Abstract
The baculum is an enigmatic bone within the mammalian glans penis, and the driving forces behind its often bizarre shape have captivated evolutionary biologists for over a century. Hypotheses for the function of the baculum include aiding in intromission, stimulating females and assisting with prolonged mating. Previous attempts to test these hypotheses have focused on the gross size of the baculum and have failed to reach a consensus. We conducted three-dimensional imaging and apply a new method to quantify three-dimensional shape complexity in the carnivoran baculum. We show that socially monogamous species are evolving towards complex-shaped bacula, whereas group-living species are evolving towards simple bacula. Overall three-dimensional baculum shape complexity is not related to relative testes mass, but tip complexity is higher in induced ovulators and species engaging in prolonged copulation. Our study provides evidence of postcopulatory sexual selection pressures driving three-dimensional shape complexity in the carnivore baculum.
Highlights
The baculum bone is located within the glans penis of many modern mammals [1] and is extremely divergent in size and shape among closely related species [2]
We investigated whether carnivore social systems are characterized by different evolutionary optimal values of alpha complexity using an evolutionary model selection approach
Comparison between AICc scores for all evolutionary models fitted to alpha shape complexity values shows strong support for OUMV and BMS models, in which either/or θ and σ2 are allowed to vary between social systems
Summary
The baculum bone is located within the glans penis of many modern mammals [1] and is extremely divergent in size and shape among closely related species [2]. Recent experimental work found a correlation between baculum morphology and reproductive success in rodents [3,4], yet the causative link between shape and reproductive output remains unclear. A correlation between baculum robustness (a metric incorporating size and shape) and intromission duration was found in carnivores; taxa engaging in prolonged copulation are characterized by more robust bacula [7]. Some taxa possess divergent bacula shape while size remains relatively consistent across the group [8]. The evolution of baculum shape has not been fully explored
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