Hunting for birds in trees, young boa constrictors sometimes have to scale the slenderest of branches to trap their prey. So what strategies do they use to move as they slither along flexible boughs that are a fraction of their girth? Greg Byrnes and Bruce Jayne decided to find out how young boa constrictors ascended slack and taut vertical ropes ranging from 3 mm to 9 mm diameter ().
Luring the snakes to climb the ropes by placing a dark cosy refuge at the top, Byrnes and Jayne filmed the animals' ascents while measuring the tension on the rope as the animals coiled and uncoiled their grip. ‘All the climbing velocities were extremely slow,’ the duo says, ranging from 0.5 to 1 cm s–1 and the snakes only reached their top speeds on the thickest and tensest ropes. All of the snakes gripped the rope with some parts of their bodies as they translated other parts up, but Byrnes and Jayne noticed the animals used different movement patterns depending on whether they were scaling the thinner ropes (3 mm and 6 mm) or the thicker (9 mm) rope.
When ascending the thinner ropes, the snake's entire body moved along the same path, maintaining contact with the same section of rope until the tail slid past. However, when mounting the thicker rope, the animal ‘concertinaed’ upward, holding on with certain parts of its body as it dragged free parts of the body up.
Analysing the effect that varying the rope's tension had on the boa constrictors' movements, Byrnes and Jayne saw that the snakes found it harder to climb on the slacker, thinner ropes, slipping backwards often. They also noticed that the snakes switched their grip strategy depending on the rope's flexibility and thickness.
On thinner ropes, the snakes' bodies formed a zigzag, so that the inward directed grip forces occurred at different points along the rope's length. Unopposed, the forces caused the rope to bend and kink as the snake ascended. However, as the boa constrictors mounted the thicker rope, their bodies formed C-shaped clamps, holding on like a gripping hand, so that the inward directed forces cancelled each other out leaving the rope straight and undeformed. Byrnes and Jayne suspect that although the tail may be able to form C-shaped clamps on all thicknesses of rope, thicker regions of the snakes' bodies may not be able to bend enough, forcing the animals to use the less effective zigzag strategy when scaling slender boughs.
