Final answer:
Fish can allocate more of their food intake to growth than mammals due to being ectotherms, which means they do not expend energy to heat themselves, and due to the reduced energy expenditure for movement and bodily support afforded by the buoyancy of water. Diverse metabolic strategies also play a role in how different species manage their energy for growth and other functions.
Step-by-step explanation:
Fish can use more of their food intake for growth than mammals, largely because they have different metabolic rates and live in an environment that imposes different energetic demands. In particular, fish do not need to maintain a constant body temperature as mammals do. Mammals are endotherms, which means they must use a significant part of their energy intake to regulate body temperature. Fish, on the other hand, are ectothermic; their body temperature varies with the environment, so they don't expend energy on heating themselves. Additionally, the buoyancy of water reduces the energy fish need to expel on bodily support and movement in contrast to land mammals that expend energy against gravity.
Another factor is related to the different metabolic strategies used by various species to support growth, reproduction, and homeostasis. For example, cold-adaptation mechanisms like increasing unsaturated fatty acids in cell membranes or producing antifreeze proteins indicate varied adaptations in different organisms to manage energy demands in challenging environments.