Phenotypic flexibility of the avian gizzard: Rapid, reversible and repeated chances of organ size in response to changes in dietary fibre content

Evolutionary biology presumes that organ capacities match their natural loa ds. Therefore, in fluctuating conditions, organ systems are expected to sho w a reversible, repeatable and rapid phenotypic response that is directiona l and scaled. In this study, phenotypic responses of the gizzard of adult J apanese quail (Coturnix japonica) to experimental mismatches of load and ca pacity 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 of the hypothesis that there is matching between loads and capacities. (1) The observed pheno typic 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 ND F was followed by a decrease of gizzard size. (3) Phenotypic responses coul d be elicited repeatedly in three successive trials. (4) Excess capacities were downregulated and insufficient capacities mere upregulated. (5) The re sponses followed changes of loads with almost no time lag, with size change s measurable within 24 h.