Balancing on a skinny branch is tricky for most animals at the best of times. Although small lizards and snakes have few problems, larger primates may struggle to remain erect as they clamber on slender supports. But some primates seem to have got round the problem by simply slinging their bodies from all four limbs beneath branches. Michael Granatosky, Cameron Tripp and Daniel Schmitt from Duke University, USA, describe how this bizarre form of locomotion appears remarkably similar to walking on all fours and add that the unconventional orientation may have provided an essential evolutionary stepping stone for animals that eventually began swinging through the trees on two arms. However, Granatosky says, ‘While many studies have explored the anatomy of suspensory species, no study has compared the movements of animals that walk below branches with all four legs versus those that swing with just the forelimbs’. Intrigued by the unconventional inverted motion, the trio set two species of lemurs – the ring-tailed lemur that prefers to walk on the ground and the ruffed lemur that frequently suspends itself from all four limbs – the task of trotting along a pole and then slinging themselves beneath it to find out how the two forms of motion compare.
Rigging up a pole with sections attached to force plates to measure the forces exerted by the animals, Granatosky and Tripp then enticed the lemurs to proceed along the pole as they filmed the animals’ movements. However, Granatosky admits that convincing the cunning animals to suspend themselves from the pole was easier said than done. ‘At first we tried covering the runway with fruits, but most of the time the lemurs just knocked the food off and ate their meal on the ground’, he recalls. Eventually, the team enticed the animals to hang beneath the pole by hanging some fruit below it. ‘Once they learned that being upside-down means treats, it was smooth-sailing from there’, he chuckles.
Having collected force measurements for 303 foot contacts as the lemurs advanced along, the duo dissected out the different force components to find how the two styles differed. And when the results were in, they were pleased that despite first appearances, the suspended form of walking was completely different from a conventional lemur stroll.
Describing how the animals elegantly picked their way along the pole while walking on the top surface, the team explains that the animals carried most of their weight on their hindlimbs, which also produced the forces that propelled them forward. However, the team was impressed to see this pattern reversed when the animals slung themselves underneath, with the lemurs using their arms to pull themselves forward in addition to taking most of the weight. They explain that this switch to forelimb weight bearing is unusual in primates, as most carry their weight on their hindlimbs to free their hands to manipulate food and other objects. However, they also point out that this load-bearing shift could have driven the evolution of specific adaptations to the limbs that would then allow them to begin swinging through the trees using their forelimbs alone.