overlap of the two monocular fields (binocular overlap). We determined the horizontal and vertical visual fields of three hammerhead shark species as well two closely related “typical” shaped sharks.
What we found was a surprise! Hammerhead sharks do have binocular vision, and even more surprising, the extent of binocular overlaps was greater than found in the “typical” shaped sharks. The largest binocular overlap was found in the winghead shark, the hammerhead with the widest head, and is a result of the positioning of the eyes on the end of the head.
The eyes of the hammerheads are tilted slightly forward and this was key in facilitating binocular vision. Our findings support the hypothesis that vision may have played a role in the evolution of the strange hammerhead.
How does hammerhead vision compare to human vision? I’m trying to get a comparative sense of how well they can see.
Binocular vision evolved to facilitate depth perception. Each eye sees a slightly different image and it’s the subtle differences within these images that are used for depth cues. So the wider separation of the eyes in hammerhead sharks, as compared to humans, enhances the stereoscopic effect and depth perception. This translates into a visual advantage for a predator like the hammerhead shark in its ability to accurately judge the distance of moving objects like prey.
After we incorporated the degree of eye and head movement into the visual field we found that the hammerheads can see behind and have a full 360 degree view around the head. This may be beneficial to smaller hammerhead sharks that are potential prey to larger sharks.
We found that all sharks have dorsal and ventral visual overlaps. This means they can see a full 360 degrees around their heads above and below as they are swimming! We also found that all sharks have binocular overlaps in front of their heads and that the hammerheads overlaps were larger and corresponded to the expansion of the head in a range of hammerhead species.
As a result of your team’s research, do we have a definitive answer for why hammerhead sharks heads are shaped the way they are?
Our results support the idea that vision may have played a role in the evolution of the hammerhead, however, there are several other hypotheses that remain, some of which are untested. Therefore, it is possible that several factors in combination may have led to the evolution of the head shape.
There are several working hypotheses about the functionality of the hammerhead shape other than vision and include:
1. The head acts as a bow plane to provide hydrodynamic lift during swimming.
2. Possible enhanced electrosensory ability due to the larger size of the head and the potential for possession of more electrosensors in the head.
3. Enhanced olfactory ability due to the wide separation of the olfactory organs and naris (nostrils) on the head.
4. The head shape may help hammerheads pin down prey items like stingrays. However, this is not strongly supported because the diet of many hammerhead species (there are 8 species of hammerhead) do not include stingrays.
Were you surprised by the outcome of the research? Why or why not?
Yes we were! When we originally discussed this project we thought we were setting out to dispel the myth that hammerheads possessed binocular vision. We had no idea that hammerheads would have even larger binocular overlaps than normal sharks!
How did you get interested in sharks?
I saw the movie JAWS when I was young and it scared me so much that I could barely swim in a pool without thinking a shark was going to get me. Because I was so afraid of sharks I began to read about them and slowly realized they were not the monstrous creatures I had thought they were. I have continued to learn all that I can about sharks and also study closely related fish like stingrays, even ones that inhabit the freshwater of the Amazon.
The media response to your research was huge. Why do you think people were so interested in it? What do you hope people came away with after reading about it?
I think people enjoy sharks because they are mysterious and there is so much about them we still do not know. I hope people understand that hammerheads are the most recently evolved sharks in the ocean and we have demonstrated that their visual abilities have been enhanced as a result of their strange expanded head.
Have you ever swum with sharks? If not, do you want to?
I have had the pleasure of travelling to a lot of different countries over the world and I have had the privilege of swimming with many sharks species. I have swam with great white shark off the coast of South Africa, bull sharks in Australia, hammerheads in Hawaii, lemon sharks in Florida, and Caribbean reef sharks in the Bahamas. Every time I see a shark while I am in the water I get a thrill and I feel so lucky and humbled that I am sharing space with them.
What do you think is the biggest problem facing sharks today? Why is it important for humans to protect them?
The cruel and wasteful practice of shark finning is the biggest threat to shark populations worldwide. Over 100 million sharks are killed each year for their fins which are dried and used to make shark fin soup, a flavorless broth, which is an Asian delicacy and status symbol.
This overharvest is unsustainable since sharks reproduce and grow slowly. The oceans are vital to life on our planet and sharks have been in the oceans for 400 million years, playing a pivotal role in ecosystem balance.
The human wholesale removal of shark species from the ocean has negative cascading effects upon the food web. Since we know so little about oceanic species interactions, coupled with the overharvest of so many other fish species, the true impact of the loss of these sharks is unknown.
What is the one thing you wish everyone knew about sharks? What do you love about sharks?
I wish everyone knew how critically endangered sharks are from shark finning and that they can make a difference by spreading the word not to buy shark products.
I love that there are so many different shark species in the oceans ranging from huge whale sharks, strange hammerheads, beautiful tiger sharks, to tiny dwarf lantern sharks! They are truly incredible!
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