Pattern of Cranial Ontogeny in Populations of Gorilla and Pan
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PATTERN OF CRANIAL ONTOGENY IN POPULATIONS OF GORILLA AND PAN A DISSERTATION SUBMITTED TO THE FACULTY OF THE UNIVERSITY OF MINNESOTA BY JASON S. MASSEY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY KIERAN P. MCNULTY June 2018 © Jason S. Massey, 2018 Acknowledgments I am so deeply indebted to my advisor Dr. Kieran McNulty. I have known Kieran for many years, and in that time, he has provided me with every opportunity to succeed. My first field experience was at his site in Kenya, and since then he has guided me through stimulating areas of study, introduced me to colleagues that have expanded my research interests, and challenged me to be a better and focused researcher. I am an anthropologist because of him. Likewise, I could not have accomplished this without a strong and dedicated committee. I am so grateful for Drs. Martha Tappen, Michael Wilson, David Fox, and Anthony Weinhaus. The interesting discussions, challenging questions, access to data, and support over the many years have been integral to my success. I am also indebted to Drs. Bernard Wood and Shannon McFarlin. This dissertation would not exist without their dedicated support, funding, and patience. They have given me access to needed specimens and many summers of exciting fieldwork. I thank the Department of Anthropology and the community therein for so many years of emotional and academic support. In particular, I would like to thank Kara Kersteter, Nora Last, Megan Whaley, and Barbara London. Additionally, many summers of data collection and fieldwork were funded by numerous block grants and the Graduate Research Partnership Program. I would like to thank the Evolutionary Anthropology Labs, and in particular John Soderberg and Matt Edling, for access to collections, facilities, and so much technical support with software and 3D scanners. i Thank you to thank the Department of Integrative Biology and Physiology for opening my eyes to teaching anatomy. The Human Anatomy Program, Drs. Anthony Weinhaus, and Mark Cook, did not just allow me to teach anatomy, they guided me toward being an effective and thoughtful educator. They provided teaching when funding was low as well as at times when I needed to write. I would like to thank the University of Minnesota for supporting my research in many ways. Funding for travel and writing was given through the Thesis Travel Research Grant and the Doctoral Dissertation Fellowship. Additional funding, including needed travel to Gombe National Park, Tanzania, was provided by the Leakey Foundation. Thank you for 50 years of funding projects such as this one. I thank the curators and staff of the many museums and institutions that let me visit and collect data: The Royal Museum for Central Africa, Tervuren, Belgium; Powell- Cotton Museum, Birchington-on-Sea, United Kingdom; Royal College of Surgeons, London, United Kingdom; Natural History Museum, London, United Kingdom; Cleveland Museum of Natural History, Cleveland, Ohio. I would also like to thank the numerous field assistants, researchers, and other staff associated with the following organizations: Mountain Gorilla Skeletal Project, Rwanda Development Board – Department of Tourism and Conservation, Dian Fossey Gorilla Fund International’s Karisoke Research Center, Mountain Gorilla Veterinary Project, Tanzania Wildlife Research Institute, Tanzania Commission of Science and Technology, Tanzania National Parks, and the Jane Goodall Institute’s Gombe Stream Research Centre. ii And, of course, I could not do this without unwavering support from my friends and family, particularly my parents and brother. I could not have stayed the course without their unconditional love and encouragement. iii Abstract The biology of African apes provides an important comparative framework for interpreting the evolutionary unfolding of our own species, Homo sapiens. Understanding the ontogeny of our closest living relatives is critical, as evolutionary transformations in adult shape ultimately arise through changes in the timing and patterning of growth and development. However, our current knowledge of African ape skeletal ontogeny is deficient in two important respects. First, wild-shot museum specimens lack the single datum necessary for a comprehensive study of ontogeny: specimen age. Second, skeletal research on African apes invariably uses samples that are pooled at the subspecies, species, or even genus level. This work aims to rigorously quantify and compare the patterns of cranial ontogeny in African ape populations, utilizing two important new skeletal samples with known ages. Unlike most previous research, this study will use samples assigned to specific populations in order to characterize intraspecific variation in development. This work aims to answer the following questions: 1) how do African ape populations differ in their ontogeny? 2) to what degree do ontogenetic models based on pooled samples diverge from population-level ontogenies? 3) how does ontogeny mediate sexual dimorphism in Gorilla and Pan?, and 4) which aspects of anatomy provide insight into heterochronic relationships between extant taxa? iv Table of Contents Acknowledgments .............................................................................................................. i Abstract ............................................................................................................................. iv Table of Contents .............................................................................................................. v List of Tables .................................................................................................................... ix List of Figures .................................................................................................................. xii 1 Introduction ............................................................................................................... 1 1.1 Background .......................................................................................................... 2 1.1.1 Evolutionary significance of juvenile fossils ................................................ 3 1.1.2 Evolution of the human life history pattern .................................................. 4 1.1.3 Sexual dimorphism ....................................................................................... 9 1.1.4 The complicated case of heterochrony ....................................................... 11 1.2 Evolutionary Changes in Morphology ............................................................... 17 1.2.1 The integration of ontogeny and evolution ................................................. 17 1.2.2 Integrating population-level thinking ......................................................... 19 1.3 Hypotheses to be Tested ..................................................................................... 21 1.3.1 Testable hypotheses: Ontogenetic variation at the population-level .......... 22 1.3.2 Testable hypotheses: The ontogeny of sexual dimorphism ........................ 24 1.3.3 Testable hypotheses: Finding heterochrony ................................................ 25 2 Materials and Methods ........................................................................................... 27 2.1 Sample ................................................................................................................ 27 2.1.1 Sample used in analyses .............................................................................. 27 2.1.2 Determination of population taxonomy and population history ................. 28 2.2 Digitization Protocols ......................................................................................... 29 2.2.1 Scanning ...................................................................................................... 29 v 2.2.2 Landmarking ............................................................................................... 31 2.3 Statistical Analyses ............................................................................................ 32 2.3.1 Procrustes superimposition ......................................................................... 32 2.3.2 Developmental and functional modules...................................................... 33 2.3.3 Geometric morphometric analyses utilized in this study ............................ 34 3 Population-level thinking in ontogenetic variation............................................... 38 4 Population-level ontogenetic variation in Gorilla and Pan .................................. 39 4.1 Introduction ........................................................................................................ 39 4.1.1 Growth and development ............................................................................ 39 4.1.2 Variation below the species-level ............................................................... 43 4.1.3 Population-level analyses............................................................................ 44 4.2 Methods .............................................................................................................. 46 4.2.1 Sample......................................................................................................... 46 4.2.2 Data collection ............................................................................................ 48 4.2.3 Statistical Analysis .....................................................................................