Global elongation and high shape flexibility as an evolutionary hypothesis of accommodating mammalian brains into skulls

Weisbecker, Vera; Rowe, Timothy; Wroe, Stephen; Macrini, Thomas E.; Garland, Kathleen L. S.; Travouillon, Kenny J.; Black, Karen; Archer, Michael; Hand, Suzanne J.; Berlin, Jeri C.; Beck, Robin M.D.; Ladevèze, Sandrine; Sharp, Alana C.; Mardon, Karine; Sherratt, Emma

Abstract

Little is known about how the large brains of mammals are accommodated into the dazzling diversity of their skulls. It has been suggested that brain shape is influenced by relative brain size, that it evolves or develops according to extrinsic or intrinsic mechanical constraints, and that its shape can provide insights into its proportions and function. Here, we characterise the shape variation among 84 marsupial cranial endocasts of 57 species including fossils, using 3D geometric morphometrics and virtual dissections. Statistical shape analysis revealed four main patterns: over half of endocast shape variation ranges between elongate and straight to globular and inclined; little allometric variation with respect to centroid size, and none for relative volume; no association between locomotion and endocast shape; limited association between endocast shape and previously published histological cortex volumes. Fossil species tend to have smaller cerebral hemispheres. We find divergent endocast shapes in closely related species and within species, and diverse morphologies superimposed over the main variation. An evolutionarily and individually malleable brain with a fundamental tendency to arrange into a spectrum of elongate-to-globular shapes – possibly mostly independent of brain function - may explain the accommodation of brains within the enormous diversity of mammalian skull form.

• Journal Article Type: Article
• Acceptance Date: Dec 13, 2020
• Online Publication Date: Feb 2, 2021
• Publication Date: Mar 21, 2021
• Deposit Date: Dec 15, 2020
• Publicly Available Date: Jan 22, 2022
• Journal: Evolution
• Print ISSN: 0014-3820
• Electronic ISSN: 1558-5646
• Publisher: Wiley
• Volume: 75
• Issue: 3
• Pages: 625-640
• Book Title: Global elongation and high shape flexibility as an evolutionary hypothesis of accommodating mammalian brains into skulls
• DOI: https://doi.org/10.1111/evo.14163
• Keywords: General Agricultural and Biological Sciences, Genetics, Ecology, Evolution, Behavior and Systematics
• Publisher URL: https://doi.org/10.1111/evo.14163
• Related Public URLs: http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1558-5646/
• Additional Information: Access Information : This is the peer reviewed version of the following article: Weisbecker, V., Rowe, T., Wroe, S., Macrini, T.E., Garland, K.L.S., Travouillon, K.J., Black, K., Archer, M., Hand, S.J., Berlin, J.C., Beck, R.M., Ladevèze, S., Sharp, A.C., Mardon, K. and Sherratt, E. (2021), Global elongation and high shape flexibility as an evolutionary hypothesis of accommodating mammalian brains into skulls. Evolution, 75: 625-640., which has been published in final form at https://doi.org/10.1111/evo.14163. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. A pre-print version can be read at: https://doi.org/10.1101/2020.12.06.410928; Funders : Australian Research Council;ATM MNHN "Biodiversite actuelle et fossile";University of Adelaide Fellowship;National Science Foundation (NSF); Grant Number: DE120102034, DP170103227, FT180100634; Grant Number: DP170101420; Grant Number: IIS 9874781, IIS 0208675, DEB 9873663, DEB 0309369