Angular Momentum and Arboreal Stability in Common Marmosets (Callithrix jacchus) Brad A. Chadwell 1,2 and Jesse W. Young 1,2,3 * 1 Department of Anatomy and Neurobiology, Northeast Ohio Medical University (NEOMED), Rootstown, OH 44272 2 Skeletal Biology Research Focus Area, NEOMED, Rootstown, OH 44272 3 School of Biomedical Sciences, Kent State University, Kent, OH 44240 KEY WORDS balance; torque; center of mass; asymmetrical gaits; primate locomotor evolution ABSTRACT Despite the importance that concepts of arboreal stability have in theories of primate locomotor evolution, we currently lack measures of balance per- formance during primate locomotion. We provide the first quantitative data on locomotor stability in an arbo- real primate, the common marmoset (Callithrix jacchus), predicting that primates should maximize arboreal sta- bility by minimizing side-to-side angular momentum about the support (i.e., L sup ). If net L sup becomes exces- sive, the animal will be unable to arrest its angular movement and will fall. Using a novel, highly integra- tive experimental procedure we directly measured whole-body L sup in two adult marmosets moving along narrow (2.5 cm diameter) and broad (5 cm diameter) poles. Marmosets showed a strong preference for asym- metrical gaits (e.g., gallops and bounds) over symmetri- cal gaits (e.g., walks and runs), with asymmetrical gaits representing >90% of all strides. Movement on the nar- row support was associated with an increase in more “grounded” gaits (i.e., lacking an aerial phase) and a more even distribution of torque production between the fore- and hind limbs. These adjustments in gait dynam- ics significantly reduced net L sup on the narrow support relative to the broad support. Despite their lack of a well-developed grasping apparatus, marmosets proved adept at producing muscular “grasping” torques about the support, particularly with the hind limbs. We con- tend that asymmetrical gaits permit small-bodied arbo- real mammals, including primates, to expand “effective grasp” by gripping the substrate between left and right limbs of a girdle. This model of arboreal stability may hold important implications for understanding primate locomotor evolution. Am J Phys Anthropol 156:565–576, 2015. V C 2014 Wiley Periodicals, Inc. The origins and subsequent diversification of primates are intimately linked to arboreality. Because the arbo- real habitat is inherently discontinuous, multidimen- sional, and frequently unstable, primates face a host of locomotor challenges not encountered by more terrestrial mammals—a fact attested to by the frequency of long bone traumas due to falling in free-ranging arboreal pri- mates (Lovell, 1991). As such, for nearly 100 years anthropologists have presented the need to move safely in an arboreal environment—specifically a narrow- branch arboreal environment—as one of the primary selective pressures shaping primate locomotor morphol- ogy and behavior (Wood Jones, 1916; Le Gros Clark, 1959; Napier, 1967; Cartmill, 1972; Larson, 1998). Despite the central role that concepts of arboreal sta- bility have played in theories of primate locomotor evolu- tion, we currently lack empirical measures of balance performance during primate locomotion. In this study, we provide the first quantitative in vivo data on locomo- tor stability in an arboreal primate, the common marmo- set (Callithrix jacchus). In general, stability can be defined as the ability of a mechanical system to mini- mize the probability of catastrophic perturbations (Alexander, 2002; Full et al., 2002). We operationalize this concept by asserting that a primary index of arbo- real stability is the control of the angular momentum of the center of mass (CoM) about the support (see also Lammers and Zurcher, 2011). Angular momentum has proved a useful metric of stability in a variety of sys- tems, including insect hexapedalism, mammalian quad- rupedalism, human bipedal walking, and robotics (Full et al., 2002; Goswami and Kallem, 2004; Herr and Popovic, 2008; Lammers and Zurcher, 2011). We assume that primates seek to maximize arboreal stability by minimizing side-to-side rolling angular momentum about the support (abbreviated below as L sup ). If L sup becomes too large, the animal will be unable to arrest its angular movement about the support and will fall (Cartmill, 1985; Preuschoft, 2002; Lammers and Zurcher, 2011). We describe a novel, highly integrative experimental protocol that integrates kinematic, kinetic, and gait data to directly measure L sup during locomotion on narrow branch-like substrates. Our system expands on Lammers’ and colleagues research on torque production and arboreal stability in small, non-primate mammals (Lammers and Gauntner, 2008; Lammers and Zurcher, Additional Supporting Information may be found in the online version of this article. Grant sponsor: National Science Foundation; Grant number: BCS-1126790. *Correspondence to: Jesse W. Young, Department of Anatomy and Neurobiology, Northeast Ohio Medical University, 4209 State Route 44, P.O. Box 95, Rootstown, OH 44272. E-mail: jwyoung@neomed. edu Received 31 August 2014; revised 20 November 2014; accepted 24 November 2014 DOI: 10.1002/ajpa.22683 Published online 19 December 2014 in Wiley Online Library (wileyonlinelibrary.com). Ó 2014 WILEY PERIODICALS, INC. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 156:565–576 (2015)