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Entries in turtles (4)


To see the world in a grain of sand - movement from a turtle hatchling's perspective

This post was chosen as an Editor's Selection for

(with apologies to William Blake).  A grain of sand represents many things to a baby turtle.  While still within the egg, sand represents a roof over your head, protection from the desiccating sun and from predators, and a blanket to keep you warm and level until its your turn to break free of the nest and do that mad nocturnal dash down the beach to the safety (yeah, right!) of the sea.  From the moment of hatching, however, sand presents a range of obstacles to a baby turtle, and believe it or not, the way they overcome those obstacles tells scientists a lot about how things can and should move through and across granular substrates, and maybe offers a few solutions to human problems of this kind too.  That’s because, while they look like little clockwork toys ceaselessly flapping their way to some unseen destination, they’re actually engaging in several different types of locomotion, and adapting them on the fly to best suit the substrate they happen to be on.  Discovered by Nicole Mazouchova and Nick Gravish from Daniel Goldman’s biomechanics lab at Georgia Tech, these adaptations show us that baby turtles are much cleverer than perhaps we gave them credit for, and they may even explain to some degree why turtles nest on some beaches and not others.

Immediately after hatching a foot or more below the surface of the beach, a baby turtle must get to the surface to draw breath and begin its journey down to the water.  That high up a beach, the sand is usually dry and loosely packed.  Governed by the laws of physics, sand of this type can act either as a liquid or as a solid, depending on the force applied to it. (if you ever want to see what I mean, add a little water to some corn starch in the palm of your hand until its like whipping cream, and then rub the surface with your other finger.  If you rub slowly, your finger will wet, but if you rub fast, the surface will appear dry and your finger will slide right across).  To move across this kind of sand, the turtle reaches forward with its front flipper and places it flat on the sand, then digs the leading edge down until the flipper is perpendicular to the surface and mostly buried.  By pushing back with just the right amount of force, the sand behind the flipper doesn’t yield, but solidifies like the corn starch in your hand, providing a solid point of leverage against which the turtle can gain traction and push further forward.  It then repeats the process on the other side, lurching forward one push at a time with alternating strokes, rather like a rock climber makes progress up a wall.  The key thing is that if the turtle pushes too hard or fast, the sand will fluidise and the flipper will pass through it like a liquid, producing no traction; they have to push just the right amount for the physics of the sand to work in their favour.  Nick, Nicole and Dr. Goldman observed this in wild loggerhead hatchlings, then showed why this is mathematically, and then created robot turtles that recreated exactly the scenario in the lab to confirm their mathematical model (what, you mean you don’t have a robot turtle hatchling in your lab?).  You might think that this kind of motion is pretty inefficient since you effectively stop after each push, but it works well for the hatchlings; they can move three body-lengths per second or more across the sandy surface.  That’s the equivalent of a human running at a full sprint, and they’re doing it on dry sand.  Good luck matching that effort!

Farther down the beach, the turtle meets a different kind of sand.  Wetted and a sorted by the tide, this sand is flat and compacted and the hatchling would be unable to dig its flipper in, so it changes strategy.  Instead of digging in and pushing against a block of solidified sand, it jams just the claw on the leading edge of its flipper into the surface like a spike and (with some help from the back flippers) pushes off it, rotating around the point as a pivot, to jam the next point in a little further ahead, just like a skier planting their stocks in the snow as the pivot point for turns.

Farther still, the turtle meets the water.  At this critical point, the game changes completely.  Instead of moving across a granular surface, the hatchling is now supported by a liquid medium that will never solidify or allow them to gain traction like thy did on the beach.  That’s OK, though, because now the turtle gets the benefit of “inertial movement”.  That is, it can build up momentum from repeated strokes, unlike on the sand, where as soon as you stop pushing, you stop moving.  Movement through this sort of medium requires a totally different motion, so the turtle switches again, this time to the familiar symmetrical flapping that it will use for the rest of its life, creating lift and thrust with every stroke of the paired front flippers.

These biomechanical adaptations to different substrates may have a role to play in why turtles nest on some beaches and not others.  That’s because not every sand behaves so predictably.  Sands where all the grains are of similar size behave differently from those where the grains vary; and well sorted sands behave differently from poorly sorted sands.  You know this is you’ve ever walked on a “squeaky” beach - those sounds come from the friction of sand grains all being the same size (try squeezing a bag of marbles and you’ll see what I mean).  Taken together, these adaptations show a remarkable flexibility of locomotion for an animal just in its first hours of life.  It must be working well for them, though, because the sea turtle lineage has been doing just fine on this planet for over 200 million years.  To borrow from Blake once more:

Every night and every morn
Some to misery are born,
Every morn and every night
Some are born to sweet delight.

Mazouchova, N., Gravish, N., Savu, A., & Goldman, D. (2010). Utilization of granular solidification during terrestrial locomotion of hatchling sea turtles Biology Letters, 6 (3), 398-401 DOI: 10.1098/rsbl.2009.1041


The solution to Bit-o-Critter round 20

Juliebug identified the round 20 Bit-o-critter as the green turtle Chelonia mydas.  With that, she takes a commanding lead in the BoC stakes.  Will no-one challenge her supremacy?  Ya gotta be in it to win it folks...


Field locations you have loved

In this thread I want to hear about field locations YOU have loved, and WHY.  Here's a couple of mine to get the ball rolling:

Kedron Brook, Brisbane, Australia.  A choked little stretch of suburban creek on the north east side of Brisbane Australia was a key field location for my PhD research, which was all about introduced (exotic) species and their parasites in rivers and streams in Australia.  At one point just above the tidal influence - stylishly named KB216 for its map reference - this creek is basically completely exotic: plants, invertebrates, fish, the whole shebang.  There aren't many parasites there, but those that were present were introduced hitchhikers.  Not sexy, but a veritable Shangri-La for a student on the hunt for ferals...
Heron Island, Queensland, Australia.  Where I met and fell in love with marine biology.  A patch of sand and guano-reeking Pisonia forest 800m long, on a reef 10 times that size, crawling with noddies, shearwaters, turtles, grad students and squinting daytrippers or more wealthy sunburned resort guests.  Too many firsts for me there to even list (but no, not that one - get your mind out of the gutter!).  Absolute heaven, hands-down.  How do I get back?

Throgs Neck, NY, USA.  You generally wouldn't think of the junction of Queens and the Bronx as a biologically interesting in any way (except maybe on the subway), but actually the western part of Long Island Sound was the epicenter of a lobster holocaust that started in (well, before, if you ask me) 1999.  When we were out on the RV Seawolf, the Throgs Neck bridge marked your entry into the East River and the start of one of the most unique and strangely beautiful urban research cruises around, right down the East side of Manhattan, past the Statue of Liberty and out into the Lower NY bays.  We would pass through on our way to do winter flounder spawning surveys off the beach at Coney Island (its that or go around Montauk).  Proof that not all interesting biology takes place in Peruvian rainforests...

In the comments, tell us about a field location YOU have loved and why.  Post links if you can find them.


Not the Dry Tortugas, as such...
...more like the Floating Tortugas.  An article in Science Now and The Journal of Experimental Biology reports on mysterious gatherings of loggerhead turtles in the Mediterranean, resembling an island made out of turtles.  It seems the critters gather to soak up the midday sun.  We probably should be surprised given how common basking is among reptiles, but loggerheads are known to spend most of their time on the bottom, so its a bit unusual.  What I find even more cool about this story is that its only now just being described, even though its in the middle of one of the busiest bits of sea in the world; how did no-one ever report on this before?  I love that there's still new stuff to discover everyday, and 90% of it is in the ocean, even the most familiar stretches.  A virtual island raft composed entirely of turtles - can you just imagine?

Knight, K. (2010). LOGGERHEADS STAY AT SURFACE TO SOAK UP SUN Journal of Experimental Biology, 213 (8) DOI: 10.1242/jeb.043901