Environmental Drivers of Terrestriality in Primates

Abstract

This dissertation investigates the evolutionary and ecological drivers of primate terrestriality and, more broadly, the relationships between forest structure and mammal diversity. I adopt an integrative approach, combining large-scale phylogenetic analyses, fine-scale LiDAR surveys, and multi-year camera trap data. In Chapter 1, I introduce these topics, detail the methodological approaches I used, and give an overview of each of the three data chapters. In Chapter 2, I establish a broad foundation by reviewing terrestriality across the primate order. Through ancestral state reconstructions and comparative modeling of hundreds of taxa, I show that terrestrial behavior has evolved multiple times, correlating with body mass and habitat changes, aligning with widely accepted hypotheses of primate terrestriality. This chapter highlights the critical need for site-specific, detailed data to capture the fine-grained habitat features that primates navigate. In Chapter 3, I zoom in on Gunung Palung National Park, in West Kalimantan, Indonesia, to analyze how forest structural heterogeneity, quantified via terrestrial laser scanning (TLS), influences mammal diversity. I demonstrate that metrics such as canopy roughness, vertical point distribution, and vegetation volume are strong predictors of species richness and functional diversity, especially when aggregated at ecologically defined forest type partitions. These findings largely support the “habitat heterogeneity hypothesis” and underscore the importance of spatial scale in revealing how structural complexity fosters biodiversity. In Chapter 4, I turn again to primate terrestriality, focusing on a case study of three arboreal primates in Gunung Palung: Bornean orangutans, red leaf monkeys, and long-tailed macaques. I combine camera trap observations, TLS metrics, population density estimates, and environmental variables to uncover the factors driving ground use. While predator presence had less influence than expected, forest structural complexity and weather conditions (rainfall, temperature) played central roles. I find that dense understory vegetation likely encouraged terrestrial foraging, while prolonged dry spells or high temperatures prompted primates to descend for water or thermoregulation. In this dissertation, I illustrate how the interplay between habitat structure, resource distribution, and other ecological variables shape the daily and evolutionary patterns of mammal behavior. My results challenge common assumptions that large carnivores necessarily deter arboreal primates from using the ground, and that forests with high connectivity inherently reduce terrestrial forays. Moreover, I highlight the promise of LiDAR-based methods in capturing structural attributes critical for explaining both primate ecology and broader mammal community composition. By bridging global, phylogenetic perspectives and focused, high-resolution ecological analyses, this work shows that terrestrial behavior in primates, and biodiversity patterns in tropical forests, cannot be explained by any single variable. Instead, species- and site-specific interactions among body size, predation risk, forest structure, food availability, and weather conditions all converge to shape how mammals occupy and exploit forest strata. These insights contribute to a deeper understanding of primate ecology, reveal new facets of mammal community structure, and demonstrate the value of integrative, multi-level research approaches in tropical ecosystems.