Computational protocol: Landing on branches in the frog Trachycephalus resinifictrix (Anura: Hylidae)

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[…] To analyse the high-speed video recordings, we used the landmark tracking plug-in MTrackJ (Meijering et al. ) for the image analysis software Image J 1.48 (available at http://www.imagej.nih.gov/ij/). For each experiment, we adjusted the pixel to cm ratio based on the video recordings of the ruler. Each landing video sequence was then analysed frame by frame, which at a recording frequency of 1000 frames per second resulted in a temporal resolution of 1 ms. For the experimental trials in which the frogs were free hanging at the stick, we tracked the position of the toe pads attached to the stick, to identify sliding movements. However, it turned out that in all experimental trials, the toe pads remained stationary once they are attached and sliding was never observed. For landing trials we traced the position of the rostral tip of the nasal capsule and the position of the toes in lateral view. MTrackJ outputs the x- and y-coordinates of each track of landmarks, as well as the travelled distances, and velocities over time. The velocity data in MTrackJ corresponds to the travelled distance per frame, i.e. per ms. For further analysis, we imported these data into the statistical computation software R 3.1.1 (available at http://www.r-project.org). For each trial we calculated a linear regression of distance over time for times ranging from the onset of the recorded videos until the frogs came in contact with the target. The slope of this regression is a measure of the average velocity of the frogs during the approach, which is different from the frame-by-frame velocities that MTrackJ calculated (Fig. a; for regression statistics see Supplementary Table 1). Further, for trials in which the frogs were attached to the wooden stick with their forelimbs, we further calculated a linear regression of changes in velocity over time for the deceleration phase, i.e. after the frogs made contact with the stick but before oscillation movements occurred (Fig. b; Supplementary Table 2). The slope of this regression is a measure for the deceleration the frogs experienced, which multiplied by the mass of the animals gives an estimation of the forces acting on the toe pads during this phase of landing.Fig. 3 […]

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