For terrestrial animals, whose lives are firmly anchored on the ground, arboreal lifestyles seem precarious. But brown tree snakes thrive in this complex three-dimensional habitat, stretching and twisting across gaps in the tree canopy to hunt small birds and mammals. Yet, little is known about the stresses and strains exerted on the bodies of these snakes as they slither between branches. Intrigued, Greg Byrnes from Siena College, USA, and Bruce Jayne from the University of Cincinnati, decided to film brown tree snakes as they bridged gaps ranging from simple drops and vertical ascents, to wide horizontal head-on chasms and sidewise steps, to analyse the mechanical forces exerted on the reptiles' sinuous bodies (p. 2611).

The snakes were most successful at negotiating the head-on vertical climbs and drops, straddling gaps that were 65% wider than head-on horizontal gaps. And when they compared the snakes' ability to span sideways gaps – where they had to twist by 90 deg – with head-on gaps, the snakes were able to cross head-on gaps that were 13% larger than the equivalent sideways gap. Some snakes even negotiated gaps that were longer than their bodies, by manoeuvring their heads close to their destination perch before lunging to cover the last few centimetres.

Calculating the torques – turning forces – exerted on the snakes' bodies as they precariously extended their heads, the duo says, ‘The orientation of the gap significantly affected all of the estimated torques’. Effectively, the snakes found sideways horizontal gaps harder to bridge than sideways steps – where they had to rise to reach the perch – while the sideways horizontal gap was the hardest of all to cross. The duo suggests that the snakes' ability to traverse gaps is restricted by their ability to keep their bodies rigid and the large torques that threaten to topple them when extending across the gaps.

Considering the snakes' agility in conservation terms, the duo explains that brown tree snakes were inadvertently introduced to the island of Guam after the Second World War. Since then, they have proliferated unchecked, decimating the island's indigenous bird and small mammal populations. Given that the Guam snakes can reach 3 m in length, Byrnes and Jayne warn that the snakes could be capable of spanning 1.5 m wide horizontal gaps and 2.2 m vertical gaps. They recommend that vegetation should be trimmed, producing chasms that are too wide for the animals to bridge if the island is to prevent their dispersal to other vulnerable locations.

B. C.
The effects of three-dimensional gap orientation on bridging performance and behavior of brown tree snakes (Boiga irregularis)
J. Exp. Biol.