title: 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.
authors.affiliation: *Molecular and Computational Biology, University of Southern California, 1050 Childs Way, Los Angeles, CA 90089, USA.
authors.forename: Nicholas G
authors.initials: NG
authors.lastname: Schultz
authors.affiliation: *Molecular and Computational Biology, University of Southern California, 1050 Childs Way, Los Angeles, CA 90089, USA.
authors.forename: Michael
authors.initials: M
authors.lastname: Lough-Stevens
authors.affiliation: West Adams Preparatory High School, 1500 W Washington Blvd, Los Angeles, CA 90007, USA.
authors.forename: Eric
authors.initials: E
authors.lastname: Abreu
authors.affiliation: Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112-0840, USA.
authors.forename: Teri
authors.initials: T
authors.lastname: Orr
authors.affiliation: *Molecular and Computational Biology, University of Southern California, 1050 Childs Way, Los Angeles, CA 90089, USA matthew.dean@usc.edu.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Dean
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001842:Bone and Bones / Q000033:anatomy & histology*; D005260:Female; D008297:Male; D008322:Mammals / Q000033:anatomy & histology* / Q000145:classification; D010413:Penis / Q000033:anatomy & histology*; D010802:Phylogeny; D012098:Reproduction