Baculum shape complexity correlates to metrics of post-copulatory sexual selection in Musteloidea

Abstract

The penis bone, or baculum, is present in many orders of mammals, although its function is still relatively unknown, mainly due to the challenges with studying the baculum in vivo. Suggested functions include increasing vaginal friction, prolonging intromission and inducing ovulation. Since it is difficult to study baculum function directly, functional morphology can give important insights. Shape complexity techniques, in particular, are likely to offer a useful metric of baculum morphology, especially since finding homologous landmarks on such a structure is challenging. This study focuses on measuring baculum shape complexity in the Musteloidea-a large superfamily spanning a range of body sizes with well-developed, qualitatively diverse bacula. We compared two shape complexity metrics-alpha shapes and ariaDNE and conducted analyses over a range of six different coefficients, or bandwidths, in 32 species of Musteloidea. Overall, we found that shape complexity, especially at the baculum distal tip, is associated with intromission duration using both metrics. These complexities can include hooks, bifurcations and other additional projections. In addition, alpha shapes complexity was also associated with relative testes mass. These results suggest that post-copulatory mechanisms of sexual selection are probably driving the evolution of more complex-shaped bacula tips in Musteloidea and are likely to be especially involved in increasing intromission duration during copulation.

Keywords: Musteloidea; baculum; evolution; genitalia; sexual selection; shape complexity.

Figure 1

Scatter plots of PCA results for both complexity metrics (a) Alpha shapes PCA results (PC1 and PC2) for musteloid species identified by subfamily. Representative species displayed as 3D mesh and labelled (bones not to scale). (b) AriaDNE PCA results (PC1 and PC2) for musteloid species identified by subfamily. Representative species displayed as 3D mesh (bones not to scale). PCA, principal component analysis; 3D, three‐dimensional.

Figure 2

Scatter plots depicting the relationship between shape complexity metric and sexual selection mechanism. (a) Alpha shapes PC1 and intromission duration. (b) AriaDNE PC1 and intromission duration. (c) residual alpha shapes PC1 and residual body mass. (d) residual alpha shapes PC1 and residual testes mass. Solid line indicates linear model best fit, dotted line indicates PGLS best fit. PGLS not plotted in a, c and d as λ values were identical.

Figure 3

Box plots depicting alpha shapes and ariaDNE PCA results, split between morphological character scores. Results indicate that both alpha shapes PC1 and ariaDNE PC1 are positively correlated with post‐copulatory selection mechanisms. Further statistical analyses on these data found significant associations between alpha shapes PC1 and ariaDNE PC1 and four baculum characters. Alpha shapes PC1 was associated with (a) abrupt bend in distal tip (representative species in order shown—Meles meles, Mustela putorius, Mustela nudipes), (b) presence of distal hook (representative species in order shown—Lyncodon patagonicus, Mustela kathiah) and (c) and shape of median portion at cross section (representative species in order shown—Mellivora capensis, Mustela lutreola, Potos flavus). AriaDNE PC1 was associated with (d) Complexity at the distal tip (representative species in order shown—Enhydra lutris, Martes flavigula). Lines indicate significant results for Kruskall–Wallis test. Significant statistical results found between scores using a pairwise—Wilcoxon and results are indicated via asterisk. Renderings below each boxplot exemplify typical bacula for the result. Bacula mesh models are depicted in the lateral plane. (c) Depicts the median slices taken from the .raw data. PCA, principal component analysis.