The “island rule” is a generalization which states that over evolutionary time on islands, small-bodied species become larger because of reduced predation, and large-bodied species become smaller due to severely limited food resources. However, it is unclear how certain anatomical parts change with the evolution of smaller body sizes. Therefore, my dissertation project aims to (1) establish whether or not island dwarfing is universal among large-bodied Southeast Asian mammals (pigs, deer, gibbons and long-tailed macaques), (2) determine how brain size scales to body size on the mainland versus islands of various sizes, and (3) analyze little-studied changes in dimensions of the skull between mainland and island-living individuals. An overarching goal of my research is to reconstruct a within-species phylogeny for gibbons and long-tailed macaques in order to take individual relatedness into account in the analyses.
Southeast Asia and museums
Southeast Asia is a spectacular region for studies of island dwarfing because there are thousands of islands of various sizes and types. Specifically, there are oceanic islands (land surrounded by water and never connected to the mainland) and continental islands (continental shelf elevations isolated by water that may connect with the mainland when sea levels are low). The remains of large-bodied Southeast Asian mammals are kept in state-of-the-art natural history museums around the world, with major collections located at the Smithsonian Institute (Washington, D.C.), the American Museum (New York), the Natural History Museum (London, UK), the Raffles Museum of Biodiversity Research (now called the Lee Kong Chian Natural History Museum, Singapore), and The Field Museum (Chicago). So I spend quite a bit of time behind the scenes at these museums to collect data.
Island dwarfing has often been assumed to be universal, and the degree of dwarfing depends on island size. So I am interested in testing whether or not island dwarfing is a general rule for mammals from Southeast Asia, specifically pigs, deer, gibbons and long-tailed macaques. Additionally, I am using allometric scaling, the study of size relationships, to analyze how brain size scales to body size in island mammals. Since the discovery of the small-brained Homo floresiensis, a new hominin species, on the island of Flores, in Indonesia, scientists have debated over hypotheses for why it has such a tiny brain. Through my research, I hope to shed some light on the specimen by answering a sidelined but fundamental question: How does brain size scale to body size in island-living mammals generally?
3D Geometric morphometrics
Geometric morphometrics is a method that allows me to analyze the morphology of museum specimen skulls by collecting 3D coordinates of designated landmarks on the skull using a microscribe. I then use MorphoJ to standardize and extract shape information from the landmark data using Procrustes superimposition, which allows for all shapes to be centered, scaled to uniform size and rotated in the same direction. Once the shapes are standardized, I aim to analyze differences in skull morphology in relation to sex, island size and island type using principal components analyses.
Studies of island dwarfism have rarely taken evolutionary relationships into account, as is needed to exclude effects of closely related species. It is clear that relatedness between and within species is a key issue when analyzing comparative data, and the need to use these phylogenetic techniques for comparative data in evolutionary anthropology and biology have been stressed by Felsenstein (1985), Nunn (2011) and Stone et al. (2011). Therefore, I am using next generation techniques in Dr. Ripan S. Malhi’s sterile ancient DNA lab at University of Illinois Urbana-Champaign along with The Field Museum’s Pritzker Lab facilities to sequence the mitochondrial genomes of Southeast Asian gibbon and long-tailed macaque individuals. The tissues used for this project are collected by scraping residual dried tissue flakes from 50-80 year old museum specimens. I hope to tease apart relationships between island populations when analyzing body size, brain size and skull morphology, which is an essential step when studying a process that occurs only within species or very closely related species.
References you may be interested in
Felsenstein, J. (1985). Phylogenies and the comparative method. Am Nat, 125(1), 1-15. doi: 10.1086/284325
Foster, J. B. (1964). Evolution of mammals on islands. Nature, 202, 234-235. doi:10.1038/202234a0
Lomolino, M. V. (1985). Body size of mammals on islands: The island rule reexamined. Am Nat, 125, 310-316. doi: 10.1086/284343
Nunn, C. L. (2011). The Comparative Approach in Evolutionary Anthropology and Biology. Chicago, IL: University of Chicago Press.
Stone, G. N., Nee, S., Felsenstein, J. (2011). Controlling for non-independence in comparative analysis of patterns across populations within species. Phil Trans of R Soc B, 366, 1410-1424. doi: 10.1098/rstb.2010.0311