A “scepter” (the baculum) and a “gem” (the baubellum): occurrence and shape tell the evolutionary history of genital bones in primates

Unmatching data and omissions have long characterised the neglected topic of genital bones in primates. Based on a thorough analysis on primary literature vs. recent reviews, several conflicting data, inaccurate references, and questionable claims have emerged. Both occurrence and form of anatomical structures are affected by selection pressures and this likely is the case of the penile and clitoral bones (i.e., bacula and baubella). To investigate the hypothesis of genital bone symplesiomorphy in primates we therefore generated a binary data matrix of genital bone occurrence based on (1) primary literature (back to the beginning of nineteenth century) covering 268 and 58 species for males and females, respectively (2) an updated taxonomy (tracing back to the species taxonomic history if necessary), and (3) new data from 148 specimens (opportunistic sampling from cadavers at Italian Istituti Zooprofilattici Sperimentali; wet primate collections at American Museum of Natural History, New York, and National Museum of Natural History, Washington D.C.) covering 12 (males) and 20 (females) species. We almost doubled occurrence dataset used so far, with N=280 species for males and N=78 species for females (new data for 29 species). The ancestral state reconstruction (ASR) analysis conclusively demonstrated baculum symplesiomorphy, tentatively extended to baubellum as well. Our occurrence data indirectly supported the baculum/baubellum homology (i.e., for each baubellum a baculum was invariably present), and directly demonstrated an occurrence variability at the intra-specific level. Limited to bacula, both anatomical and shape features were coded into different matrices for phylogenetic inferences. Analyses performed demonstrated that baculum filled the entire length of the penis and was “stick”-shaped as ancestral state. From this anatomical and morphological condition, several character transitions have occurred in the primate evolutionary history, which brought to the actual form variation affecting the baculum.