Browsing by Subject "Phylogenetic comparative methods"

Primates span incredible behavioral and ecological diversity, and this diversity is reflected in the shape of the skull. This dissertation asks two questions surrounding the evolution of primate skull shape: 1) what are the macroevolutionary correlates of primate skull shape? And 2) what is the pattern of phenotypic integration in the primate skull at a macroevolutionary scale? To address these questions, I compiled a broad comparative dataset of anatomical landmarks identified from 3D scans of primate skulls and analyzed this data using statistical methods that combine geometric morphometrics and phylogenetic comparative methods. To investigate the macroevolutionary correlates of skull shape, I used multivariate phylogenetic generalized linear models to test for relationships between skull shape and several variables that are predicted to be correlated with skull shape: allometry, typical diet, tree gouging behavior, activity pattern, and sexual dimorphism. I found strong phylogenetic signal for primate skull shape, confirming the need for phylogenetic comparative methods. Allometry was a significant predictor of skull shape, with larger primates having relatively small, convergent orbits, and anteroposteriorly short skulls compared to small primates. Sexual dimorphism was associated with a dramatically lengthened rostrum, probably to facilitate a large gape in aggressive displays. Folivory was associated with deeper mandibles, which may improve mechanical advantage and strain resistance. To investigate patterns of phenotypic integration in the skull, I performed hierarchical clustering analyses on phylogenetically corrected inter-landmark covariance matrices. In contrast to previous research, I did not find evidence for distinct phenotypic modules in the primate skull, and I argue that this discrepancy is due to methodological shortcomings of past research that biased results towards identifying different anatomical regions as discrete modules. This dissertation represents one of the first investigations of primate skull shape at a macroevolutionary scale, and demonstrates that the combination of geometric morphometrics and phylogenetic comparative methods can yield novel insights into evolutionary morphology.

What makes our minds human? How did they evolve to be this way? This dissertation presents data from two complementary lines of research driven by these orienting questions. The first of these explores the `what' of human cognitive evolution through comparative studies with chimpanzees and bonobos. The general aim of these studies is to understand which aspects of cognition are unique to humans, and which are shared with our closest living relatives. Chapters 2-3 test the hypothesis that humans have unique cognitive skills for reasoning about the attention of other individuals (theory of mind), and unique motivation to use these skills in cooperative contexts with conspecifics. In Chapter 2 I show that understanding others' attention is unlikely to be the `small difference that makes the big difference', as some researchers have proposed. However, my data support the possibility that species differences in the ontogeny of these skills may have robust consequences for the adult cognitive phenotype. In Chapter 3 I show that (contrary to previous reports) nonhuman apes are also motivated to engage in some simple triadic social activities, which resemble those characteristic of human children. Again however, I identify important differences between humans and other apes in their spontaneous preferences for these types of activities, and their attitudes toward a partner when cooperative behaviors are interrupted. The second half of this dissertation (Chapters 4-5) explores the `why' and `how' of cognitive evolution. Chapter 4 outlines the kind of research questions and methods that comparative psychologists will need to embrace in order to use the comparative method to its full potential in the study of cognitive evolution. Chapter 5 provides a proof of principle for this approach using a dataset including 33 species tested on two cognitive tasks measuring inhibitory control. Here I show that cognitive skills for inhibitory control are closely related to phylogeny across species, and strongly predicted by absolute (but not relative) brain size. Further, I show that two of the other leading hypotheses put forth to explain primate intelligence, namely sociality and diet, do not predict cognitive skills on these tasks. These data illustrate the power of the comparative method for understanding cognitive evolution, and provide a starting point for future studies embracing this approach. Collectively, this research refines our understanding of how human cognition differs from that of other primates, and illustrates the utility of studying cognitive evolution from an explicitly phylogenetic comparative framework.