No Directional Scapular Asymmetry among Tamarines of the Genus Saguinus (Primates: Callitrichidae)

Bilateral asymmetry is defined as a deviation of a whole organism or a part of it from a perfect symmetry, and different categories can be recognized. One is the fluctuating asymmetry, defined as the random developmental variation of a trait (or character) that is expected to be perfectly symmetrical on average, and the other one is directional asymmetry, which occurs when one of the sides shows stronger morphological structures or marks than the other. The aim of this study was to determine the kind of scapula asymmetry in Saguinus scapulae. On lateral surface of each right and left scapula, a set of 5 landmarks and 3 curves with semi-landmarks along the margins, on a sample of 16 pairs from different Saguinus species, were considered. Asymmetries (fluctuating and directional) on size and shape of the scapulae were analysed by means of geometric morphometric methods. Directional asymmetry was not detected, demonstrating no side scapular shape bias. The absence of significant directional asymmetry may indicate a similar contralateral pattern of employment of the shoulder, at least for one-arm vertical suspension, as it needs stronger forces than those for terrestrial locomotion and thus would cause more asymmetry in case side loadings were different. To our knowledge, this is the first investigation on the symmetrical/asymmetrical nature of scapulae in Saguinus. Our findings increase knowledge and understanding of humeral joint and arboreal locomotion in primates.