Publication Date:
2017-12-21
Description:
Understanding the evolutionary forces that influence sexual dimorphism is a fundamental goal in biology. Here, we focus on one particularly extreme example of sexual dimorphism. Many mammal species possess a bone in their penis called a baculum. The female equivalent of this bone is called the baubellum and occurs in the clitoris, which is developmentally homologous to the male penis. To understand the potential linkage between these two structures, we scored baculum/baubellum presence/absence across 163 species and analyzed their distribution in a phylogenetic framework. The majority of species ( N = 134) shared the same state in males and females (both baculum and baubellum present or absent). However, the baubellum has experienced significantly more transitions, and more recent transitions, so that the remaining 29 species have a baculum but not a well-developed baubellum. Even in species where both bones are present, the baubellum shows more ontogenetic variability and harbors more morphological variation than the baculum. Our study demonstrates that the baculum and baubellum are generally correlated across mammals, but that the baubellum is more evolutionarily and developmentally labile than the baculum. The accumulation of more evolutionary transitions, especially losses in the baubellum, as well as noisier developmental patterns, suggests that the baubellum may be nonfunctional, and lost over time. Our research summarizes the evolutionary history and development of the clitoral bone, the baubellum. Our meta-analysis shows that this bone arose multiple independent times in mammals, and its history has more transitions and its development is more variable than the penis bone, the baculum. We conclude that although the baubellum is at least partially linked to the baculum, it is not completely constrained by it, leading to its increased lability in length, presence, and developmental trajectories.
Electronic ISSN:
2045-7758
Topics:
Biology
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