Body size and shape in insular environments and applications of the island rule in biological anthropology
Authors:   , Young Colleen Brennan 
Published: December 2021

The discovery of small-bodied hominin fossils in 2004 on the island of Flores, Indonesia, unearthed a large debate within biological anthropology. This debate has exemplified that there are questions and research areas that biological anthropologists do not understand about island evolution. To improve understanding on the causes and products of evolution within island areas for biological anthropologists, this dissertation addresses three overarching research areas relevant to the biological anthropology community. The first is an analysis of how primate body sizes vary on islands, with interpretations that are anchored in the evolutionary history of body sizes of primates. Primates that initially evolved body sizes to survive within a frugivorous niche, with elongated life spans to improve survival in unpredictable environments, have body sizes distributed among islands in relation to the presence of absence of these pressures. Smaller islands contain more large, bodied primates overall, whereas larger islands contain more small-bodied ones. Second, an analysis of island fox body size and shape indicates that island foxes have reduced body sizes and divergent skeletal traits compared to mainland, closely related counterparts. Distinct body proportions are likely due to selection because allometric scaling of limb lengths to body mass are divergent for the island fox. Further, the island fox is not a scaled down version of the mainland fox, with limbs decreasing in size at a faster rate compared to the mainland. Last, an investigation on the diversity of two human populations in the Baja California peninsula demonstrates that Amerindians who migrated to and survived in these regions were impacted by ecogeographic pressures in different degrees, likely related to access to resources. Heat-adapted skeletal traits are apparent in both human populations who inhabited this hot desert, but body size is distinct for the two groups. Body size is smaller for individuals with less access to marine resources and increased susceptibility to periods of drought and starvation. Body size is larger for humans with convenient access to oceanic and terrestrial resources. These studies demonstrate that primates, omnivores, and humans are not immune to the effects of insularity as has been suggested. Rather, interpreting body size and shape alterations requires contextualizing the organism with their evolutionary histories and subsequent interactions within the island areas. Body size alterations are the result of shifting selective pressures from competing with other community members to competing with other individuals within a population over finite resources. As such, body shape can also be divergent compared to closely related mainland counterparts due to adaptation to local ecogeographic pressures. Skeletal traits of organisms need to be interpreted in relation to their migratory journeys and adaptation to local ecogeographic pressures within the island. For humans, contextualizing these variables with cultural and behavioral characteristics is imperative to understand a body size response within a sociocultural omnivorous niche.