Swimming like a Turtle

Body flexibility is distinctly advantageous when performing complex, unsteady movement patterns such as turning. For us this is easy due to the range of motions the human body can achieve. As such we are considered flexible, as are most soft-bodied mammals.  Flexibility allows us to combine the abilities of being highly maneuverable and incredibly agile.  Here we shall define maneuverability as being the proficiency with which an animal may turn in a limited space and agility as being the speed with which this motion is achieved.

In predator-prey interactions, the chances of escape and survival are drastically aided by the ability to negotiate obstacles and weave through the environment while being voraciously hunted by your unrelenting attacker. As such, turning is a critical component of any animals “survival toolbox”, as essential as an Escape Rope is to a lost Ash in a cave without Flash. Can we therefore lend a thought to the only rigid-bodied aquatic tetra-pods, the turtle. Turtles must account for predators such as sharks and whales which can easily crush through their shells which do not confer as much protection as one might think. Being rigid-bodied, turtles are theoretically placed at a severe disadvantage during predator-prey interactions.

Rigid-bodied animals have been shown to be highly maneuverable or highly agile, rarely both. Boxfish, for example, are able to turn in a confined space (i.e. they are maneuverable) but can only achieve this slowly (i.e. they are not agile).Whirligig beetles, conversely, are highly agile but not very maneuverable. They can achieve high angular velocities but have large turning radii. However, studies have shown that turtles do not excel at either component of turning (maneuverability or agility) yet combine good levels of each to produce a well-rounded turning style.

Graph turtle

This graph depicts how turtles compare to other species (or submarines… (USS Albacore)) and we can see that they approach the top-left of the graph. This indicates a high turning rate (agility) and a low turning length (maneuverability) almost comparable to that of fish! Damned good for a rigid-bodied tetrapod. Gimme me some fin!

So, how do they achieve this? It’s all in the flipper-action. To turn, there must be a net difference in the forward forces on the left and right sides. Simplistically, a turtle can control its flippers individually in one of 4 patterns. 1) Continue to move it as during normal swimming 2) Modify the style of movement to produce a reverse propulsion 3) Retract the limb across the body to reduce drag or 4) extend the limb to increase drag. A right handed turn while moving forwards is performed by retracting the front-right flipper across the body while the front-left flipper remains in phase with the hind-limbs which are acting to propel the turtle forwards. However, while moving backwards (propelled by the simultaneous protraction of the hind-limbs) the front-limbs remain static, with the left flipper protracted and the right flipper retracted. See below!

Turtle turning

Hopefully you could now, if prompted, be able to demonstrate how a turtle is able to turn so proficiently underwater. With this new found knowledge, I hope you can join me on Sunday 26th January 2014, for the first ever national “Pretend to be a Turtle” Day!

Until next time,

Rip it, roll it, punch it!

James

Rivera G, Rivera AR, Dougherty EE, Blob RW. (2006). Aquatic turning performance of painted turtles (Chrysemys picta) and functional consequences of a rigid body design. Journal of Experimental Biology DOI: 10.1242/jeb.02488

Original featured image: http://www.lovepixar.com/wp-content/uploads/2012/09/squirt-sea-turtle-finding-nemo.jpg

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