Postcopulatory sexual selection influences baculum evolution in primates and carnivores

Abstract

The extreme morphological variability of the baculum across mammals is thought to be the result of sexual selection (particularly, high levels of postcopulatory selection). However, the evolutionary trajectory of the mammalian baculum is little studied and evidence for the adaptive function of the baculum has so far been elusive. Here, we use Markov chain Monte Carlo methods implemented in a Bayesian phylogenetic framework to reconstruct baculum evolution across the mammalian class and investigate the rate of baculum length evolution within the primate order. We then test the effects of testes mass (postcopulatory sexual selection), polygamy, seasonal breeding and intromission duration on the baculum in primates and carnivores. The ancestral mammal did not have a baculum, but both ancestral primates and carnivores did. No relationship was found between testes mass and baculum length in either primates or carnivores. Intromission duration correlated with baculum presence over the course of primate evolution, and prolonged intromission predicts significantly longer bacula in extant primates and carnivores. Both polygamous and seasonal breeding systems predict significantly longer bacula in primates. These results suggest the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection.

Ligamentous structures in human glans penis

“…It has been argued that the morphology of male intromittent organs has been subject to more‐rapid evolutionary divergence than any other structure in the animal kingdom (Kahn et al. ; Simmons & Firman, ; Brindle & Opie, ). The baculum (also called the os penis) is an extraskeletal bone with a low mineral density that is derived from connective tissue and is found within the distal end of certain mammals, including many primates, rodents, bats, carnivores and some insectivores (Sharir et al.…”

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.…”

Cross‐Sectional Geometry and Scaling in the Baculum of Cuban Hutias (Rodentia: Capromyidae)

“…Similarly, the argument concerning the existence of a role due to the energetic costs associated with the risk of fracture seems illogical. Brindle and Opie () considered their results to support the hypothesis of prolonged intromission, at least in Primates and Carnivores. The question is whether a global adaptive role is retained.…”

“…A less parsimonious hypothesis assumes that the baculum is a therian plesiomorphy lost several times during mammalian evolution (Patterson and Thaeler Jr, ). A reconstruction of the evolution of the baculum using a Bayesian phylogenetic framework (Brindle and Opie, ) concluded that the baculum was not present in the ancestral mammal but rather appeared after placental and nonplacental mammals split about 95 millions of years.…”

“…In both cases, the baculum would prevent the obstruction of sperm flow (Dixson et al, ). However, results are controversial (Brindle and Opie, ).…”