Endocrine control of life-cycle stages: A constraint on response to the environment?

Most organisms live in seasonal environments that fluctuate on a predictabl e schedule and sometimes unpredictably. Individuals must, therefore, adjust so as to maximize their survival and reproductive success over a wide rang e of environmental conditions. In birds, as in other vertebrates, endocrine secretions regulate morphological, physiological, and behavioral changes i n anticipation of future events. The individual thus prepares for predictab le fluctuations in its environment by changing life-cycle stages. We have a pplied finite-stale machine theory to define and compare different life-his tory cycles. The ability of birds to respond to predictable and unpredictab le regimes of environmental variation may be constrained by the adaptabilit y of their endocrine control systems. We have applied several theoretical a pproaches to natural history data of birds to compare the complexity of lif e cycles, the degree of plasticity of timing of stages within the cycle, an d to determine whether endocrine control mechanisms influence the way birds respond to their environments. The interactions of environmental cues on t he timing of life-history stages are not uniform in all populations. Taking the reproductive life-history stage as an example, arctic birds that have short breeding seasons in severe environments appear to use one reliable en vironmental cue to time reproduction and they ignore other factors. Birds h aving longer breeding seasons exhibit greater plasticity of onset and termi nation and appear to integrate several environmental cues. Theoretical appr oaches may allow us to predict how individuals respond to their environment at the proximate level and, conversely, predict how constraints imposed by endocrine control systems may limit the complexity of life cycles.