[immibis]

Note that none of the known laws of physics say that excess energy is saved by converting (which raw materials?) into fat and storing them.

[grecy]

Sure, a mammal does not HAVE to store excess energy.

But that is entirely irrelevant when a mammal eats less energy than they consume for a while. In that instance they MUST burn some stored reserves or break the laws of physics

[immibis]

An equally plausible hypothesis is that the mammal feels lethargic and does not consume more energy than it eats.