Size doesn't matter, at least for hammerheads and swimming performance

Sharks come in all shapes and sizes and perhaps the most unusual is the hammerhead shark, easily recognized by its oddly shaped head. Most research on hammerheads has focused specifically on their laterally expanded heads, or cephalofoil, and how they use it to see and smell as well as its effects on hydrodynamics and sensory efficiency. There are about nine known species of hammerhead sharks with dramatic differences in their body shape including the shape and size of their heads. While much is known about the variations in their electroreception, olfaction and vision, very little is known about whether or not their shape differences affect their swimming performance.

Researchers from Florida Atlantic University have conducted the first study to examine the whole body shape and swimming kinematics of two closely related yet very different hammerhead species: the Bonnethead and the Scalloped hammerhead, with some unexpected results.

Adult Bonnetheads are about 2 to 3 feet long and their head width makes up about 18 percent of their body length; adult Scalloped hammerheads are closer to 12 feet long and their head width makes up about 30 percent of their body length. Despite these differences, results of this new study, featured on the cover of the current issue of the Journal of Experimental Biology, find that in the end, size or shape really doesn't matter, at least when it comes to swimming.

Using an interdisciplinary approach in the Biomechanics Laboratory in FAU's Charles E. Schmidt College of Science under the direction of Marianne E. Porter, Ph.D., assistant professor of biological sciences and co-author of the study, the researchers set out to test their hypothesis. Different head shapes and different body sizes of hammerhead sharks should result in differences in their swimming performance.

Prior to starting the study, Porter, Sarah L. Hoffmann, lead author and a Ph.D. student of biological sciences, and Steven Matthew Warren, co-author and a senior in the Department of Mechanical and Ocean Engineering in FAU's College of Engineering and Computer Science, reviewed CT scans of both species of hammerhead sharks. Because sharks are made up entirely of cartilage that is heavily mineralized, they were able see the marked differences in the two species of the sharks' physiology from these scans.

To test their hypothesis, they focused on undulation, which is how a shark moves its body and tail from side-to-side to propel itself forward. The goal: to figure out if the movement of the body changes between these two species with very different head shapes.

Beginning in 2015, they viewed hours of video of Scalloped hammerheads and Bonnetheads swimming. They looked at tail beat frequency and tail beat amplitude. They analyzed video sequentially to select clips in which sharks completed at least three full tail beat cycles of straight, steady swimming. Warren analyzed and then condensed the videos into a minute-long segment to enable the research team to use the measurements to compare swimming mechanics between the two species. They were able to get all of the measurements they needed from that condensed one minute of video footage.

"One of the more unique aspects of our study is that we were able to observe these sharks swimming in large tanks moving around naturally," said Porter. "Most studies place sharks in flumes, which are basically underwater treadmills that force them to move. We are interested in learning how these animals move in and of themselves for both conservation efforts and real world applications such as bioinspired engineering."

Results of the study revealed that the Bonnetheads swing their bodies further in and out and therefore have a larger amplitude of undulation. On the other hand, Scalloped hammerheads bend faster and have a higher frequency of undulation.

"When we corrected for their body size, we discovered that they actually swam at the same speed to get to points A and B, but did so in different ways," said Hoffmann. "Even though they are different, they get to the same destination at the same time; they're just using different body mechanics."

A key finding from their study is that in both species the head is moving at a different rate than the rest of the body. In fact, it is actually moving back and forth a lot faster than the rest of the body. Similar to sturgeons, a species of fish, the researchers discovered that these hammerheads have a double oscillating system. They speculate that it is because of an increased ability for sensory perception.

"There is no way that we could have anticipated the double oscillation system in this species," said Hoffmann. "The head movement being different than the rest of the body movement is something that's almost impossible to see with the naked eye."

The researchers point out that with the double oscillation system, the shark's head is moving at a much quicker rate than the rest of their body essentially to scan more of the substrate of their environment.

"Think of it like a metal detector as you move it back and forth," said Hoffmann. "They're going to be covering more territory for electroreception and olfaction and they need to be able to do that at a greater rate than the rest of their body."