Microspines in tropical climbing plants: a small-scale fix for life in an obstacle course

Many climbing plants exhibit microspines on their stems, which facilitate attachment and prevent slipping and falling from host plant supports. Extending forwards via growth through complex environments and anchoring stems to substrates with minimal contact forces are key benefits to climbing plants. Microspines are also highly desirable features for new technologies and applications in soft robotics. Using a novel sled-like device, we investigated static and sliding attachment forces generated by stems in 10 species of tropical climber from French Guiana differing in size and climbing habit. Eight species showed higher static and sliding forces when their stems were pulled in the basal direction against a standard surface than in the apical direction. This anisotropic behaviour also suggests that tropical climbers have evolved different ratchet-like mechanisms which allows easy sliding forwards but is resistant to slipping downwards. The presence of a downwards "stick-slip" phenomenon, where static attachment is not significantly stronger than maximal sliding attachment, was present in most species apart from three representing relatively weak attachment by microspines. This indicates that diverse microspine attachment strategies exist in climbing plants. This diversity of functional properties in tropical climbers offers a range of potential design specifications for climbing strategies on different substrates for artificial climbing artefacts.