The effect of food restriction on morphological and metabolic development in two lines of growing Japanese quail chicks

To investigate whether there is a relationship between the inherent maximum growth rate of a chick and its degree of developmental plasticity, we studied the effects of food restriction on morphological and physiological development in P-line (selected for high growth rates) and C-line (nonselected) chicks of the Japanese quail between 2 and 64 d of age. In each line, three groups were created; one group in which the chicks were fed ad lib. and two other groups in which the chicks received 70% and 40% of the line-specific ad lib. feeding levels, respectively. In all three groups, chick survival rates were very high, and no relationship was detected between feeding level and chick mortality. The experimental feeding levels strongly affected the chicks' attainment of body mass at all ages. In addition, at most ages the growth of the structural components (wing, tarsus, and head) were also significantly affected by the feeding experiment but to a much smaller extent than the body mass. The feeding levels also affected the achievement of female sexual maturity such that, at the end of the experiment, most of the ad lib.-fed females and none of the females that received 40% of the ad lib. feeding level were laying eggs. In chicks of both lines that were fed ad lib., homeothermy was achieved at 7 d of age, but in chicks that received 40% of the line-specific ad lib. levels, homeothermy was achieved after 15-22 d of age. Poikilothermic chicks (P- and C-line) that were relatively heavy for their age exhibited relatively high resting metabolic rates (RMRs) and peak metabolic rates (PMRs) for their age. In addition, a positive relationship was found between the chicks' relative mass and RMR (based on mass-based predictive equations). No relationship was detected between the chicks' relative body mass and their relative PMR (relative to mass-based predictive equations). With respect to the metabolic scope (i.e., PMR-RMR), Japanese quail chicks exhibited remarkable developmental plasticity: chicks with body masses that were 50% lower than the normal growing chicks did exhibit a normal capacity of thermogenic heat production for their mass. Because no differences were found with respect to the level of plasticity of morphological and physiological development between the two strains, we conclude that the selection for high postnatal growth rates in P-strain chicks has not resulted in a in a higher vulnerability of these chicks to food restrictions.