The results of this dissertation will provide a new hypothesis for the process by which locomotion evolved in Old World Monkeys. The dataset to be collected is extensive and includes four postcranial elements of 49 species. Data will be made accessible online to other researchers after completion of the dissertation. An online database will allow access to people who may not have the resources to collect such data on their own, such as undergraduates at small institutions or students in developing countries. The research also incorporates undergraduates and is training a female scientist.

The Old World monkeys (OWMs)- monkeys that are distributed in Africa and Asia - are unique among primates for having several ground-dwelling, or terrestrial, species across many lineages. Even extant lineages that are almost strictly tree-dwelling, or arboreal, have highly terrestrial members in their fossil record. Therefore, the ancestor of OWMs is most commonly thought to have been at least partly terrestrial, with many groups secondarily evolving arboreal behavior. Support for this conclusion has been identified in the fossil species Victoriapithecus, a close relative of living OWMs that exhibits many traits thought to be adaptive for terrestriality. However, the process by which OWM locomotion evolved (i.e. the timing and number of locomotor transitions) remains unclear. Thus, the aim of this dissertation is to test the hypothesis that OWMs are ancestrally terrestrial, and to determine when and how many times locomotor transitions occurred. To accomplish this goal, measurements will be taken on four postcranial elements, the humerus, femur, astragalus, and calcaneus, of a diverse sample of living and fossil OWMs at several domestic and international museums. Statistical methods used will seek to discriminate arboreal and terrestrial species based on differences in their postcranial elements. The benefit of such an analysis is that fossil species, for which the locomotor mode is unknown, can be assigned to a group by the analysis providing the most likely locomotor behavior of the fossil. Measurements will then be mapped onto a phylogeny, a tree of proposed relationships among species, of living and fossil species to determine the most likely locomotor mode of the ancestor of OWMs.

Project Report

The goal of this study is to determine whether the ancestor of African and Asian monkeys lived a primarily arboreal or primarily terrestrial lifestyle. The majority of primate species have an arboreal, or tree-dwelling, lifestyle. However, the African and Asian monkeys, or Old World monkeys (OWMs), include the most terrestrial, or ground-dwelling, species of any primate group. Research on this group of primates suggests that the evolutionary history of OWMs included several locomotor transitions - i.e. from living an arboreal lifestyle to a terrestrial one or vice versa. Additionally, primatologists generally accept that the ancestral lifestyle of OWMs included terrestrial behavior based on the fossil species, Victoriapithecus. Victoriapithecus is considered by many primatologists to be the closest known relative of OWMs and exhibits several skeletal features indicative of terrestriality. Given this evidence, the current understanding of OWM evolution is that the ancestor of all OWMs was terrestrial and that arboreality re-evolved in several OWM lineages. However, this framework has been challenged by a recent description of the fossil species, Microcolobus. Microcolobus is one of the earliest OWMs and exhibits several skeletal adaptations for arboreality, suggesting a different evolutionary scenario in which at least some early OWMs were arboreal, like most other primates, and terrestriality evolved later in the group’s evolution. Figure 1 demonstrates these two evolutionary scenarios. This study has two main goals: 1) to determine if the ancestor of OWMs was arboreal or terrestrial, and 2) to understand the evolution of locomotor behaviors by determining the number and timing of locomotor transitions. This study accomplishes these goals by 1) including a broad number of OWM species in the sample, 2) using statistical techniques to determine which features of the skeleton, if any, are functionally related to arboreality or terrestriality, 3) reconstructing the likely locomotor behavior of fossil species, and 4) reconstructing the occurrence of locomotor transitions along the phylogeny (i.e. pattern of relatedness) of OWMs. The sample collected from this study includes 44 living species of OWMs and 16 fossil species, including Victoriapithecus and Microcolobus. Whenever possible, six male specimens and six females specimens were collected for each species. A total of 49 measurements were collected on the humerus (upper arm bone), femur (thigh bone), talus and calcaneus (ankle bones). Measurements were taken using digital calipers and photographs. Specimens were measured at 17 museums and institutions in the United States, Europe, and Africa. Each species of OWM was classified into one of three groups: 1) arboreal, 2) terrestrial, or 3) semi-terrestrial (i.e. engages in both arboreal and terrestrial behavior frequently). The results of this study show that several anatomical features across the humerus, femur, talus, and calcaneus show associations between skeletal anatomy and locomotor behavior. Generally, arboreal and terrestrial species are well-separated by the anatomical features and semi-terrestrial species overlap with both the arboreal and terrestrial group. When considering single elements, the humerus is best at separating arboreal from terrestrial OWMs. Improvement in separation of arboreal and terrestrial OWMs is accomplished when using a combination of anatomical features from the humerus, femur, talus, and calcaneus together. The results of this study also show that Victoriapithecus probably engaged in more arboreal locomotor behavior than previously thought, although the species was likely able to travel on the ground. Most elements of Victoriapithecus are statistically classified as arboreal but the distal humerus (i.e., the part of the upper arm bone that forms the elbow joint) is statistically classified as semi-terrestrial. This study supported previous research that suggested that Microcolobus was a primarily arboreal species, and the humerus and astragalus of this species were statistically classified as arboreal. These results suggest that the ancestor of all OWMs was probably a primarily arboreal species (that may have engaged in a modest amount of terrestrial behavior) and that Microcolobus inherited its adaptations for arboreality from this ancestor. Based on the statistical classification of the other 14 fossil OWMs, it seems most likely that terrestrial behavior evolved multiple times over the course of OWM evolution. Overall, this study supports the hypothesis depicted in Figure 1c, in which early OWMs engaged in frequent arboreal behavior, and terrestrial behavior evolved later in the group’s evolution.

National Science Foundation (NSF)
Division of Behavioral and Cognitive Sciences (BCS)
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Carolyn Ehardt
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State University New York Stony Brook
Stony Brook
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