On land, penguins seem awkward and clumsy, but in the water they move at least as gracefully and acrobatically as their flying relatives. However, the physics behind their swimming movements are very different from those of flying birds. Researchers at the Tokyo Institute of Technology have now examined swimming curves in penguins in more detail in a new study.
The physical principles of the penguin swimming style have already been investigated in a few studies. Most of them, however, focused on forward swimming rather than turning. In any case, existing studies on the underlying mechanisms in flying birds cannot be applied to penguins. Since water is 800 times denser than air, the resistance to be overcome is also much greater and requires different movement to make the turn.
Two Japanese physicists at Tokyo Tech have therefore observed swimming Gentoo penguins living at the Nagasaki Penguin Aquarium. They wanted to better understand the three-dimensional kinematics and hydrodynamic forces that enable penguins to do turns. Their findings were published in the Journal of Experimental Biology.
The two researchers installed more than a dozen underwater cameras in the penguins’ water tank and recorded the animals’ swimming movements, which they tagged with colored markers at various points on their bodies and flippers. Thanks to special technology, the direct linear 3D transformation, they were thus able to perform detailed 3D motion analyses.
The researchers then used this data to create a 3D mathematical body model of the penguins. This model included the orientation and angles of the body, the various positions and motions of the flippers during each stroke, and associated kinematic parameters and hydrodynamic forces. Statistical analyses and comparisons with the experimental data gave the researchers insights into the role of flippers and other movements of the body during turning.
The main results of the study show how penguins generate centripetal force to support their movement when turning. In part, they accomplish this by maintaining outward banking. So they tilt their body such that their belly is facing inward. In powered turns – those where the penguin flaps its flippers – most directional changes occur during the upstroke, whereas forward thrust occurs during the downstroke.
In addition, the researchers found that penguins flap the flippers asymmetrically during execution of the turn. “We found contralateral differences in wing motion; the wing on the inside of the turn becomes more elevated during the upstroke than the other,” explains Hiroto Tanaka, associate professor at Tokyo Tech and co-author of the study. “Quasi-steady calculations of wing forces confirmed that this asymmetry in wing motion with the outward banking contributes to the generation of centripetal force during the upstroke. In the following downstroke, the inside wing generates thrust and counter yaw torque to brake the turning.”
The new findings contribute to a better understanding of how penguins turn when swimming and are important not only from a biological but also from an engineering perspective. The research of the two physicists is in fact also the basis for a penguin robot. Professor Tanaka notes, however, that these results are only one piece of the puzzle: “The mechanisms of various other maneuvers in penguins, such as rapid acceleration, pitch up and down, and jumping out of the water, are still unknown. Our study serves as the basis for further understanding of more complex maneuvers.”
Julia Hager, PolarJournal
Featured image: Michael Wenger
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