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Tuesday, July 12, 2011

Life in a Sandwich Board

Imagine living in a sandwich board.  Not one you could remove, like an advertisement people parade at busy intersections, but one that is made of your own bones and will be with you for your entire life.  No bending to touch your toes.  In fact, forget about touching your toes.  Forget about curling up, sitting down, even rolling over.  You can't breathe by expanding your rib cage, instead you have to pump air into your lungs using the muscles in your throat.  Now imagine that you need to swim around in order to survive.

Red-eared slider, Trachemys scripta. Photo: J. A. Gervais

This improbable body plan is actually a pretty ancient idea, because turtles have been using it, without major changes, since the Triassic.  In other words, over 200 million years.  Good ideas simply don't go out of style, and this good idea has endured ice ages, drifting continents, the disappearance of the dinosaurs, the appearance of mammals, and the arrival of the ape with the big brain and opposable thumbs.

The shell is an extraordinary piece of biological architecture.  Formed from the ribs and the spine, it incorporates the the carapace, or top shell, the plastron, or lower shell, and the bridge, the plates that hold the two together. The hips and shoulder blades are encompassed inside the rib cage.  Ultimately, nearly half a turtle's mass is bone.
Red-eared slider, Trachemys scripta.  Photo: J. A. Gervais

Life inside the shell has its advantages, in that you carry your fortress with you.  Although the shell offers some protection, determined otters and other predators have been known to pry legs or the tail free and gnaw them off.  There are no fast getaways, and if the fortress is breached, there isn't much that the turtle can do about it.

There are other drawbacks.  Reproduction is challenging, to say the least, when both participants are dancing in suits of their own unremovable armor.  The distinctly domed carapace of the female makes things more difficult, as does the fact that most female turtles are larger than the males.  The male turtles' plastrons are slightly dished, and their front claws are extra long, to help them hang on to get the deed done.  It's an awkward affair, but one that has worked well enough for a long time.

The shell has to grow with the turtle.  Each bony plate must grow in synchrony to keep the shell in its proper shape.  Turtles lay eggs, and unlike birds, they form a clutch and lay it all at once.  All those eggs have to fit in there somewhere, which is why females have taller, more domed carapaces than males.  Too big a shell wastes energy, both in its growth and in the effort needed to haul it around.  Too small, and turtles cannot gain weight to form their eggs, carry them til laying, or get fat to survive the winter- not to mention draw deep enough inside to be safe from their more nimble, agile enemies.

Turtle shells have worked so well they've not only been around for millions of years but turtles and tortoises also occupy all continents except Antarctica, in habitats ranging from the deep ocean to deserts.  They can survive in some pretty degraded environments, appearing to be much more robust than many other vertebrates to pollution, reduced water quality, and other challenges thrown at them in recent times. 

This pond supported turtles...  Photo: J. A. Gervais

However, the ape with the big brain and opposable thumbs may be too much for them, as wetlands disappear and turtles fall victim to roadways, over harvesting, the pet trade, and other human activities.  With a little care, however, at least a few may survive for another few million years, moving through time in that same cautious, patient manner that has served them so well for so long.

1 comment:

  1. Never thought too much about the turtle's shell - and will definitely do so the next time we stop to watch the small turtles in south pond as they warm themselves in the sun on fallen trees at the edge of the water.