Branching out: perch diameter and orientation affect pull strength in chameleons (genus: Bradypodion)

Arboreal species tend to have specific adaptations allowing them to effectively negotiate the complexity of their habitats. For example, chameleons have a prehensile tail and limbs adapted to grasping branches. However, the impact of branch diameter and orientation on their movement has been poorly studied. Using ecological, morphological, and pull force data from two species of Bradypodion from different habitats, we examined if chameleons use different substrates and if perch orientation and diameter affect pull strength by quantifying their grip forces on different sized dowels. We found that the habitat of Bradypodion ventrale is homogeneous, and dominated by sparse patches of narrow, vertical perches. For this species, perch diameter significantly affected pull strength in both horizontal and vertical pull directions, with chameleons performing best on a vertical 1.5-mm dowel and a horizontal 3-mm dowel. In contrast, Bradypodion pumilum typically occurs in more variable vegetation, ranging from low shrubs to wooded habitat with high canopy. Our results show that the habitat has perches that are on average wider than for B. ventrale with a greater size range. The performance of B. pumilum was less impacted by perch diameter and pull direction suggesting that B. pumilum is able to use perches of different diameters in their more heterogeneous habitat.