Popular culture would have it that turtles are weak, flaccid, crappy organisms with dull social lives, stunted and barely functional internal organs and - it goes without saying - undersized sex organs. Right? WRONG...

Warning: the following blog post may be considered unsuitable for viewing by minors.

Believe it or don't, turtles are horrifically well-endowed, and if the thought of learning more about the genitals of these oh-so-surprising reptiles doesn't appeal to you, look away now. Last warning. Ok, here we go. To begin with, I have to confess that I actually know very little about the subject I'm writing on, and this is despite a reasonable amount of literature-based research. I have also never dissected a turtle, nor manipulated a live turtle's genitals, so if you know more about the subject than I do, and/or have any amusing anecdotes or personal adventures you'd like to relate, please do feel free to chip in.

Hydraulic intromittent male sexual organs - or dicks - are not unique to mammals among tetrapods, but are also present in squamates, archosaurs and turtles, and this phylogenetic distribution has led some authors to conclude that these organs were present in amniote common ancestors. However, in their details, the organs of these groups are all quite different and actually formed from non-homologous tissues. As shown by Kelly (2002) [and covered by Pharyngula last year], male intromittent organs therefore arose independently among tetrapods on more than one occasion. The turtle organ, for example, contains only one vascular erectile body and develops on the ventral surface of the cloaca, whereas the mammal organ contains two erectile bodies and is derived from non-cloacal tissue [in the diagram above - borrowed from Kelly (2002) - intromittent organs of turtles, birds, mammals and snakes are compared in transverse cross-section. VS = vascular space; TM = tensile membrane. Note how different the organs are in their cross-sectional structure].

Before going any further, what is it with my use of the term 'organ': why aren't I using the familiar term 'penis'? The reason is that this might not be the right word to use, though this does depend on who you ask. According to one school of thought, 'penis' should be restricted to the organ present in mammals, and the non-homologous but convergently similar organs of turtles and archosaurs should be termed a phallus instead (T. Isles, pers. comm.). Not everyone goes along with this: some biologists who have published on intromittent organs consistently term all of these organs penises (e.g., Kelly 2002, 2004, McCracken 2000). For what it's worth, I personally prefer to restrict 'penis' to mammals.

Like the mammalian penis, the turtle phallus is a hydraulic cylinder that becomes engorged by fluid and is relatively resistant to bending when erect. The single erectile body of the turtle phallus is divided into a collagenous corpus fibrosum and a highly vascularized, expandable corpus spongiosum. As a turtle's phallus inflates, its length may increase by nearly 50%, its width by 75%, and its depth by 10%. A 50% increase in length doesn't sound too impressive, so I assume that even an uninflated phallus - tucked away inside the cloaca - is large. However, the corpus fibrosum increases in length somewhat as well, and hence may contribute to the total length of the erect organ. More on the issue of size in a moment.

A pair of long retractor muscles extend for most of the length of the phallus' dorsal surface, and attach within the body cavity to the lumbar vertebrae. When at rest, the phallus is actually doubled up on itself within the cloaca, and it is the contraction of the retractor muscles that causes the phallus to un-double and protrude (Gadow 1887). Bishop & Kendall (1929) found that turtle phallus retractor muscles were 'physiologically rugged' and of 'extreme endurance'.

Collagen fibres reinforce the phallus wall and are arranged either along, or perpendicular to, the phallus' long axis, and in this respect the turtle phallus is superficially similar to the mammalian penis. However, while the mammal penis only has one layer of long-axis fibres, and one layer of perpendicular fibres, the walls of the turtle phallus exhibit multiple layers of these fibres. This array of stiffening collagenous fibres is still, however, highly similar in turtles and mammals: a fact which led Diane A. Kelly to title her 2004 paper 'Turtle and mammal penis designs are anatomically convergent' (Kelly 2004). The strong similarity observed in the erectile organs of these phylogenetically disparate groups indicates that there are few functional solutions permitting the evolution of cylindrical, inflatable intromittent organs (Kelly 2002, 2004). Kelly is well known for her previous work, widely reported in the media, on penis anatomy in armadillos (Kelly 1997) collected as roadkills near Tallahassee, Florida. Note that the title of Kelly's 2004 paper includes that most evil of words in biology: design. It is to be avoided at all costs, for reasons that I don't need to elaborate on, I'm sure. Anyway, her publications can be obtained, free, from her homepage here.

As interesting as it is from the point of view of embryology, phylogeny and microanatomy, what is particularly eye-opening (no pun intended) about the turtle phallus is how frighteningly large and formidable it is in some species. Again, I can't pretend to have much useful experience in this area, so do help out if you know more. From the images I've seen, it seems perfectly normal for some tortoise species to have a phallus that is half the length, or more, of their plastron. I would guess that in a tortoise with a total length of 20 cm, the phallus might be 8 cm long. Look at the images accompanying this article, some of which show better-endowed species/individuals than others. The organ is always dark - grey, purple or blackish - with an expanded head and a sharp spine at its tip. To date I've only seen the organs of testudinids and emydids, and would like to know if other turtles are the same in these respects.

While it might seem like a bloody stupid question, one has to wonder exactly what it is that turtles do with these sometimes enormous organs. As in other tetrapods that sport proportionally large sexual organs (including certain ducks, cetaceans and, yes, some primates), observational data suggests that turtles might employ their members in display or aggression. Honda (2001) had this to say about captive specimens of the box turtle Terrapene carolina...

Sometimes males will distend their organ neither while mating, nor while in the presence of females. Usually while bathing or drinking, the turtle will submerge the front half of his body, rise up on his back legs, and drop his organ through the cloaca. It is a sight to behold, and one that can startle both novice and experienced herpetoculturalists alike. The organ itself is large in proportion to the turtle, and dark purple in color. After several seconds, the turtle will retract the organ back through the cloaca. It may repeat this process once or twice.

I also note the title of a very interesting paper by de Solla et al. (2001): 'Penis displays of common snapping turtles (Chelydra serpentina) in response to handling: defensive or displacement behaviour?' [adjacent image shows mating snapper pair]. Unfortunately I have yet to see this paper (a pdf doesn't seem to be available), so I don't know if they concluded whether defensive or displacement behaviour better explained the phallus displays they reported. Please let me know if you know the answer (or, even better, can send me the paper). Without information to the contrary, I cannot help but imagine that some turtles might be in the habit of intimidating enemies with their erect 20-cm long, black, spike-tipped phalluses.

Now there's a thought. You might never look at a turtle the same way again.

For previous posts of mine on turtles see the series on snappers and alligator snappers here and Tortoises that drink with their noses. More on turtles in the near future, including stuff on Gilbert White's pet tortoise, J-Lo the araripemydid and pleurodire diversity, and meiolaniids.

So, tomorrow (Monday) is the big day...

Refs - -

Bishop, G. H. & Kendall, A. I. 1929. Action of formalin and histamine on tension and potential curves of a striated muscle, the retractor penis of the turtle. American Journal of Physiology 88, 77-86.

de Solla, S. R., Portelli, M., Spiro, H. & Brooks, R. J. 2001. Penis displays of common snapping turtles (Chelydra serpentina) in response to handling: defensive or displacement behaviour? Chelonian Conservation and Biology 4, 187-189.

Gadow, H. 1887. Remarks on the cloaca and on the copulatory organs of the Amniota. Philosophical Transactions of the Royal Society of London B 178, 5-37.

Honda, M. 2001. Chelonian notes. Art Journal 60 (2), 96-100.

Kelly, D. A. 1997. Axial orthogonal fiber reinforcement in the penis of the nine banded armadillo (Dasypus novemcinctus). Journal of Morphology 233, 249-255

- . 2002. The functional morphology of penile erection: tissue designs for increasing and maintaining stiffness. Integrative and Comparative Biology 42, 216-221.

- . 2004. Turtle and mammal penis designs are anatomically convergent. Proceedings of the Royal Society of London B 271 (Suppl 5), S293-S295.

McCracken, K. G. 2000. The 20-cm spiny penis of the Argentine lake duck (Oxyura vittata). The Auk 820-825.