Bridging the gap: How to design canopy bridges for arboreal guenons to mitigate road collisions

Roads affect the integrity of ecosystems worldwide as a cause of mortality to animals and a barrier to animal movement, decreasing gene flow and increasing local extinction probability. It is estimated that construction of linear infrastructure impacts up to 13% of primate species but research focusing on primate road fatalities and mitigation is not extensive and experimental research on canopy crossing designs for primates is lacking. We used the South African samango monkey (Cercopithecus albogularis) as a model species to test suitable bridge design through field experimentation and behavioural data collection for arboreal guenon roadkill mitigation and a mapped actual roadkill data in the region. We show that canopy overpasses are a viable intervention for mitigating arboreal guenon road fatalities, reducing the probability that monkeys will cross a road on the ground. Samango monkeys clearly preferred a pole bridge over a rope ladder design and canopy bridges were preferred to trees and the ground when the tree canopy was open. Pole bridges were also used by other non-guenon and non-primate species. Although samango roadkills are not predictable in time (no seasonality), adult female and immature fatalities are predictable in space, restricted to bisected riparian zones and roads close to intact forests. Adult male road fatalities can also be expected in seemingly unsuitable habitat areas. Our study shows how important correct interpretation of spatial, temporal and demographic data on road fatalities is and how experimental research prior to installing crossing structures could increase mitigation impact.