The sea pen leads a simple life. After floating on the deep-sea currents as a juvenile, it settles down onto a comfortable patch of sand and begins its largely immobile adult life, growing into a feathery shape and swaying in the water while ensnaring whatever edible morsels pass its way. It hardly moves on its own; it just passively filters the world that goes by. No choices need be made, for there are no actions to take. As such, the sea pen will not feature prominently in this book.

For other more active (and more behaviourally interesting) species, life presents a stream of decision points, at which actions must be chosen: stay or move on, ingest or pass by, approach or avoid, wait or strike, court/accept or decline. These are all essentially forms of the exploitation/exploration trade-off that organisms must balance throughout their lives, whenever resources are distributed in space or time and the individual can actively seek them. This is the realm of natural action selection. How do organisms do it?