It’s a long time since we gave up our primate lifestyle in favour of a bipedal existence, but even so, anthropologists are still intrigued by how our primate cousins move. Matthew O’Neill from Stony Brook University School of Medicine, USA, explains that quadrupedal primates do not use the same spectrum of gaits that other quadrupeds employ; they never trot. He adds that ring-tailed lemurs are also exceptional because, unlike the majority of primates that move primarily through trees, they move between foraging sites at ground level. Intrigued by the lemur’s relatively terrestrial lifestyle, O’Neill and his PhD advisor Daniel Schmitt from Duke University, USA, decided to investigate their movements to find out which gaits lemurs use while crossing the ground ().
‘The most reliable approach for determining the specific gaits of ring-tailed lemurs is to measure how their centre of mass moves’, explains O’Neill. So, the duo filmed five animals from the Duke Lemur Center moving at speeds ranging from a pedestrian 0.43 m s–1 up to a 2.91 m s–1 dash, while capturing the ground reaction forces acting on the animals as they crossed a force plate buried in the runway’s floor. Calculating the trajectory of each lemur’s centre of mass, based on the force plate measurements, and analysing the lemurs’ footfall patterns, the team found that the lemurs walked at slow speeds, switched to a canter at high speeds and broke into a gallop at their top speed. Despite their terrestrial lifestyle, the lemurs never trotted.
Curious to find out why lemurs do not trot, O’Neill built a computer model where he could simulate a lemur trotting. Modelling the forces acting on the simulated lemur’s legs as it trotted like a dog and cantered, he calculated the movement of the cyber-primate’s centre of mass and found that the trot would give the lemurs a much bumpier ride than a canter. ‘Our data show that the smoother ride is due to the fact that in a canter, the centre of mass bounces up and down once in a stride, rather than twice as in a trot’, explains O’Neill. He also realised that the centre of mass of the simulated cantering lemur bounced higher than the centre of mass of the trotting simulation. However, he suspects that in practice the additional bounce is not a problem for the cantering animals. ‘The differences between the vertical displacements of a trot and canter are only a few centimetres and may simply matter less for a lemur than some other factors, like bumpiness’, says O’Neill. In addition, O’Neill found that cantering allowed the lemur to keep one foot in contact with the ground at all time, as well as allowing the animals to move faster without increasing the peak forces exerted on them.
Essentially, the duo suspect that primates failed to evolve a trotting gait because cantering is more stable and better suited to an arboreal existence. ‘We think that cantering may have some advantages over trotting when moving in trees. So, the absence of a trotting gait in primates is likely tied to their early evolution as tree-living mammals’, says O’Neill.
