In animals that reproduce by internal fertilisation, as
humans do, you'd think a penis would be an organ you couldn't really do
without, evolutionarily speaking. Surprisingly, though, most birds do exactly
that, and now researchers reporting in the Cell Press journal Current Biology have figured out where,
developmentally speaking, birds' penises have gone.
It turns out that land fowl, which have only rudimentary
penises as adults, have normally developing penises as early embryos. Later in
development, however, the birds turn on a genetic programme that leads their
budding penises to stop growing and then wither away.
"Regulation of the balance between cell proliferation
and cell death is essential for controlled growth and development," said
Martin Cohn of the Howard Hughes Medical Institute and University of Florida,
Gainesville. "Too much cell division or too little cell death can lead to
overgrowth or mis-regulated growth, as in cancer. If the balance is tipped in
the other direction, deficient cell division or excess cell death can lead to
underdevelopment or even absence of a tissue or organ.
What the research
"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."
A critical gene in
the process is one called Bmp4. In chicken development, Bmp4 switches on and
the birds' developing genitals shrink away. In more well-endowed ducks and
emus, that gene stays off and their penises continue to grow.
It's not entirely clear why chickens and other birds would
have lost their penises, said graduate student Ana Herrera, lead author of the
study, but it may be that the loss leaves hens with greater control over their
The findings could have implications for understanding
evolutionary loss more broadly. (Think of snakes and their lost limbs, as
another example.) They might yield some answers to medical questions as well.
"Genitalia are one of the fastest-evolving organs in
animals, from mollusks to mammals," Cohn said. "It is also the case
that 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