When jumping from branch to branch, squirrels find tradeoffs between jumping distance and the flexibility of the branches. A new study shows how the little acrobats make their decisions and how they make up for mistakes to prevent fatal falls. Both the biomechanical adaptations to the habitat in the treetops and their ability to learn help them. The results could also contribute to the development of robots for demanding areas of application.
Squirrels spend most of their lives high up in the trees – a complex environment in which they have to move skillfully through a maze of branches to forage, nest, and avoid predators. This requires a combination of highly developed biomechanical skills and learned behaviors. But how exactly do squirrels decide which jump to risk and which one could lead to a potentially fatal fall?
Jump tests for croissants
This is what a team led by Nathaniel Hunt from the University of Nebraska looked at. “As a model organism for understanding the biological limits of balance and mobility, I think squirrels are second to none,” says Hunt. “If we understand how squirrels move in the treetops, then perhaps we can discover general principles of locomotion in complex terrain that could also apply to the movements of other animals and robots.”
In order to observe the jump artists in the most natural environment possible, the researchers lured some fox squirrels (Sciurus niger) to the campus of the University of California at Berkeley. They used nuts to motivate the little rodents to make daring jumps in a course designed by the researchers. They documented the movements of the animal test subjects on video and then evaluated the jump position, flight path and landing maneuver. For their experiments, Hunt and his colleagues used springboards of different lengths and flexibility. From here the croissants could reach a narrow branch on which a nut was luring in a small bowl. The further they ventured on the flexible springboard, the less far they had to jump. In return, the take-off base became more shaky with every step, which made the take-off more difficult.
Acrobatic maneuvers
The observations showed: “When the squirrels jump over a gap, they choose their jump point depending on the flexibility of the branch and the size of the gap,” says Hunt. They went far forward on stable diving boards, while on wobbly boards they preferred to take a slightly longer jump than venture too far forward. The flexibility of the jump base played a six times bigger role in the decision than the jump distance. “When they hit a branch with new mechanical properties, they learned within a few jumps to adjust their starting mechanics,” explains Hunt. “This flexibility of behavior, which adapts to the mechanics and geometry of the jump and landing structures, is important in order to jump over a gap precisely and land on a small target.”
“During the entire experiment, not a single squirrel fell while attempting to jump,” report the researchers. They made mistakes and jumped either too far or too short to land optimally on their narrow target. But every time they managed to cling to the branch with their claws and pull them up with acrobatic maneuvers. “If you jump into the air at too much or too little speed, you can use a variety of landing maneuvers to make up for it,” explains Hunt. “If they jump too far, they roll forward around the branch. If they jump too short, they land on their front legs and swing under the branch before pulling themselves up on the perch. This combination of adaptive planning behavior, learning control and reactive stabilization maneuvers helps you to move quickly through the branches without falling. “
In addition, the researchers observed an unexpected tactic by their animal test subjects: During difficult jumps, they often pushed themselves off the wall to which the branches were attached, and in this way adjusted their speed and direction – similar to human course runners. The squirrels adapted flexibly to new possibilities of their environment and quickly learned to use them to make the jumps more efficient and safer. The study shows that it is likely a synergy between biomechanical properties and trial-and-error learning that enables animals to move quickly and skillfully in the canopy. The findings could also help develop flexible robots that can safely traverse rough terrain.
Source: Nathaniel Hunt (University of Nebraska) et al., Science, doi: 10.1126 / science.abe5753