When it comes to getting around, most frogs prefer to hop; after all, years of evolution have resulted in a body plan fine-tuned for jumping. However, some members of the frog family have moved into the treetops, where jumping becomes a precarious way to move around. Instead, they resort to walking along the narrow walkways, constantly having to stabilise themselves as they navigate amongst the branches. But, they are not the only animals in evolutionary history to make this jump to arboreal life, and Anthony Herrel, a researcher at the National Natural History Museum, France, wondered how similar they are to these other arboreal species (p. 3599).

Outlining the aims of his team's study, Herrel says: ‘The two things we were interested in were the actual grip types used and the basic mechanics of arm movement during locomotion and to see how that differed between substrates.’ To do this the team turned to the South American tiger-leg monkey tree frog, Phyllomedusa azurae, famed for its monkey-like prehensile ability to wrap its tiny hands around branches. ‘We had five cameras set up around a Plexiglas tank and in the tank we had different substrates, which were essentially metal wires of different diameters [1, 4 and 40 mm], and then we could orientate the tank horizontally or at a 45 deg angle,’ says Herrel.

Herrel recalls that, at first, the frogs were very wary of their new surroundings. However, after a few months the frogs became accustomed to the tank and eventually the team were able to coax them to walk along the narrow walkway by providing them with a leafy hide-out at the other end of the wire. Having finally enticed them to move, the team could then begin to analyse the frogs' movements. They found that on the narrowest 1 mm wires, the walking frogs preferred grasping the wire between their second and third finger or between their second and fourth finger. The former grip, which the team call D23, was also their grip of choice when the narrow wire was inclined. Herrel and his team think that the D23 grip both helps stabilise the frog and can provide the most traction of all three grips identified, which, of course, is extremely important on inclines. In contrast, the third grip, where fingers three and four wrap around the branch, uses less grip force and perhaps for this reason is favoured on the horizontal and wider wires.

Although the grip types varied depending on the diameter of the walkway, the team didn't find as many differences in the way that the frogs moved. On the whole, the frogs were understandably slower on the narrower wires, taking up to twice as long to traverse the 1 mm horizontal wire. On the narrower walkways, the frogs also maximised their stability by lowering their body close to the wires as well as maintaining a good triangle of support by grasping onto the gangway with two hands and one foot or vice versa.

Overall, the frogs seem well adapted to walking on thin branches but what really stood out to Herrel is how similar these adaptations are to those of other arboreal and climbing species, including the primates: ‘It's suggesting that these substrates are really imposing strong biomechanical constraints and animals really have to conform to this because there's quite an evolutionary history between them [frogs and primates] and still they're doing very similar things.’ As this ability to grasp has evolved several times in the frog lineage, Herrel and his colleagues hope to understand exactly what drives the evolution of prehensile ability.

The effect of substrate diameter and incline on locomotion in an arboreal frog
J. Exp. Biol.