UC Berkeley

The genitalia of males evolves rapidly and divergently in most animal groups. Explanations for this pattern typically evoke mechanisms of post-copulatory sexual selection, such as sperm competition, cryptic female choice, and sexual conflict. While the fossil record often allows for the documentation of patterns of evolutionary change, the predominantly soft-tissue nature of the genitalia is rarely observed in the fossil record. However, the baculum (os penis or os priapi), a bone located at the distal end of the penis of most mammalian species, offers the opportunity of examining the evolution of male genitalia, and thus may contribute to our understanding of post-copulatory sexual selection and the evolution of reproductive biology within mammals. Bacula are rare in the fossil record and thus difficult to be identified and classified when in isolation. Nonetheless, sometimes they are preserved in association with other skeletal material allowing accurate species-level taxonomic assignments. Here, I focus on the bacula of the Canidae, which consists of three subfamilies, the extant Caninae (wolves, coyotes, jackals, foxes, etc.) and two extinct subfamilies, the Borophaginae and the Hesperocyoninae, both of which have rich fossil records that reveal high taxonomic and ecomorphological diversity.

This dissertation consists of four chapters. In the first chapter, I briefly contextualize the theory of post-copulatory sexual selection and then I review the evidence that it acts on the baculum from a combination of studies that used functional, experimental, and comparative data.

In Chapter 2 I describe the morphology of fossil bacula of five species of Borophaginae. These descriptions add substantial knowledge regarding the baculum of extinct canids, which was previously known for only two extinct species. The five bacula were found associated with other skeletal elements that were already described in the literature and have enabled identification to the species level. These borophagine bacula share many similarities with extant canines such as being robust, having a urethral groove, and a simple distal end. Because these features have been associated with reproductive strategies in extant carnivorans it is possible to use the baculum of Borophaginae to draw implications for their reproductive biology.

In Chapter 3 I quantify variation in morphology of the baculum of canids in 2D using elliptic Fourier analysis. First, I show that the baculum of 26 (out of ~35) extant species is morphologically conserved and that the intraspecific morphological variation is larger than the interspecific differences, and therefore it is difficult to distinguish among species using bacular morphology alone. However, four extinct canids that have completely preserved bacula show morphological differences from the extant canids. In particular, one morphological feature, the primary curvature of the baculum, differentiates the baculum of three stem group canids from those of extant species. I used phylogenetic ancestral state reconstruction to show that it would be unlikely to predict the arched curvature of the baculum observed in the stem fossils from the morphology observed in extant species.

Finally, in Chapter 4, I examine the relative size of the canid baculum. Using a linear regression corrected for phylogeny, I show a slightly negative allometric relationship between baculum length and body size in canids.Further, an isometric relationship cannot be rejected. These results are in contrast with the strong negative allometric relationships that have been described for other mammalian clades. Of particular interest are the relative size of fossil bacula. The baculum of Hesperocyon gregarius, the only known in the Hesperocyoninae, is significantly longer than expected for its body size and suggests a different allometric curve for this species in comparison with other canid groups. The bacula of two borophagine species are within the same allometric line of extant canids. Finally, the baculum of the extinct dire wolf (Aenocyon dirus) is longer than expected based on its body size, suggesting that its reproductive strategies could be different from those of extant canines.