I have read that turtles can breathe through their bums. Is this true, and if so, why did they evolve such a talent and what are the mechanics of this trick?
My understanding of physiology is that animals (including humans) draw in air by expanding the volume of the chest cavity. How does a turtle, with a fixed chest cavity determined by its shell, draw in air?
--E. Nolan Cooper
When turtles put their heads in their shells, what happens to their spines? Do they buckle or contract?
How turtles breathe, how they pull in their heads--these are worthy topics. But when you get to whether turtles can breathe through their butts, that's when you know you're on the cutting edge of science. We turned to George Angehr, Smithsonian ornithologist and Straight Dope curator of critters. His reply:
"With an ancestry going back more than 200 million years to the late Triassic, the 200 or so species of turtles are the most ancient surviving lineage of land vertebrates. They are also one of the most distinctive life forms on the planet. My herpetology professor started his 'Identification Key to the Reptiles' with the couplet: 'A. Turtles. Any damn fool knows a turtle. B. Other reptiles.'
"The most notable turtle characteristic is the rigid shell, which is composed of the flattened and fused ribs and vertebrae, plus bony elements that don't exist in other vertebrates. Also unlike other vertebrates, the shoulder and hip girdles of turtles are located within the rib cage instead of outside it. Many turtles partly compensate for the rigidity of the body by having exceptionally flexible necks. The two main groups of turtles are distinguished by the way they pull the neck back into the body. Most species belong to the cryptodire ('hidden-neck') branch, which can fold the neck in an S-bend in the vertical plane to fully retract the head. The pleurodires ('side-neck'), two families restricted to South America, Africa, and Australia, can only bend the neck back against the body in the horizontal plane, leaving it partly exposed.
"The rigid rib cage also places restrictions on breathing. Turtles have two special sets of respiratory muscles. One set pulls the body contents outward, toward the openings at the front and rear of the shell. This expands the body cavity and draws air into the lungs, which are located in the top part of the shell. The other pushes the viscera up against the lungs to expel the air. This system has the drawback that both inhalation and exhalation require energy--in most vertebrates, elastic energy can be recovered from the rib cage so that exhalation requires little exertion.
"Turtles have extraordinary anaerobic capacity--they have survived up to 33 hours in a pure nitrogen atmosphere. (Most reptiles have a high anaerobic capacity compared to mammals, but even they can't survive much more than 30 minutes without oxygen.) Although basically air-breathing, many aquatic species have developed ways to pick up oxygen even when submerged. Of these the most remarkable, which some turtles share with dragonfly nymphs, sea cucumbers, and certain televangelists, is the ability to breathe through one's butt. You've heard the expression 'Blow it out your after regions'? It's no mere figure of speech. Many species have a pair of sacs (bursae) opening off the cloaca (combined digestive and urogenital chamber). These are heavily vascularized to facilitate the uptake of oxygen.
"The champion in this regard seems to be the recently discovered (1973) Fitzroy River turtle Rheodytes leukops ('white-eyed stream-diver'), which is confined to its namesake river in Queensland, Australia. It lives in shallow rapids where the water is highly oxygenated. One can detect the keen sense of discovery in the account by the scientists who found it, John Legler and John Cann: 'One of our vivid early impressions of Rheodytes was that adults of both sexes swam with a widely gaping cloacal orifice (up to 30mm in diameter). The orifice remains open when individuals are out of the water. We first became aware of the large cloacal bursae when a female was examined in bright sunlight; the carapace transmitted enough light to illuminate the coelomic cavity and produce a spectacular view internally for at least 100mm, via the cloaca, revealing a large sac lined with vascular, villose mucosa....Water is pumped in and out of the bursae of captives and experimental animals at rates of 15 to 60 times per minute' (Legler and Cann 1980). Only dedicated herpetologists could characterize the vista up a turtle's gaping bunghole as a 'spectacular view.' But you can understand their enthusiasm--since the turtle's shell is only 260 millimeters long, a 100-millimeter-long bursa is relatively enormous. Up to 68 percent of the turtle's oxygen uptake is accomplished through the cloacal bursae, so it rarely needs to come to the surface to bask or breathe."
George has equally engrossing stories about the butt-breathing abilities of sea cucumbers and dragonfly nymphs, but sorry, no room. For those you'll just have to visit www.straightdope.com.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.