Tree snakes’ keel gets a grip

Abstract

To the untrained eye, most snakes look alike, but when Bruce Jayne observes a serpent, he sees many different features. ‘Sea snakes have paddle-shaped tails and gliding snakes are vertically flattened,’ he explains, adding that many species of tree snake have flattened bellies and sharp ridges running along both belly edges, known as keels. Jayne is also fascinated by how animals interact with their surroundings and is particularly intrigued by how tree snakes stop themselves from slipping on rugged bark surfaces. Reasoning that more rotund species would have problems getting a purchase on rough surfaces, as they would simply roll over bumps and ridges, while flat bellied species may be able to lodge the keel ridge against protrusions and lock themselves in place, Jayne recruited a team of students to test the tree-worthiness of three species, ranging from stout boa constrictors to corn snakes and agile brown tree snakes.First, Jayne, Steven Newman, Michele Zentkovich and Matthew Berns simulating the rugged surfaces encountered by snakes in the wild. ‘Variation in the roughness of the bark on natural vegetation is very complex and the spacing between various ridges and grooves can be quite irregular’, explains Jayne. So, instead of trying to mimic every detail on a branch, he focused on one aspect of bark structure – the height of ridges on the surface – by embedding 8 mm wide screws (at lengths of 1, 2, 10, 40 mm), spaced at 10 cm intervals, into the top face of long narrow poles that he inclined over a range of angles from horizontal to vertical. ‘The sheer number of trials was a significant challenge’, chuckles Jayne, estimating that the student trio must have videoed almost 10,000 movies of the snakes ascending poles over the course of the study.Analysing the snakes’ techniques, Jayne saw that on the steepest branches the boa constrictors and corn snakes preferred to grip the pole with one portion of the body and straighten other portions to slide the animal along using a concertina-like motion. However, the tree snake only resorted to this approach when presented with a smooth vertical pole, switching predominantly to sliding sinuously as soon as they were provided with the shortest pegs to lean on. As the pegs became longer the tree snakes dispatched with the concertina style of ascent entirely, while the boa constrictors and corn snakes continue shuffling until the pegs were quite long. Even at the shallowest pole angles, the boa constrictors seemed to struggle, preferring to shuffle along slowly; meanwhile the tree snakes slithered at higher speeds as the pegs became longer and the corn snakes’ performance fell somewhere in between. Even when the team set the tree snakes an additional challenge of negotiating a pole angled at 60 deg with the pegs positioned along the bottom face of the pole, the tree snakes seemed completely unfazed.But how were the tree snakes clinging on? ‘[They were] catching their keel on surface projections that were only 1 mm high’, Jayne says. Where the boa constrictors’ rotund bodies simply rolled over the shortest pegs – forcing them to grip the pole in their coils – the tree snakes were able to lock themselves onto the pole by running their keel ridges against the pegs like a rope passing through a pulley. And Jayne hopes that a better understanding of how tree snakes get a grip could help us design snake unfriendly surfaces, to prevent invasive species – such as the brown tree snake in Guam – from getting into places where they are not wanted.