Variability and asymmetry in the shape of the spiny dogfish vagina revealed by 2D and 3D geometric morphometrics

• Published in: Journal of zoology (1987) Vol. 308; no. 1; pp. 16 - 27
• Main Authors: Hedrick, B. P., Antalek‐Schrag, P., Conith, A. J., Natanson, L. J., Brennan, P. L. R.
• Format: Journal Article
• Language: English
• Published: London Blackwell Publishing Ltd 01.05.2019
• Subjects: Allometry; Antagonism; Asymmetry; Biology; Body size; Copulation; Dogfish; Females; fluctuating asymmetry; genital evolution; genital morphology; Genitalia; geometric morphometrics; Gonads; Marine fishes; Mathematical morphology; Methods; Morphometry; Pregnancy; Sexual selection; Shape; Sharks; Skewed distributions; Squalus acanthias; three‐dimensional geometric morphometrics; Vagina
• ISSN: 0952-8369; 1469-7998
• DOI: 10.1111/jzo.12653

Genital structures are among the most variable in nature and have been suggested to evolve at exceptionally high rates. However, the vast majority of research on genital morphology has been done on male genitalia. We present one of the few studies of female genital shape using geometric morphometrics, and the first of such studies to employ 3D geometric morphometrics, using the spiny dogfish shark, a taxon for which reproductive biology is well‐studied. In a sample of 21 adult females, we found no correlation between body size and reproductive and non‐reproductive trait size, and therefore no general allometric patterns. Furthermore, we found limited evidence for different 2D and 3D vaginal shapes in visibly pregnant and not visibly pregnant sharks, but trends were more obvious in 3D than 2D. We found high congruence between data derived using 2D geometric morphometrics with that derived from 3D methods. We also found exceptionally high asymmetry in the vagina, again more apparent in 3D than in 2D. Visibly pregnant sharks had especially high directional asymmetry (>48% of total variation) likely as a result of an asymmetric distribution of pups in the shark's paired oviducts. Therefore, this asymmetry was functional rather than developmental and presents an important consideration when studying vaginal shape. The lack of significant association between pregnancy and vaginal shape in a species with an extremely long pregnancy suggests that vaginal shape differences may be under additional selective forces such as sexual antagonism during copulation. A combination of 3D geometric morphometric methods along with assessments of asymmetry sheds further light on the growing appreciation that female genitalia is highly variable in shape and may play a substantial role in sexual selection. Female genital shape is highly variable, but this variability has only been recently appreciated. We analyzed the female genital shape of the spiny dogfish shark using a combination of traditional linear measurements and 2D and 3D geometric morphometrics to understand whether pregnancy affects vagina shape. We found limited evidence based on our sample that pregnancy affects vagina shape with 3D geometric morphometrics separating sharks somewhat better than 2D. We did find significant asymmetry in the vagina such that visibly pregnant sharks had 48% of their total shape variation summarized by directional asymmetry alone. This is likely due to the asymmetric distribution of pups in the shark's paired oviducts. Such high levels of asymmetry are an important factor to consider when analyzing vaginal shape. Due to the lack of a significant relationship between pregnancy and vaginal shape, we suggest the spiny dogfish vagina is under selective forces in addition to pregnancy.