Organization of vertebrate annual cycles: implications for control mechanisms

Abstract

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.

Optimal annual routines: behaviour in the context of physiology and ecology

“…More generally, we can investigate whether we expect to observe distinct life-cycle stages (cf. Wingfield 2008).…”

Hormonal mechanisms of cooperative behaviour

“…A single hormone may have effects on many different aspects of behaviour, which may depend on social context and the life-history stage (e.g. Wingfield 2008); a hormone may also have pleiotropic effects by affecting many different traits at the same time, thereby acting as the proximate mediator of functional trade-offs (for review, e.g. Lessells 2008).…”

Recent advances in reproductive neuroendocrinology: a role for RFamide peptides in seasonal reproduction?

“…paracrine) secretion of these peptides. Uncovering how these peptidergic systems respond moment to moment to relevant signals at multiple levels in the HPG axis will further elucidate their role in allowing for precisely timed reproductive events ( Wingfield 2008).…”

“…The effects of photoperiod on HPG axis activity have been well established in seasonal breeders (for review see Dawson et al (2001) and Goldman (2001)); however, the neuroendocrine signals relaying photoperiod and additional environmental signals (e.g. food availability) to the HPG axis remain less well understood ( Wingfield 2008).…”