Variation in Arboreal Locomotion and Hand Morphology in Modern Humans, Olive Baboons, and Chimpanzees
Open AccessThe hand provides an interface between a primate and their external environment. It is involved in almost all aspects of primate lives. Amongst a multitude of activities, it provides access to food, facilitates social interactions such as grooming, and assists with locomotion (e.g., Napier, 1967; McGrew and Tutin, 1978; Byrne and Byrne, 1993; Hunt et al., 1996; Hopkins et al., 2007; Marzke et al., 2015). Thus, the hand is subject to a wide range of different selection pressures, meaning that its morphology is a representation of the intricate compromises among different functions. Understanding how behavioral variation, including locomotion, is reflected in the hand morphology of extant primates has the potential to provide critical insight into the behavioral capabilities of fossil hominins and primates (e.g., Jungers et al., 1997; Alba et al., 2003; Kivell et al., 2011; 2015, but see Wallace et al., 2020). Arboreality plays a key role in multiple hypotheses for adaptive trends in primates (e.g., Napier, 1967; Sussman, 1991; Cartmill, 1992; Sussman et al., 2013; Boyer et al., 2017) and the potential capacity for arboreal locomotion in fossil hominins is central to many hypotheses on the origin of bipedalism (e.g., Wood Jones, 1916; Prost, 1980; Fleagle et al., 1981; Hunt, 1996; Stanford, 2006; Thorpe et al., 2007). Given the importance of the hand for multiple forms of arboreal locomotion (e.g., Preuschoft and Demes, 1984; Hunt et al., 1996; Thorpe et al., 2007) it is a logical anatomical region in which to investigate the effects of behavioral variation in arboreality.This thesis investigates aspects of the behavior-stress-form axis (Figure 1) in three different species: Modern humans (Homo sapiens), olive baboons (Papio anubis), and chimpanzees (Pan troglodytes schweinfurthii) (see Figure 1). In the long-term, my results set the stage for future studies that directly link all three aspects of the form-function relationship in each system. Chapter 1 examines the effects of support diameter size and activity on how peak pressure is experienced by the hand during suspension and vertical climbing in modern humans. Support diameter opposed to static or dynamic activity had the greatest effect on normalized peak pressure in both suspension and vertical climbing. Diameters closer to the ergonomic optimal diameter (30-40mm, e.g., Hall, 1997; Seo and Armstrong, 2008) had the greatest normalized peak pressure value compared to larger than optimal diameters. The third ray was the mode radial to ulnar location of peak pressure in both suspension and vertical climbing, highlighting its importance in identifying arboreal associated morphology more broadly. Whereas the distal to proximal location differed with support orientation and size, suggesting that within ray morphological variation may provide greater insight into arboreal locomotor capabilities. Potential interpretations of the hand morphology of Orrorin tugenensis and Homo naledi are discussed.Chapter 2 investigates morphological variation in olive baboon hand bones in relation to arboreality. The metacarpal, proximal and intermediate phalangeal curvature and phalangeal index of the third ray were compared in two samples of Papio anubis that differ in their degree of arboreality. Increased curvature and larger phalangeal indices were predicted in the more arboreal sample. However, after controlling for body mass and bone type, phalangeal indices and curvature were reduced in the more arboreal population. Differences in hand postures during arboreal and terrestrial behaviors, and terrestrial environmental differences may explain these unexpected results. This highlights the variability within arboreal associated hand morphology and that detailed behavioral and biomechanical data are needed to formally test the outlined hypotheses. Chapter 3 analyses the ecological and social correlates of arboreality in wild female chimpanzees. Behavioral data from chimpanzee mothers at Gombe National Park, Tanzania were used to examine the effects of season, maximum temperature, rainfall, rank, age, and the presence of a clinging infant on the proportion of time spent arboreally. Increased arboreality was found in the wet season and arboreality decreased at higher temperatures and in older individuals. However, no effect of rainfall, rank or the presence of a clinging infant was found. Whilst the presence offspring and rank constrain aspects of terrestrial ranging behavior (e.g., Pontzer and Wrangham, 2006; Murray et al., 2007), social factors have a limited effect on maternal arboreality. The results of this dissertation provide an understanding of the ecological and social drivers of arboreality and the variation in the morphological expression of arboreality in the hand. It highlights the importance of the manual third digit for investigating locomotor capabilities and suggests that within ray morphological comparisons can provide additional detail on an individual’s arboreal capabilities. In addition, it shows a need for investigating within species variation in arboreality and known behavioral contextualization of primate skeletal collections, which provide a valuable resource to the study of primate functional morphology. Future work should quantify within species variation in time spent arboreal and hand postures used across multiple populations, alongside within hand morphological variation in modern humans, olive baboons, and chimpanzees.
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