Postcopulatory sexual selection influences baculum evolution in primates and carnivores.

Matilda Brindle,Christopher Opie

doi:10.1098/rspb.2016.1736

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.

Highlights

The morphology of male intromittent organs is argued to be subject to more rapid divergent evolution than any other form in the animal kingdom [1]
When apes subsequently split from Old World monkeys this trend reversed and the ape baculum underwent further high rates of evolution, this time reducing in length. (See electronic supplementary material for variable-rates trees depicting carnivore baculum length evolution and primate and carnivore testes mass evolution, figures S2 –S4.)
Our results have uncovered the evolutionary trajectory of the baculum across the mammalian class, showing that the baculum first evolved after placental and non-placental mammals split around 145 million years ago (Ma), but before the MRCA of primates and carnivores evolved around 95 Ma [25]

Summary

Introduction

The morphology of male intromittent organs is argued to be subject to more rapid divergent evolution than any other form in the animal kingdom [1]. It was thought only to exist in eight of the mammalian orders: Afrosoricida, Carnivora, Chiroptera, Dermoptera, Erinaceomorpha, Primates, Rodentia and Soricomorpha [3,4]. It has recently been discovered that a Lagomorph, the American pika (Ochonta princeps), has a small baculum [5]. This discovery suggests that baculum presence may be more prevalent across mammals than historically assumed. Certain orders have a mixture of baculum presence and absence across species; these are the Carnivora, Chiroptera, Primates and Rodentia. Aside from documenting the presence and absence of the baculum across the mammalian orders, the evolutionary history of the baculum had not been studied until recently, leaving many questions unanswered

Methods

Results

Conclusion