The Baculum was Gained and Lost Multiple Times during Mammalian Evolution

Area of Science:

• Evolutionary Biology
• Comparative Anatomy
• Mammalian Phylogenetics

Background:

• Male genitalia, including the baculum (penis bone), show rapid evolution across mammals.
• Baculum morphology is highly diverse but lacks consistent correlation with mating systems.
• Previous comparative studies assume baculum homology, which may be flawed.

Purpose of the Study:

• To test the hypothesis that the mammalian baculum is not a homologous structure.
• To investigate the independent evolutionary origins and losses of the baculum across mammals.
• To explore the relationship between baculum presence and diversification rates.

Main Methods:

• Phylogenetic modeling of baculum presence/absence data for 954 mammalian species.
• Utilized three bootstrapping methods to ensure robustness against species sampling variations.
• Analyzed evolutionary rates of diversification in relation to baculum evolution.

Main Results:

• The baculum evolved independently at least nine times and was lost at least ten times during mammalian evolution.
• Results were robust across different species sampling scenarios.
• Mammalian clades possessing a baculum exhibited significantly higher rates of diversification.

Conclusions:

• The mammalian baculum is a classic example of convergent evolution, not a single homologous structure.
• This non-homology explains previous inconsistencies in linking baculum evolution to mating systems.
• The evolution and loss of the baculum are associated with altered diversification dynamics in mammals.