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Male birds have the largest genital diversity of any class of animals because their sex chromosomes make it easy to pass malehelping mutations down the line.

Thanks also to our supporters on   / minuteearth  
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To learn more, start your googling with these keywords:
Sex chromosome: A chromosome involved in determining the sex of an individual.
Cloaca: A posterior orifice that serves as the only exit for the gastrointestinal, urinary and genital tracts.
Mutation: An alteration in the DNA sequence that makes up a gene.
Sexuallyselected Trait: A trait based on a mutation that confers a reproductive advantage.
BMP4: A protein whose presence halts the growth of bird penises in the egg.
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If you liked this week’s video, you might also like:
An article from The Week about some of the strangest penises in the animal kingdom: http://theweek.com/articles/474417/12...
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Credits (and Twitter handles):
Script Writer: David Goldenberg (@dgoldenberg)
Script Editor: Kate Yoshida (@KateYoshida)
Video Illustrator: Ever Salazar (@eversalazar)
Video Director: Emily Elert (@eelert)
Video Narrator: Emily Elert (@eelert)
With Contributions From: Henry Reich, Alex Reich, Peter Reich
Music by: Nathaniel Schroeder:   / drschroeder  


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References:

Brennan, P. and Prum, R. (2015). Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection,Mate Choice, and Sexual Conflict. 121. Cold Spring Harbor Perspectives in Biology. Retrieved from: http://cshperspectives.cshlp.org/cont...

Brennan, P. (2013). Genital Evolution: CockaDoodleDon’t. Current Biology. R523R525. Retrieved from: http://www.sciencedirect.com/science/...

Brennan, P., Birkhead, T., Zyskowski, K., van der Waagand, J., and Prum, R. (2008). Independent evolutionary reductions of the phallus in basal birds. Journal of Avian Biology, 39: 487492. Retrieved from: http://onlinelibrary.wiley.com/doi/10....

Briskie, J. and Montgomerie, R. (1997). Sexual selection and the intromittent organ of birds. Journal of Avian Biology. 28: 7386. Retrieved from: https://www.jstor.org/stable/3677097?...

Herrera, A., Brennan, P., and Cohn, M. (2014). Development of Avian External Genitalia: Interspecific Differences and Sexual Differentiation of the Male and Female Phallus. Sexual Development. 9: 4352. Retrieved from: https://www.ncbi.nlm.nih.gov/pubmed/2....

Reinhold, K. (1998). Sex linkage among genes controlling sexually selected traits. Sexual Selection. 44:17. Retrieved from: https://link.springer.com/article/10.....

Image Credits:

Mandarin Ducks Francis C. Franklin
https://commons.wikimedia.org/wiki/Fi...

Gorillas Based on photo by Wikimedia user No escape
https://commons.wikimedia.org/wiki/Fi...

Lake Duck Penis Kevin McCracken (Published in Nature 2001)