Western painted turtles at Wood Lake, Winfield

I used to wonder how freshwater turtles survive winters in a frozen world.  After all, like all reptiles, their skin is impermeable to water and gas exchange, unlike amphibians.  A frog can breathe through its skin, a turtle cannot.  And they don't overwinter out of the water.  They would need a den deep enough to prevent freezing.  Many mammals do this, but not turtles.  And they certainly don't migrate south.  So, how do they do it?

The answer is stranger than you may think.  Cloacal respiration, otherwise known as butt breathing.

The western-painted turtle (photographed two days ago at Wood Lake, BC) hibernates in the winter by burying itself in benthic sediments (aka mud) at the lake's bottom.  With its rectum sticking out, water passes into and out of a portion of the lower digestive tract.  There, the important business of gas exchange takes place; oxygen in and carbon dioxide out.  There may very well be an exchange of metabolic waste products as well.

There is another interesting fact related to metabolic waste products.  Reptiles are ectotherms, or cold-blooded organisms, and metabolism is related to environmental temperature.  The water temperature at the bottom of an ice-covered lake is 4° C.  The turtle's metabolism is incredibly low and there is little need for much in the way of gas exchange.  If there is little oxygen about, a turtle's system may switch to anaerobic respiration, which is what happens to our muscles when blood and oxygen supplies run out (this produces lactic acid which produces the painful "stitch" in your side if you exercise too much).  

The acidic byproducts are a problem for the turtle's system, but it can pull calcium carbonate out of the shell to act as a buffer reducing the pH imbalance that results.  Together, with the fat reserves built up from summer foraging, the creature is able to withstand its winter predicament.

Not all turtles can do this, and I wonder if the ones that do can stay underwater during the summer months longer by tying into this system.  I have not been able to find any information on this.  If you happen to know, please pass the knowledge on.

As always, thanks for reading.

Eric Svendsen     www.ericspix.com