How marmoset monkeys (Callithrix jacchus) cope with stability challenges during arboreal locomotion

For arboreal mammals, problems of stability increase as branches become smaller and more compliant (i.e., move more under the animal’s weight). Changing the size and compliancy of a substrate should cause arboreal mammals to change their gait to facilitate balance. We tested this by having two marmosets (Callithrix jacchus) cross differently sized stable and compliant arboreal supports (5cm, 2.5cm, and 1.25cm in diameter). We filmed the marmosets with four high-speed cameras, testing how they would change their gait in order to maximize stability on thin and unsteady branches. Overall, the marmosets put more limbs on the substrate and for longer periods of time when substrate diameter decreased and when the substrate was compliant. Specifically, duty factors (the percent of time that a limb contacts the support during a stride) increased when the substrate size decreased. There was also a difference in duty factor between the stable and compliant substrates, such that the duty factor increased when the marmosets were on the compliant substrate. The average number of supporting limbs contacting the substrate at any one time also increased when the size of the substrate decreased and when the substrate became compliant. These findings show that marmosets respond to variation in substrate size and substrate compliancy in similar ways, in both cases increasing the average number of limb contacts and the duration of limb contact on a substrate in order to maintain stability.