Figure 1.
(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).
Figure 2.
Examples of genital covariation in waterfowl.
(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.
Figure 3.
Relationships between male phallus length and female vagina.
(A) Phallus length vs. number of vaginal pouches. (B) Phallus length vs. number of vaginal spirals. Points are the averages for each species studied.
Figure 4.
Hypothesis of the phylogenetic pattern of evolution in phallus length, based on the phylogeny proposed in Fig. S1, using the minimized squared change parsimony algorithm in MacClade, and three size classes (1–6 cm, 6–12 cm, and 12–18 cm).
Phallus length >12 cm has evolved three times independently within these waterfowl- in Oxyura, Clangula, and Anas. All three of these lineages show correlated evolution of anatomical counter measures in the female reproductive tract.