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New research sheds light on why some birds have lost their penises over the course of evolution.
Land fowl, such as chickens, have normally developing penises as early embryos, but only have rudimentary organs as adults.
A study in Current Biology shows that these birds initiate a genetic "programme" during development that stops the budding penises from growing.
The loss of the organ could have given hens more control over reproduction.
Co-author Dr Martin Cohn, from the University of Florida in Gainesville, said: "Our discovery shows that reduction of the penis during bird evolution occurred by activation of a normal mechanism of programmed cell death in a new location, the tip of the emerging penis."
The team discovered that a gene called Bmp4 plays a critical role in this process. In chicken development, Bmp4 switches on and the birds' developing male genitals shrink away.
In ducks and emus, which retain their penises, the gene stays switched off and the organs continue to grow.
Given the absence of a penis, chickens and other land fowl have had to develop a method of fertilisation that does not rely on penetration.
Both male and female birds have an orifice known as a cloaca. When the cloacae are touched together, sperm is transferred to the female reproductive tract. This is known as a "cloacal kiss".
However, the reasons why the penis was lost remain less clear. Lead author Ana Herrera, also from the University of Florida, speculated that the loss could leave hens with greater control over their reproductive lives.
The study could also provide clues to other evolutionary conundrums, such as how snakes lost their limbs. And it might yield some potential answers to medical questions, say the researchers.
Dr Cohn commented: "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."
-BBC
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