NDVI and elevational (y-axis, meters) migration of the ten elephants equipped with GPS-collars in the Marsabit area. Color map in the background shows NDVI as a function of height and time (x-axis, in days since the start of the experiment). Dots show the observed elevations of the elephants and the bold lines show the predictions of 3 models. Red line marks the pure surf model (Table 1 model 1), black line marks the individual surf model (Table 1 model 2) and cyan line marks the most justified model we found, which include linear modification of a single "Surf"-NDVI with linear effects of positive and negative temporal changes to NDVI (Table 1, Model 8). Each modeled individual is shown is a different panel. The bottom right panel shows the overall goodness of fit of two of the models, model 8 in cyan and model 2 in black.