Hit or miss: branch structure affects perch choice, behaviour, distance and accuracy of brown tree snakes bridging gaps

The effects of branch size, shape and orientation on arboreal animalsʹ movement across gaps or their choice of destinations when doing so are poorly understood compared to the well-documented effects of habitat structure on the locomotor behaviours and maximal speeds of animals moving on solid surfaces. Some highly arboreal species of snakes, such as the brown tree snake, Boiga irregularis, cross gaps using either a slow-speed crawling or a high-speed lunging behaviour. We expected that wider destination perches would enhance either the speed or maximal distance crossed while bridging a gap, because reaching and touching larger objects requires less precise motor control. Thus, for B. irregularis, we tested whether branch size and shape affected perch preference, maximal distance and the behaviour used to bridge gaps. The snakes usually preferred wider destination perches, some of which significantly increased both maximal distance and the amount of high-speed lunging, which contributed to maximal distance. Cylinders with pegs that simulated secondary branches enhanced lunging success rate across gaps compared to cylinders without pegs. The use of high-speed lunges revealed trade-offs between the speed and the accuracy of head placement when first contacting the destination perch on the far side of a gap. Thus, the structure of destination perches on the far side of a void can have important consequences for what destination is chosen, the distance of the gap that is crossed and how the void is traversed.