A major evolutionary transition explained

Photo from the BBC website

It's difficult to compete with the Higgs Boson story, but here's a strong second, a hot breaking story on the BBC web site about erections in big birds (note, the picture shows the birds, so you'll know what we're talking about, but the erect structures in the picture are necks).  The story starts out:

Ostriches have bloodless erections, according to researchers.

The large birds were previously thought to have blood-based erection mechanisms similar to humans.

But scientists from Yale University, US, have now confirmed that the birds actually enlarge their penises with lymph fluid.  

All other birds with a penis achieve erections in this way, leading scientists to believe the mechanism evolved in their ancient ancestors.

The paper itself is published in the Journal of Zoology.  The authors explain in the abstract why it's so important:

Because the penis in all other described birds has a lymphatic erection mechanism, clarifying that the erection mechanism of ratites [large flightless birds] is of great importance to understanding one of the major evolutionary transitions of penis morphology within amniotes. Here, we show that the erection mechanism of ratites is lymphatic, confirming that the evolutionary transition to lymphatic erection occurred in the last common ancestor of Aves.

You'd always wondered about the major evolutionary transitions of penis morphology, hadn't you?
(Actually, though, this may not have been as pressing a question as it seemed at the time, given another recent breaking story: men don't think about sex as often as their reputation has lead us all to believe, proven by a study that asked men how often they thought about sex. 

"The absolute number of sexual thoughts is dramatically less than the urban legend that men think about sex every seven seconds," says study researcher Terri Fisher, PhD, professor of psychology at The Ohio State University at Mansfield.

Men were asked to push a button on a counter every time they had a sexual thought.  But, we wonder, isn't it possible that the count was artificially reduced by the subjects' thumbs tiring of pushing the freaking button?) 

But, back to the story at hand, ratite penises.  There was a suggestion long ago of lymphatic involvement in the erections of flightless birds, but it remained unconfirmed until now.  After all, who dared to look?  These guys bite hard!  (Hard to do it, but we're resisting the temptation to make a pecker joke here.)  Instead, the authors were sent ostrich and emu penises from birds of reproductive age (that is, certified to be adults, if you're worried that we're writing this from Penn State), and they dissected them -- for methodological and anatomical details, see the paper, but suffice it to say that their work confirms the old suspicion. 

It is rather surprising that this wasn't known until now, and the authors of the paper briefly describe why this is so, including something tantalizing about a reference gone missing.  Indeed, there are a lot of citations in the paper of "unpublished data" on bird penile morphology.  So, it's even more surprising that all it took to answer this question was a bit of dissection of the relevant organs.  Now done. 

The story on the BBC reports that there are still unanswered evolutionary questions, however (there must be, or the specialists would be out of a job).

Similarities have been drawn in the past between bird and reptile penises but the latter use blood for erections, as do mammals.

"The reason why the change between blood vascular and lymphatic took place remains a mystery," said Dr Brennan.

"The lymphatic system is a low pressure system, so this means that erection cannot be maintained, and this has some important implications for how birds actually copulate," said Dr Brennan.

Some species of bird, such as ducks, are known for their "explosive" erections achieved when lymph fluid is forced into the penis to increase pressure for a short time.

But ornithological reproduction expert Prof Tim Birkhead from the University of Sheffield suggests that the structure of ostrich penises could make up for the shortcomings of the lymph system.

"Ostriches and rheas appear to have additional muscles that help to maintain a rigid phallus," he explained.

Ok, so is this actually of interest, other than as a footnote to the sexual life of birds?  Because, after all, however erections happen in these birds, the mechanism has worked for a very long time (except for the dodo and the roc).  The authors say this is an important contribution to the understanding of the evolution of penis morphology.  We have to take their word for it, though this does seem to be a rather specialized interest -- yet another science paper detailing perfectly good work, but that didn't warrant all this fuss.

Is there even a general lesson here?  Well, it's yet another of the myriad examples of variation underlying a single trait or mechanism -- if it does the job, evolution can support it.  Nothing new here.  Indeed, the genetics underlying many examples of what's called 'phenogenetic drift' -- many genetic pathways to a given trait -- are well-documented (Kazu Kawasaki in our lab has done a lot of work on the evolution of genes for mineralization in multiple lineages, for example, a beautiful example of phenogenetic drift; same trait, even under strong selection, yet produced by different genes in different individuals, populations, or times).  So the fact that there are different erectile mechanisms isn't at all a surprise, just another variant in the wide spectrum of how organisms reproduce.

What makes this story manage to stand tall in the news makes it harder, to fathom.