Tikki changes, assuming her four-legged form. At the start, she won’t have it
any other way; in the end, it doesn’t matter. What matters is that she’s felt
his teeth gripping the back of her neck, his claws pressuring her hide, that she’s met the animal part of him, felt his power. That’s when she
begins to grasp what is really happening, it has nothing to do with whether she 
likes him or he likes her, nothing to do with love or any other romantic idea. 
It’s something primal and fierce and wholly inexorable, reaching into their
animal cores as forcefully as the moon, and yet also grazing their higher 
natures, their minds, their emotions, till they feel almost welded together, two 
halves of a single creature.