Computer simulation. A–D , In each set, left four panels are examples of tracks of real animals ( A ) or simulated animals ( B–D ). See Materials and Methods for selection of representative tracks. The right-most panels show density plots of the animals' occurrence at each point. The darkness of each pixel is proportional to the fraction of time spent by an animal in that pixel relative to the total time, so that points frequently visited by the animals are darker. Results of the following numbers of animals were pooled. A , Real animals (n = 67). B , Computer simulation with the pirouette mechanism alone (n = 500). C , Computer simulation with the weathervane mechanism alone (n = 500). D , Computer simulation with both mechanisms (n = 500).