Tag Archives: swimming

Do Hammer-Shaped Heads Help Sharks Swim?

With their sandpaper skin, cartilage skeleton, electroreceptive sensors, and rows of dangerous teeth, sharks fascinate many people. However, even within this distinctive group the hammerhead sharks that make up the Sphyrnidae family have attracted a special attention due to the unusual shapes of their namesake heads, called cephalofoils. Several evolutionary benefits of the cephalofoil have been proposed by researchers. The wide hammer-shaped head may allow the shark to house more sensory receptors in its snout, to bludgeon prey, and to move and maneuver through the water more easily. Here we will address the question posed by the third theory: Does the cephalofoil found on hammerhead sharks provide an advantage in moving and maneuvering underwater?

Great Hammerhead Shark
Image of “Great Hammerhead Shark” by Wendell Reed showing a close-up of the cephalofoil. https://search.creativecommons.org/photos/3fa18a9b-9085-4867-93e1-15a88b01389b

Many advancements in the aviation and nautical industries have been developed from the study of sea creatures. There is certainly some potential that research into the mobility of sharks could someday be used as inspiration to advance locomotion technologies. In addition, a deeper understanding of the physiology and behavior of hammerhead sharks could help us to better preserve their habitat and species from endangerment – a crisis which some of them are already facing.

The theory that the cephalofoil provides advantages in forward swimming to hammerhead sharks relies on it supplying some hydrodynamic lift similar to the wing of an aircraft. Aircraft wings provide lift partly by creating a pressure difference between the top and bottom wing surfaces. An area of higher pressure on the bottom surface of a wing will generate upward lift. One study by Matthew Gaylord, Eric Blades, and Glenn Parsons applied a derivation of the Navier-Stokes equations – a set of partial differential equations for analyzing fluid flow – to water flow around digitized models of the heads of the eight most common species of hammerhead. It was found that in level, forward swimming there was some pressure differential that developed between the dorsal (top) and ventral (bottom) surfaces of the cephalofoil, but for each species it was very small and often in the direction to produce negative lift. The drag coefficients of the cephalofoil of each hammerhead species were then calculated and shown to increase as the size of the cephalofoil increased. The drag created by a cephalofoil was always much greater than the drags caused by the heads of a control group of non-hammerhead Carcharhinidae sharks.

Images of the pressure contours at zero angle of attack on the dorsal (top image) and ventral (bottom image) sides of the cephalofoil. The eight leftmost sharks are the hammerheads. Taken from Gaylord, Blades, Parsons.

However, the same study showed that the pressure difference between either surface of the cephalofoil did significantly increase in some species if the shark raised or lowered its head. This extra hydrodynamic force caused by the pressure differential at nonzero angles of attack would help the shark to turn its head up or down very quickly. This more explosive maneuverability was particularly present in the hammerheads that commonly feed on fish and less present in the species that feed predominantly on slower bottom dwellers. Another study by Stephen Kajiura, Jesica Forni, and Adam Summers theorized that the unbalanced, front-heavy cephalofoil may provide extra stability during tight turns by preventing banking. The unbalanced head would create a moment – or torque – to counteract the force from the tail that causes most sharks to roll into their turns. Not banking around turns could be important to some hammerhead sharks that often swim so close to the seafloor that banking into a turn could cause their head or fins to bump into the floor. It is likely that hammerhead sharks evolved the cephalofoil at least in part to provide more explosive and stable maneuvering.

Featured image “Hammerhead Shark” by bocagrandelasvegas.

Dolphin Magic or Dolphin Muscle?

Because of the film Bee Movie, many people at one point were intrigued by the idea that bumblebees should not physically be able to fly due to their large bodies and tiny wings. But, they fly anyway. Technology is advanced enough to study bee wing movement and determine that they produce enough lift to allow them to fly, disproving the previous notion. Similarly, Gray’s Paradox for a long time inferred that dolphins should not be able to swim nearly as fast as they do. But, they still consistently swim at speeds over twenty miles per hour. It was not until recent history that advancements allowed researchers to determine why they are able to reach such high speeds.

Gray’s Paradox

All the way back in 1936, Sir James Gray observed the high speeds dolphins could reach in the ocean. He calculated an approximation of the amount of power the dolphins would need to produce to sustain these speeds, based on the drag force on the dolphin as it travels through the water. Gray compared this to the amount of power he expected the dolphin to be able to produce. In order to compute this, Gray used muscle power data from oarsmen. When he compared the muscle mass of these oarsmen compared to dolphins, he determined that the power dolphins could produce was only about one seventh what was needed to travel at the high speeds of which they are capable.

Force Diagram, showing that the same forces that the swimming mammal applies to water are applied back on it. Allows observation of max speed to determine these forces.
This diagram shows that the drag force, D, thrust force, T, and net axial force, Fx, must be equal for the swimmer and the fluid. The lateral velocity, u, can be used to determine the resulting drag force, allowing researchers to estimate how much thrust is needed. Credit: [2]

And now we have arrived at Gray’s Paradox. What allows dolphins to move so quickly? To Gray and other researchers for most of a century, this was a mystery. If the assumptions they had made were correct, that would mean dolphins have some way of travelling through water more efficiently than was thought to be possible. This sparked a large amount of speculation into how dolphin skin could reduce the drag force of the water, which was originally believed to be the way Gray’s Paradox would be resolved.

Answering Questions while Creating More

Finally in 2008, Timothy Wei’s research team was able to definitively disprove Gray’s Paradox. He set up an experiment that would allow the force that dolphins exert to be measured. This mainly consisted of having dolphins swim through a curtain of bubbles in a tank. By recording at high resolution the movement of these bubbles as the dolphins swam by, the researchers determined the speed of the water around the dolphin as it traveled. With this information, Wei’s team showed that dolphins are able to produce over 300 pounds of force at one moment, and over longer periods of time 200 pounds of force. This is approximately ten times more force than Gray estimated.

Wei’s findings resolve Gray’s paradox by showing that dolphins have the ability to produce sufficient power from their tail movement to overcome the strong drag force of the water as they move at high speeds. However, this does not explain how dolphins produce so much power with their amount of muscle mass, which is still being examined. One idea is that this is caused by anaerobic muscle fibers that behave in different ways than in humans, and allow more power to be generated than Gray expected.

Future Plans: Investigating Force Generation

Timothy Wei plans to continue examining force generation in the swimming of other marine animals. This has the potential to provide more understanding of how marine animals evolved in their swimming aptitude. On the level of microbiology, this research could improve understanding of how dolphin and other animal muscles can perform such high levels of power generation over sustained periods of time.

Additional Reading and Sources

Swimming Fast and Slow: What We Know About the Sailfish’s Iconic Fin

Indo-Pacific Sailfish
Indo-Pacific Sailfish. Credit: D. Corson/Shostal Associates

Sailfish, or Istiophorus platypterus, are one of the most recognizable fishes in the ocean due to their large sail-like dorsal fin. But, did you know that they are also iconic because they are one of the fastest swimmers in the ocean? Sailfish top speeds have been recorded to be up to 30 m/s, which roughly translates to 67 miles per hour. When researchers examined the sailfish swimming out in the ocean, they discovered that they have the unique ability to retract and deploy their sail and other fins. Furthermore, they saw that when swimming at top speeds, swordfish retract their sail, and when the fish are hunting prey, they deploy it. How does retracting the sail help them swim fast, and how does deploying the sail help them hunt? 

Sailfish swimming with both the sail retracted (0:24) and extended (0:40). Disregard the silliness of the video. Credit: Pew

Speeding Up

When it comes to swimming fast, the answer may have to do with drag. Due to their hydrodynamic characteristics, sailfish have a very low drag coefficient, which is the quantity used to describe the resistance of an object moving through fluid. Sailfish have a drag coefficient of 0.0075, which is similar to smaller fish such as pike, dogfish, and small trout. Additionally, due to their size, sailfish are able a generate much more force with each swimming motion than their smaller peers. The combination of these two factors allows them to move at such high speeds. The thing that truly sets sailfish apart, though, is this ability to retract their sail and pectoral fins. Studies show that when the sail and other fins were retracted, sailfish are able to reduce their drag by about 18%. With less drag to worry about, the fish can be more efficient in generating thrust from its swimming motion, which allows it to speed ahead of the competition. To get a sense of just how fast these fish move, watch the end of the video above, where they only get up to 40 mph!

Slowing Down

Sailfish usually feed on smaller fish that swim in schools, like sardines. These small fish “exhibit higher performance than large fish in unsteady swimming,” which is to say making quick lateral movements. So, if a sardine senses a sailfish coming at it at full speed, it will likely have no problems avoiding it. With the sail dorsal-fin raised, the increase in drag slows the fish down dramatically and steadies its swimming path.

In the figure below, sub-figures C and D show how much the tail oscillates (in grey) and the direction in which the fish is moving (in black) when the sail is retracted and deployed, respectively. Zero on the Y-axis represents the tail being directly in line with the rest of the body and the fish is moving straight forward. While the sail is up, the tail moves far less aggressively and the fish more consistently moves in a straight line.

Side by Side comparison of Sailfish swimming with and without sail
Affect of Sail on stability while swimming. Credit: Stefano Marras

This allows the sailfish to become more controlled in its movements and help pursue a sardine performing evasive maneuvers. When it gets within striking distance, the sail helps counterbalance the fish’s swift lunge at its prey, which provides much-needed accuracy. Biologists have also theorized that the large size of the sail helps herd the school closer together, but no official research has been done on this.

Take-Aways

The sailfish’s ability to retract and deploy its sail dorsal-fin gives it unique advantages in both long-distance swimming and hunting. Minimizing drag is one of the most important concepts for travel through any fluid (air or water), so understanding how the sailfish is able to reduce drag could provide a new perspective on things. The sail’s ability to provide stability at lower speeds has potential use in any sort of water travel, whether it be with cargo ships (like the Evergreen in the Suez), boats, or submarines.  

If you are interested in another unique feature of the Sailfish, its bill has been a popular topic as of late, especially with its feeding habits. The bill is often used like a sword, either spearing or stunning prey, and also has been thought to have helpful drag-reducing qualities. Click here for more information.

Sources

Woong Sagong, “Hydrodynamic Characteristics of the Sailfish (Istiophorus platypterus) and Swordfish (Xiphias gladius) in Gliding Postures at Their Cruise Speeds,” Plos One, https://doi.org/10.1371/journal.pone.0081323

Stefano Marras, “Not So Fast: Swimming Behavior of Sailfish during Predator–Prey Interactions using High-Speed Video and Accelerometry,” Integrative and Comparative Biology,” https://doi.org/10.1093/icb/icv017

P. Domenici, “How sailfish use their bills to capture schooling prey,” The Royal Society Publishing, https://doi.org/10.1098/rspb.2014.0444


The Ship of Pearl – Jet Propulsion in the Chambered Nautilus

In the aptly titled poem The Chambered Nautilus, Oliver Wendell Holmes Sr. praises the eponymous cephalopod for its elegant shape and vibrant colors. The ship of pearl, as Wendell calls it, might not be the swiftest vessel; but Thomas R. Neil and Graham N. Askew’s research indicates that the chambered nautilus might be among the most energy efficient ships in the seven seas.

The Nautilus pompilius is constantly moving to depths up to 700 m. At such depths, oxygen concentration decreases significantly: only 30% of that available at the surface. In such harsh environments, the nautilus must find a way to use its oxygen reservoir most efficiently while still being able to carry out metabolic functions. As such, the nautilus has adapted to use jet propulsion in a most efficient manner.

Jet propulsion mechanism of the chambered nautilus
modified from Neil & Askew, Royal Society Open Science (2018)

Jet propulsion in the nautilus is achieved by the simultaneous contraction of the head retractor and funnel muscles (shown in diagram above). The compression of the mantle cavity causes a pressure difference with its surroundings, resulting in a jet of water being expelled from the mantle cavity via the funnel. The nautilus uses its flexible funnel to control its swimming direction as shown in the video. Slower swimming is powered by rhythmic contractions of the funnel flaps that generates a wave that travels along the funnel wings. This produces a unidirectional flow across the gills. Although jet propulsion is less efficient than undulatory swimming (think of the swimming motion of a ray), the Nautilus converts chemical energy into hydrodynamic work for motion more efficiently than fellow cephalopods such as the squid and even salmons (at lower speeds).

 

In their study, Neil and Askew used particle image velocimetry (PIV) to study the wake of the nautilus’s jet. PIV consists of spreading particles of aluminum oxide in a water tank and shinning a laser on them. As the nautilus moves, the particles in the tank move with the jet. A high-speed camera is used to take multiple pictures of the floating particles and the relative position of these between snapshots is used to determine a velocity profile of the nautilus’s water jet.

Nautilus and its jet wake as seen in PIV
Photo by: Simon and Simon Photography, University of Leeds. Taken from Greenwood, The New York Times (2018)

 

 

 

 

 

The results from the study show that the whole cycle propulsive efficiency and thrust generation are related to the swimming orientation of the nautilus. The propulsive efficiency ranged between 30% and 75% for posterior-first swimming and 48% to 76% for anterior-first swimming (orientations defined in diagram above). Moreover, efficiency increases with greater speed in posterior-first orientation but decreases in anterior-first. This might have to do with energy losses associated with the re-orienting of the funnel that turns back on itself to move in the anterior orientation. Moreover, at low speeds, the nautilus spends more time jetting water than refilling, resulting in a lower jet speed but overall more efficient propulsion.

Although not covered in their research, Neil and Askew’s findings about jetting duty cycles and efficient propulsion at low speeds could be potentially applied to the design of more efficient hydrojets to be used in underwater vehicles. Engineering seeks to imitate nature’s most intelligent designs; and as Wendell puts it in his work, the nautilus proves to be a truly awe-inspiring creature worthy of imitation.

Women in Endurance Athletics: The Further, the Faster

In the majority of athletic events, men have long outperformed women. This is due to a combination of factors including physiological differences, societal norms, and legislation. But in the last few decades, there has been a noticeable swing in the realm of endurance athletics. Now more than ever, women are closing the gap with respect to their male counterparts in ultra-long distance races, including running, biking and swimming. In some cases, women are even outperforming men at the elite level, winning a number of top-tier events. So what are the reasons for this changing of the guard, and why is it happening now?

A woman fights up a steep hill in a mountain biking raceImage courtesy of Pixabay

One of the major reasons for this transition in endurance athletics performance boils down to athletics becoming more inclusive. Since the passing of Title IX in 1972, the number of women participating in a wide array of athletics has increased dramatically. Before Title IX, only about 300,000 girls participated in high school sports, whereas now that number has climbed to around 3.3 million annually. Many experts believe that about a third of the difference in performance between male and female athletes can be attributed to the door opening for more women to compete. But why does this effect endurance athletics the most?

In endurance athletics, there are five major factors that contribute to an athlete’s performance: heart size, VO2 max (the efficiency of oxygen delivery to muscle), lean muscle mass, central drive, and movement economy. Men are typically better suited than women when it comes to the first three. But central drive, or how well the nervous system can send continued signals to maintain muscle performance over time, and movement economy, or efficiency of form, allow women to close the gap. These two factors can be improved through practice with monumental results. In ultra-long-distance swimming, where efficient body control is perhaps most critical to building speed and saving energy, women perform better than men in two out of the three most elite races in the world.

Woman swimming in open water
Image courtesy of Free-Photos on Pixabay

There are a number of other physiological advantages for women at long distances. Women’s muscles tend to be smaller than men’s, but over long distances this means that they do not tire as quickly since their hearts do not have to work as hard to pump as much blood. Women have been found to recover faster than men, utilize fat stores for energy more efficiently than men, and hold a consistent pace nearly 20% better than men. All of these things add up over the length of high endurance races of all kinds, allowing women to perform better compared to men than they do at shorter distances.

There are still many factors in this area of biomechanics research which are unsure, but one thing is sure. As more women continue to participate in athletics and especially high endurance athletics, there is no telling the limit to how fast and how far they may go.

 

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