Final answer:
In mouse development, the increase in metabolic heat production is likely due to the maturation of brown fat and an accelerated metabolic rate to maintain constant body temperature amidst their higher surface area-to-volume ratio.
Step-by-step explanation:
Developmentally, during mouse development, the increase in metabolic heat production is most likely due to the development of brown fat and an increase in metabolic rate.
Mice, being small mammals with a high surface area-to-volume ratio, lose heat quickly and thus have a higher metabolic rate than larger animals.
To maintain their body temperature, especially in colder environments, they rely on brown fat, which is rich in mitochondria and specialized for heat production through uncoupling proteins within the mitochondria that allow protons to re-enter the matrix without ATP production. This mechanism generates significant heat, helping to compensate for the rapid heat loss. Furthermore, the shivering reflex and muscle contractions can contribute to heat generation as a byproduct of ATP utilization in muscle actions.