The spectacle of a tightrope walker apparently suspended in mid-air on a thread-like cable has mesmerised crowds for generations, from Charles Blondin's first crossing of the Niagara Gorge in 1859 to Philippe Petit's 1974 death-defying ‘coup’ traversing a 61 m-long cable, 400 m above the ground between the twin towers of the World Trade Center. Yet awe-inspiring aerial displays are routine for many tree-dwelling species. ‘The ease with which primates can walk and run over narrow and compliant branches has long fascinated both scientists and the public’, says Jesse Young from Northeast Ohio Medical University, USA, who has spent the last 5 years investigating how tree-dwelling species negotiate their complex surroundings. ‘We've amassed a large dataset on how primates adapt their walking and running patterns to cope with variation in perch diameter’, he says. Yet, little was known about how primates tackle the challenge of bounding along a springy tree limb. Intrigued by this knowledge gap, Young and Brad Chadwell decided to investigate how marmosets take pliable surfaces in their stride.
‘Figuring out how to motivate the animals was the most difficult part’, says Young, recalling how he and Chadwell encouraged the marmosets to scamper atop tubes ranging in diameter from 1.25 to 5 cm with puffs of air aimed at the tail and tempting fruity yoghurts – ‘Which they would get all over their faces’, he chuckles. ‘They were also a little freaked out the first time we transitioned them from the stable to the compliant substrates’, adds Young, describing how he and Chadwell eventually hit on the idea of supporting the tubes with super-cushioning polyurethane foam to simulate the movement of a bouncing bough as a marmoset scurried along. Then, having fine-tuned the marmosets’ running track, the duo filmed the monkeys in 3D as they scampered from one end to the other.
Analysing the bucking motion of the tube and the details of the marmosets’ strides, Bethany Stricklen, Young and Chadwell realised that the animals’ hindfeet remained in contact with the pipe for longer when it flexed: ‘Increasing substrate contact time may improve stability on narrow and compliant supports by giving the monkeys more time to react to perturbations and use substrate reaction forces to redirect the centre of mass appropriately’, says Young. The team also noticed that the marmosets were able minimise the deflection of the springy trackway by lowering their bodies closer to the tubes and moving more slowly: ‘Even a moderate level of compliance exerts a significant influence on locomotor mechanics’, says Young. And when the team analysed how the marmosets adapted to the different diameter runways, it was evident that they stopped bounding and switched to a slower gait that allowed them to always keep at least one foot in contact with the narrower tubes.
Referring to the animal's natural tree-top homes, Young says, ‘Adjustments to gait mechanics, like those documented here, are undoubtedly one of the many “solutions” arboreal quadrupeds use to achieve locomotor stability in precarious locomotor environments’. He also adds, ‘These results clearly show that compliance is an important determinant of locomotor performance that shouldn't be ignored in future studies of arboreal animals’. And Young is eager to learn more about the movements of other primates that have better-developed grasps than those of the marmosets, while hoping eventually to transfer this study into the forest to learn how the natural variation in stiffness from sturdy boughs to bendy twigs affects the animals’ movements in their tree-top homes.