Selection for high growth displaces costs of digestion from SDA to SMR in divergent ecotypes of rainbow trout

To understand the drivers of variation in digestive performance and their effects on growth, we examined relationships among food consumption, digestive metabolism, food processing efficiency, and growth rate in juveniles of fast-growing piscivore versus slow-growing insectivore ecotypes of rainbow trout reared at satiation rations on the same diet (i.e., commercial food pellets). Relative to slow-growing insectivores with lower basal metabolism, and despite a much larger maximum food ration, faster-growing piscivores presented an unexpected pattern of higher digestive efficiency through a reduction in the absolute costs of postprandial metabolism coupled with shorter or similar gut residence time and higher assimilation efficiency. These results suggest that the increase in digestive metabolism following the ingestion of larger meals can be mitigated by displacing the costs of digestion from SDA to SMR. Reducing total digestion costs (SDA + SMR) while maintaining higher assimilation efficiency may be possible through potential adaptations including (1) increased intestinal absorption capacity; (2) economies of scale that shorten gut transit time with increasing ration level; or (3) a permanently larger digestive tract that increases maintenance costs but reduces the need for cyclic upregulation and associated overhead costs.