Evolutionary biology presumes that organ capacities match their natural loads. Therefore, organ systems are expected to show a reversible, repeatable and rapid phenotypic response to fluctuating conditions that is directional and scaled. In this study, phenotypic responses of the gizzard of adult Japanese quails Coturnix japonica to experimental mismatches of load and capacity were tested by a series of diet-switching experiments (involving an increased content of non-digestable fibre, NDF, in the diet). The results of all experiments were in accordance with the predictions made from the hypothesis that there is matching between loads and capacities: (1) the observed phenotypic responses are directional and scaled to the demands, i.e. increasing NDF elicits an increase in gizzard size. When the proportion of NDF in the diet was raised from 1 % to 45%, the gizzard was more than twice as large as in the control group; (2) size responses were reversible, and reduced NDF was followed by a decrease of gizzard size; (3) phenotypic responses could be elicited repeatedly in three successive trials; (4) excess capacities were down-regulated and insufficient capacities were up-regulated; (5) the responses followed changes of loads with almost no time lag, with size changes measurable within 24 hours.
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