As well as their long legs, a particularly striking feature of a frog’s skeletal anatomy is a sharp bend in their lower back. Underlying this bend is the ilio-sacral (IS) joint – a hinge-like pivot which allows the frog to control the angle between its upper and lower body. The IS joint is folded when a frog is sitting at rest to allow the frog to crouch closely to its perch. During the explosive early moments of a jump, a frog’s muscles extend the IS joint to rapidly straighten the back. Until now, biologists believe the IS hinge was an essential part of the anatomy, allowing a force transfer between the upper and lower body of the frog which enabled the jump.

However, a recent study conducted by the Royal veterinary College (RVC) was able to find that, contrary to prior understanding, this IS extension is not required for jumping but more of an evolutionary innovation for fine-tuning a frog’s jump performance.

The research team, which consisted of Dr Christopher Richards, Dr Amber Collings and Enrico Eberhard, developed a simulation of a frog jumping using 3D computer technology. This simulation model was unique because it allowed the researchers to calculate the frog’s push off force from the ground, rather than directly measuring it. From further analysis the team was also able to isolate the influence of the IS joint by changing its action, whilst maintaining the consistency of all other body motions.

This research was funded by a European Research Council Starting Grant and will feature in a new paper published this week in the Royal Society journal Biology Letters.

Dr. Christopher Richards, Research Fellow in Paleorobotics at the RVC, said:

“We are particularly excited about this study because we think our methods open new doors for exploring how specific anatomical features affect animal locomotion.”