ABSTRACT:
Comparisons of 3D shapes have recently been applied to diverse anatomical structures
using landmarking techniques. However, discerning evolutionary patterns can
be challenging for structures lacking homologous landmarks. We used alpha shape
analyses to quantify vaginal shape complexity in 40 marine mammal specimens including cetaceans, pinnipeds, and sirenians. We explored phylogenetic signal and the
potential roles of natural and sexual selection on vaginal shape evolution. Complexity
scores were consistent with qualitative observations. Cetaceans had a broad range
of alpha complexities, while pinnipeds were comparatively simple and sirenians were
complex. Intraspecific variation was found. Three-dimensional surface heat maps
revealed that shape complexity was driven by invaginations and protrusions of the
vaginal wall. Phylogenetic signal was weak and metrics of natural selection (relative
neonate size) and sexual selection (relative testes size, sexual size dimorphism, and
penis morphology) did not explain vaginal complexity patterns. Additional metrics,
such as penile shape complexity, may yield interesting insights into marine mammal
genital coevolution. We advocate for the use of alpha shapes to discern patterns of
evolution that would otherwise not be possible in 3D anatomical structures lacking
homologous landmarks.

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