RAPID REVERSIBLE CHANGES IN ORGAN SIZE AS A COMPONENT OF ADAPTIVE-BEHAVIOR

Organ structures and correlated metabolic features (e.g. basal metabol ic rate) have often been taken as fixed attributes of fully grown indi vidual vertebrates. When measurements of these attributes became avail able they were often used as representative values for the species, di sregarding the specific conditions during which the measurements were made. Evidence is accumulating that the functional size of organs and aspects of the metabolic physiology of an individual may show great fl exibility over timescales of weeks and even days depending on physiolo gical status, environmental conditions and behavioural goals. This fle xibility is a way for animals to cope successfully with a much wider r ange of conditions occurring during various life-cycle events than fix ed metabolic machinery would allow. Such phenotypic flexibility is lik ely to be a common adaptive syndrome, typical of vertebrates living in variable environments.