The majority of vertebrates have a life span of greater than one year. Therefore individuals must be able to adapt to the annual cycle of changing conditions by adjusting morphology, physiology and behaviour. Phenotypic flexibility, in which an individual switches from one life history stage to another, is one way to maximize fitness in a changing environment. When environmental variation is low, few life history stages are needed. If environmental variation is large, there are more life history stages. Each life history stage has a characteristic set of sub-stages that can be expressed in various combinations and patterns to determine state at any point in the life of the individual. Thus individuals have a finite number of states that can be expressed over the spectrum of environmental conditions in their life spans. Life history stages have three phases-development, mature capability (when characteristic sub-stages can be expressed) and termination. Expression of a stage is time dependent (probably a minimum of one month), and termination of one stage overlaps development of the next stage. It follows that the more life history stages an individual expresses, the less flexibility it will have in timing those stages. Having fewer life history stages increases flexibility in timing, but less tolerance of variation in environmental conditions. To varying degrees it is possible to overlap mature capability of some life history stages to effectively reduce 'finite stage diversity' and maximize flexibility in timing. Theoretical ways by which this can be done, and the implications for neuroendocrine and endocrine control mechanisms are discussed. Twelve testable hypotheses are posed that relate directly to control mechanisms.
