The Baculum was Gained and Lost Multiple Times during Mammalian Evolution

Abstract

The rapid evolution of male genitalia is a nearly ubiquitous pattern across sexually reproducing organisms, likely driven by the evolutionary pressures of male-male competition, male-female interactions, and perhaps pleiotropic effects of selection. The penis of many mammalian species contains a baculum, a bone that displays astonishing morphological diversity. The evolution of baculum size and shape does not consistently correlate with any aspects of mating system, hindering our understanding of the evolutionary processes affecting it. One potential explanation for the lack of consistent comparative results is that the baculum is not actually a homologous structure. If the baculum of different groups evolved independently, then the assumption of homology inherent in comparative studies is violated. Here, we specifically test this hypothesis by modeling the presence/absence of bacula of 954 mammalian species across a well-established phylogeny and show that the baculum evolved a minimum of nine times, and was lost a minimum of ten times. Three different forms of bootstrapping show our results are robust to species sampling. Furthermore, groups with a baculum show evidence of higher rates of diversification. Our study offers an explanation for the inconsistent results in the literature, and provides insight into the evolution of this remarkable structure.

The baubellum is more developmentally and evolutionarily labile than the baculum

“…(2016). Presence/absence of the baubellum was scored through literature searching and online museum records from August 2015 to January 2017.…”

“…(2016) and was trimmed down to include only species where both the baculum and the baubellum were scored, resulting in 163 species. We then applied stochastic mapping as implemented in the function make .…”

“…This same approach was employed by Schultz et al. (2016) to model baculum evolution. We summarized baculum and baubellum gains and losses from 1,000 iterations of stochastic mapping across each of the four strategies described above, using only the 163 species for which both baculum and baubellum were scored.…”

Copulatory and Postcopulatory Sexual Selection in Primates

“…Thus, it is present in most prosimian and anthropoid species but has undergone secondary reduction, or loss, in a number of cases (e.g., in Ateles, Chiropotes, Cacajao, Tarsius and Homo [Dixson, 2012]). Bacula also occur in some species belonging to the following orders of placental mammals: Chiroptera, Rodentia, Eulipotyphla, Lagomorpha, Carnivora, Pinnipedia and Afrosoricida [Brindle and Opie, 2016;Schultz et al, 2016]. The baculum forms by ossification of the distal region of the corpora cavernosa and extends forwards into the glans penis (Fig.…”

Comparative Morphology of the Penis and Clitoris in Four Species of Moles (Talpidae)

“…The absence of the bone precursor could account for the absence of an os penis and os clitoris in star‐nosed and Japanese shrew moles, thus making androgens (whether derived from testes or ovotestes) irrelevant for this feature. The European mole ( T. europaea ) has an ovotestis and possesses an os penis but the status of an os clitoris in the females is unknown (Schultz et al., ). The status of the os penis and os clitoris in other species of European moles ( T. occidentalis and T. romana ) having ovotestes has not been reported (Matthews, ; Beolchini et al., ; Zurita et al., ).…”