NAUWELAERTS, S.; AERTS, P.; University of Antwerp, Belgium; University of Antwerp, Belgium: Takeoff and landing forces in jumping frogs

Most morphological adaptations in frogs are interpreted in the context of their propulsive ability. One selection pressure is their escape behaviour. The most common measure of performance for anurans is jumping distance. However, to escape is to realize a large distance between predator and prey, as fast as possible . It is plausible that a high horizontal velocity is more relevant than realizing the longest possible jump. Also, during the phase spent in the air, the trajectory of the frog is predictable, making it easy to intercept the frog. Performing a series of small jumps increases the possibility of changing direction and velocity (i.e. manoeuvrability). An additional complication is that the increase of the takeoff velocity inevitably concurs with a larger impact. During landing, the front limbs touch the ground and form a pivot about which the rotation of the hind limbs and pelvis occurs. Since the forelimbs are considerably shorter than the hind limbs, impact forces on the front limbs are expected to be high. The landing phase could therefore be a limiting factor in the jumping capacity of a frog. We examined the role of the take-off angle on the total jumping distance, the impact velocity and the horizontal velocity through a mathematical model. The three different strategies all have different optimal takeoff angles: jumping distance ~35-55�, impact velocity ~80-90� and horizontal velocity <20�. We also investigated whether a frog can achieve a larger horizontal velocity by using slower jumps with a smaller takeoff angle. This is only the case when recovery time is large and greatly influenced by the impact velocity. In addition to this, we looked at the takeoff and landing forces of jumping Rana esculenta at a range of jumping distances and evaluated the role of the forelimbs in landing. When jumping further, the frog increases only the force component in the direction of the jump, opting for a larger horizontal velocity. The landing forces are almost four times as large as the propulsion forces. The forelimbs have a limited capacity of braking, causing two types of landing force profiles, which are correlated with the size of the landing forces. Recovery time decreases with jumping distance, causing a larger jump to be optimal for escape.