Shapeshifters and Body Mass

If your shapeshifter creatures are real, not magic, one problem you need to consider is the physical logistics. If a man can change into a mouse and back to a man, can the mouse weight less than a pound and a man weight much more?

In a magical fantasy, that is acceptable since magic by its nature defies the laws of nature and physics, but if you've created a real race of werewolves that exist in this world with its natural laws, you can't make the weight go away and come back.

According to the laws of physics, mass can be lost, but it can't be regained in a closed system.

If a man turns into a wolf, the wolf will weigh just a little less than the man weighed because some of the weight was burned off as the energy needed to make the physical change. Think of it as the energy equivalent of running a marathon. To change back into the man, even more energy would be burned.

The question of mass makes for interesting possibilities in a story. In my short story, "The Werewolf Whisperer," the protagonist is trapped in wolf form in an animal shelter. The only way out of the shelter requires his being neutered. The change from wolf to man heals most injuries, but it can't replace lost mass. If he's neutered as a wolf, he will be neutered as a human.

My protagonist definitely doesn't want that to happen so he must find another way out of the shelter.

Here's other interesting questions to consider in your stories. If mass is burned in the change, and the wolf or man is totally lean with nothing in his stomach, where will the burned mass come from?

Doesn't that mean that somehow the wolf must have a full stomach before the change comes, or he could die of starvation, be very weak from loss of muscle, or be crippled somehow?

If a man changes into a dragon, how big will his wings have to be to handle an adult human male's weight?

As I said, interesting possibilities. Logical worldbuilding offers more interesting possibilities than sloppy worldbuilding ever will. Take the extra effort to think out all aspects of your creatures' existence, and you'll have a better story.