Final answer:
The snake will receive less than 1000 units of energy from eating the mouse due to the energy used by the mouse's metabolism and other processes which prevent 100% energy transfer efficiency.
Step-by-step explanation:
If a mouse stores 1000 units of energy in its body, the amount of energy a snake can extract from it when it eats and digests the mouse will be less than 1000 units. This reduction in energy is due to the energy transfer efficiency between trophic levels, which includes the energy the mouse has used for its metabolism and other life processes before being consumed by the snake. Essentially, this is analogous to other ecological relationships where predators cannot gain all the energy their prey had stored. When considering the metabolic rate and energy utilization for warmth or activity, we can infer that not all energy stored in an animal will be passed on to its consumer.
Similarly, in a human context, if someone consumes a food item, such as a 2500 kcal burrito, the energy expended to fuel metabolism results in a mass loss that's negligible compared to the mass of the food. Animals like snakes, which are ectotherms, have a standard metabolic rate that is much lower than endotherms like mice.