Disclosing 3D Shape of Ossa Genitalia in Primates

Selection pressures on copulatory systems rapidly shape the anatomy of external genitals, affecting both the occurrence and form (size+shape) of baculum (os penis) and baubellum ( os clitoridis ) in primates. These heterotopic bones are usually located within the distal end of the penis and clitoris. However, the pattern of occurrence within the primate phylogenetic tree and the multitude of forms shown cannot be easily merged into a univocal functional interpretation. This study aims to develop a methodological protocol to supply a suitable dataset of bone virtual volumes. These data are useful in morphological approaches exploiting both traditional and geometric morphometric techniques. We provide results on samples from both captive individuals (i.e. fresh samples from corpses coming from zoos or research laboratories) and museum samples (i.e. the primate collection of “La Specola” Natural History Museum, Florence, IT). We followed a three-step protocol which entails progressive sophistication: (1) manual palpation (efficient for fresh tissues, much less so for museum tissues made stiff by either alcohol or formalin); (2) X-rays (confirming or not the palpation); (3a) if absence is confirmed, histological analysis is performed focused on hypothetical traces of cartilage or osteocytes; (3b) if presence is confirmed, high resolution 3D images are obtained by using a non-invasive microtomography system which allows the exploration of both the external and internal structure of bones. The main objectives of the research were (1) completing the primate genital bone occurrence matrix at the specific level, and (2) providing morphological data to test evolutionary hypotheses by overlapping form variation to reproductive and ecological correlates.