Skull asymmetry in the harbour porpoise, Phocoena phocoena

The main aim of this research was to find out if the harbour porpoise, Phocoena phocoena, possessed any sexually dimorphic, directional asymmetry of the skull. The cetacean group Odontoceti, which the harbour porpoise is classified under, are known to possess different degrees of asymmetric skulls. This topic was chosen because of how little is known about the rather unusual sexual dimorphism that the harbour porpoises possess, in that the females grow larger than the males, and whether this could be applied to their unique use of echolocation. In this study the skulls were CT-scanned, and each 3D-image manually landmarked before the asymmetry was calculated. Overall, the female skulls possessed slightly more asymmetry than their male counterpart and the region with the most general asymmetry measured was the dorsal nasofacial region, known to be related with bisonar production. However, the results of whether sexual dimorphism is present show a general trend of not being statistically significant, with the model 𝐴𝑠𝑦𝑚𝑚𝑒𝑡𝑟𝑦 ~ 𝑁𝑜𝑛𝑒 being the only one with a p-value of less than 0.05. For directional asymmetry the only regions with significant results were the dorsal nasofacial region and the rostrum, of which both regions relate to how the harbour porpoises use echolocation. The rostrum did show some sexual dimorphism, with the model 𝐷𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑠𝑦𝑚𝑚𝑒𝑡𝑟𝑦 ~ 𝑆𝑒𝑥 having the lowest AIC value, and that the male harbour porpoise possessed more left-hand side directional asymmetry than the females. If indeed true that the unusual sexual dimorphism of size in harbour porpoises also extends to a sexual dimorphism in skull directional asymmetry, then this could mean the different uses of echolocation between males and females have an impact on their skull asymmetry and implies a direct link between skull asymmetry and use of echolocation in the harbour porpoise.