71 - Wild North American River Otters, Lontra Canadensis, as a Model of Sexual Size and Shape Dimorphism of the Skull and Endocranial Cavity

Poster Board Number: 71
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Co-authors:

Kristin Marsh - Midwestern University; Dominik Valdez - Midwestern University; Alexander Claxton - Oklahoma State University Center for Health Sciences; Christopher Heesy - Midwestern University

Abstract Body : The diversity of mammalian skull morphology and function is founded on complex interactions between the brain and skull during development and evolution. Key insights into the evolutionary development of skull-brain interactions are available, yet under-studied, in natural populations, which are under myriad selective pressures. Sexually dimorphic wild species provide an ideal model for studying skull and brain evolution as males and females within a species are frequently subjected to different selective pressures. Studying evolution within a sexually dimorphic species reduces the intrinsic variable of genetic variation while still maintaining the breadth of extrinsic environmental, dietary, and behavioral selective pressures. Using the North American river otter (Lontra canadensis), a sexually dimorphic species, as a model, we sought to quantify the correlation between cranial and brain shape between sexes. Male L. canadensis are larger in body size, hunt larger prey and thus have a higher quality diet, and navigate larger, more environmentally heterogeneous territories. Each of these variables are hypothesized to select for more robust skulls and increased encephalization in mammals. We microCT scanned the skulls of 30 individuals, 15 male and 15 female, collected from Arkansas within a single year. We quantified skull and endocast shape using 3D geometric morphometrics. We found significant differences between sexes in both skull shape and endocast shape, with males exhibiting wider zygomatic arches, enlarged coronal gyri and narrowed pseudosylvian gyri and sylvian sulci. Interestingly, we noted that the greatest variation in skull shape was a rostral-caudal shortening of the braincase and ventrally oriented occipital bone in males. This directly correlated to rostral-caudal compression and increased curvature in the endocast, resulting in distinct operculization. These results suggest that selective pressures associated with males hunting larger prey, which requires larger muscles of mastication and more robust neck musculature has the greatest influence on skull shape. Brain shape may then follow the trajectory of skull development rather than dictate it. Future studies focusing on a developmental series will elucidate the selective drive for this unique morphology.