Explosive erections, vaginal dead ends, seasonal penises, and vaginas in VR? It must be time for Rachel's favorite Naughty Nature episode so far: waterfowl and their co-evolving genital cold war. Hold onto your butts, because it's time for a deep dive.
Antagonistic Sexual Co-Evolution
Brennan PL, Prum RO, McCracken KG, Sorenson MD, Wilson RE, Birkhead TR (2007)
Here's the BIG paper you should read, where the figures in the rest of this post are taken from. Dr. Patty Brennan is one of the big minds behind duck genitalia research, AND the beautiful duck VR app. It's an oldie but a goodie, published in 2007.
From the introduction:
“In many taxa there is evidence that females respond to manipulating male strategies with behavioural counter-strategies to retain control over fertilization [reviewed in 12]. In several invertebrates, the female response to male reproductive strategies involves changes in genital anatomy , –, although in general female genitalia are less variable than male genitalia . The avian vagina has invariably been described as a short, narrow muscular duct, folded upon itself and covered with connective tissue  and no variation in this basic design has been reported. These data represent the most elaborate known case of genital coevolution in vertebrate animals.”
Explosive, Withering Penises
Brennan, P.L.R., Clark, C. and Prum, R. 2010. Explosive Eversion and Functional Morphology of Waterfowl Penis Supports Sexual Conflict in Genitalia. Proc. Royal Society B. 277(1686):1309-14.
High speed video showing the explosive eversion of a duck penis. Slowed 10x, real-time eversion speed is a third of a second.
Figure 2. Examples of genital covariation in waterfowl. [Brennan, 2007; citation above] (A) Harlequin duck (Histrionicus histrionicus) and (B) African goose (Anser cygnoides), two species with a short phallus and no forced copulations, in which females have simple vaginas as in Fig 1a. (C) Long-tailed duck (Clangula hyemalis), and (D) Mallard Anas platyrhynchos two species with a long phallus and high levels of forced copulations, in which females have very elaborate vaginas (size bars = 2 cm). ] = Phallus, * = Testis, ★ = Muscular base of the male phallus, ▹ = upper and lower limits of the vagina.
Forced copulation in waterfowl: observations about male behaviors, examples of interspecies forced extra-pair copulations.
McKinney, Frank, et al. “Forced Copulation in Waterfowl.” Behaviour, vol. 86, no. 3/4, 1983, pp. 250–294. JSTOR, www.jstor.org/stable/4534287. Accessed 21 Mar. 2020.
Female consequences of forced extra-pair copulations.
Brennan, P.L.R., and Prum, R. 2012.Sexual conflict in the narrow sense: New insights from waterfowl biology. Philosophical transactions of the Royal Society. 367: 2324-2338.
Figure 1. Avian vaginal morphology. [Brennan, 2007; citation above] (A) Typical tubular avian vagina from domestic Pheasant (Phasianus colchicus) (connective tissue removed). Note the lack of any elaborations. (B) Vagina (V) of Pekin duck (domestic Anas plathyrhynchos) (connective tissue removed). Note the complexity of the structure. (C) Longitudinal dissection of Pekin Duck vagina showing structural complexity. Pockets (*) are closer to the cloaca (Cl) and their lumen in shown between the traces lines. Spirals (white arrows) are closer to the uterus (or shell gland) (U). S.S. = Area of sperm storage tubules. (Scale bar in all pictures = 2 cm).
How do you woo a female duck?
Dance and look pretty, of course. Here's a video by Cornell Lab of Ornithology illustrating some of their courtship behaviors. They have an entire article about it.
And last but not least, the interview with Schilthuizen, author of "Nature's Nether Regions." How Corkscrew Vaginas Evolved.