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Abstract ObjectivesAn accident during arboreal locomotion can lead to risky falls, but it remains unclear that the extent to which primates, as adept arborealists, change their locomotion in response to the perceived risk of moving on high supports in the tree canopy. By using more stable forms of locomotion on higher substrates, primates might avoid potentially fatal consequences. Materials and MethodsUsing high‐speed cameras, we recorded the quadrupedal locomotion of four wild lemur species—Eulemur rubriventer,Eulemur rufifrons, Hapalemur aureus, and Lemur catta(N = 113 total strides). We quantified the height, diameter, and angular orientation of locomotor supports using remote sensors and tested the influence of support parameters on gait kinematics, specifically predicting that in response to increasing substrate height, lemurs would decrease speed and stride frequency, but increase stride length and the mean number of supporting limbs. ResultsLemurs did not adjust stride frequency on substrates of varying height. Adjustments to speed, stride length, and the mean number of supporting limbs in response to varying height often ran counter to predictions. OnlyE. rubriventerdecreased speed and increased the mean number of supporting limbs on higher substrates. DiscussionResults suggest that quadrupedal walking is a relatively safe form of locomotion for lemurs, requiring subtle changes in gait to increase stability on higher—that is, potentially riskier—substrates. Continued investigation of the impact of height on locomotion will be important to determine how animals assess risk in their environment and how they choose to use this information to move more safely. 

Much research on primate locomotor performance in arboreal settings focuses on how primate morphology allows them to navigate substrates that vary in diameter, orientation, and compliance. However, little prior research has considered how these and other environmental factors - such as substrate height and light availability - may also affect locomotor behavior by altering how risky a given substrate is perceived to be. To investigate the relationship between risk perception and locomotor performance, we video-recorded four species of wild lemur (Ranomafana National Park), three species of wild cercopithecoid monkeys (Kibale National Park), and four species of captive lemur (Duke Lemur Center). We test the general hypothesis that primates should change their gaits and engage in exploratory behaviors – using touch and sight as guides – to increase stability in precarious settings. Augmenting our prior study showing that some lemurs change their locomotion when moving high in the canopy, we present new data showing that wild lemurs and monkeys frequently cross gaps between substrates and transition between locomotor modes without pause. In the investigation on captive lemurs, we examine whether variations in branch diameter, compliance, orientation, and light availability influence the paths lemurs choose to take. Preliminary results suggest that lemurs tend to avoid the most precarious substrates (i.e., the most narrow and compliant) regardless of lighting conditions. Overall, this research indicates that primates are able to make quick and accurate judgements about locomotor safety in the context of ongoing arboreal locomotion. 

Research on primates’ aptitude for navigating fine, compliant, and oblique branches has often focused on their postcranial morphology and locomotor mechanics. Here we aim to understand how primates perceive risk and make informed judgments to move safely. We video-recorded and digitized the locomotion of four lemur species (Ranomafana National Park) and 3 cercopithecoid monkeys (Kibale National Park). We test the general hypothesis that primates should change their gaits and engage in exploratory behaviors – using touch and sight as guides – to increase stability in precarious settings. Augmenting our prior study showing that some lemurs change their locomotion when moving high in the canopy, we present new data on the behavior of wild lemurs and monkeys as they cross gaps between substrates or switch between locomotor modes. They frequently cross gaps and transition between modes without pause, meaning they can accurately gauge their locomotor capacity before moving onto a new substrate. In an investigation on four species of captive lemurs (Duke Lemur Center), we examine how variations in substrate diameter, orientation, and compliance influence the paths lemurs choose to take. Preliminary results suggest that lemurs will tend to avoid the most precarious substrates in their paths, and future analysis will examine the role that light availability plays as well. Overall, this research highlights the importance of risk perception for robust locomotor performance while moving in arboreal environments.