The baubellum is more developmentally and evolutionarily labile than the baculum

Abstract

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.

The evolution of female genitalia

“…The baubellum was found to be much more labile than the baculum in terms of its evolution and development; the baubellum showed many more gains and losses than the baculum and exhibited greater developmental variation (Lough‐Stevens et al., ). The baubellum was also shown to be significantly more variable in morphology than the baculum (Lough‐Stevens et al., ). Together, these results suggest that the baubellum may not be a target of sexual selection but rather a vestigial and nonfunctional trait in females.…”

“…A more recent study used a phylogenetic approach to examine the presence or absence of genital bones across 163 mammalian species (Lough-Stevens, Schultz, & Dean, 2018). The baculum is the bone found inside the penis of many mammalian species, and the baubellum is the equivalent bone found within the clitoris of females (Lough-Stevens et al, 2018).…”

“…A more recent study used a phylogenetic approach to examine the presence or absence of genital bones across 163 mammalian species (Lough-Stevens, Schultz, & Dean, 2018). The baculum is the bone found inside the penis of many mammalian species, and the baubellum is the equivalent bone found within the clitoris of females (Lough-Stevens et al, 2018). The baubellum was found to be much more labile than the baculum in terms of its evolution and development; the baubellum showed many more gains and losses than the baculum and exhibited greater developmental variation (Lough-Stevens et al, 2018).…”

“…The baculum is the bone found inside the penis of many mammalian species, and the baubellum is the equivalent bone found within the clitoris of females (Lough-Stevens et al, 2018). The baubellum was found to be much more labile than the baculum in terms of its evolution and development; the baubellum showed many more gains and losses than the baculum and exhibited greater developmental variation (Lough-Stevens et al, 2018). The baubellum was also shown to be significantly more variable in morphology than the baculum (Lough-Stevens et al, 2018).…”

The ultimate database to (re)set the evolutionary history of primate genital bones

“…Afterwards, several authors dedicated their studies to the genital anatomy in primates 6 and some of them were specifically interested in genital bones 7 – 25 . Recently, some authors shifted the attention from the descriptive anatomy to the evolution and adaptive meaning of such bones in several mammal orders, primates included 26 – 29 . Based on different datasets and analyses, these studies obtained conflicting results.…”