Hmmmn...how to explain. Aha!
Okay, you know that humans NEVER use 100% of their brain at once? Seriously. Using more than 60% of your brain (JUST the 'thinking' part, not counting the sensory areas and the centres of balance and symbol/word recognition) at a time is what doctors call "epileptic seizure".
I think that when they morph an animal, they use the parts of the brain that aren't occupied at the time. NOTHING uses all its brain at once. EVER.
So...what do you do when there is no brain to use?