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| How did the cockerel lose its penis? |
One of the great mysteries in bird evolution is why the males in all but about three percent of species have lost their penises during their evolution, even though fertilisation takes place internally. What makes this a little more of a puzzle is why a few species, such as ducks, geese, ostriches and rhea have retained one - and we now know that it has been retained rather than having evolved independently.
In the majority of bird species sperm is transferred to the female during a 'cloacal kiss' when the single openings for the digestive, urinary and genital tract, the cloaca, of the mating pair are pressed together. Mating for most birds lasts just a second or two and is often performed frequently during the breeding season.
One theory, which requires a lot more work to validate, is that in ducks and geese at least, sex is more forceful, almost amounting to rape, and so requires little or no cooperation on the part of the female. It has been suggested that the loss of a penis gave females more control of the selection of a mate. Birds are renowned for producing stunning examples of female sex selection such as the peacock's tail and of using mating rituals involving song and plumage. Geese usually show little sexual dimorphism - a sign of sex selection - but ducks, on the other hand, usually show considerable plumage difference between sexes.
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| The red area shows an embryonic chicken penis, before it starts to regress. |
So we now know that whatever drove the loss of the penis in most species during bird evolution, it involved the genetic mechanism which activates Bmp4 in this critical stage and in those cells in the developing embryo. What we now need to understand is why this led to greater reproductive success for most species, but not all of them.
As one of the authors, Dr Cohn, explained:
Genitalia are affected by birth defects more than almost any other organ. Dissecting the molecular basis of the naturally occurring variation generated by evolution can lead to discoveries of new mechanisms of embryonic development, some of which are totally unexpected. This allows us to not only understand how evolution works but also gain new insights into possible causes of malformations.As a passing nod to biblical Creationists and especially to their under-cover branch, the 'Intelligent Design' advocates, where is the intelligence in starting to grow a penis then creating a mechanism for killing it in the developing embryo?
Note: you have to explain why this is an intelligently designed mechanism. It won't do to simply use the standard cop-out that your god's ways are ineffable because you aren't supposed to admit that the 'intelligent designer' is your god as that gives the game away and undermines the Discover Institutes Wedge Strategy. You have to say why this is what an intelligent designer, not an ineffable one, would come up with.
In your own time.
References:
Developmental Basis of Phallus Reduction during Bird Evolution; Ana M. Herrera, Simone G. Shuster, Claire L. Perriton, Martin J. Cohn; Current Biology - 06 June 2013'via Blog this'
Study reveals how birds lost their penises. BBC News - Science & Environment.
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