How do organisms really move, and why?

Animals must move in order to eat and mate. They may also need to escape their predators. The details of their movement may depend on many factors such as climate, temperature, concentrations of pheromones, or the local density of other organisms (including humans) [172, 365]. Although such factors may affect the velocity, sinuosity, or specific trajectory taken, they do not change the primary reasons underlying the movement: the biological necessity of interactions or “encounters” with other organisms.

Given the ubiquity of moving organisms, a number of important questions arise naturally. For example, the priority in the order of driving factors that determine animal movement is not yet well understood and may even depend on the specific activity an organism is performing at a given time. However, there has been progress in understanding how organisms move, i.e., what patterns the trajectories follow. This is the main focus of this book. As an illustrative example, Figure 1.1 shows how spider monkeys move in the Yucatan Peninsula when they are allowed to roam freely. What factors determine the shape and the statistical properties of such trajectories? If we know the answers to these questions, we can venture beyond phenomenological descriptions and ask about causation: for a specific species of organism, why do the organisms actually move as they do? Cui bono, i.e., what advantage or benefit do they gain from such behavior?