You're becoming a gorilla!
You can tell by the heavy brow forming on your forehead. Your nose is flattening, and your skin is darkening. You feel a terrible pain in all your joints, and you see yourself growing taller. In the mirrored ceiling, you see the restraints that were at your wrists have moved almost up to your elbows, and the ones around your calves have dropped to near your ankles. Your head which had been 10" from the end of the table is now partly hanging over the edge. You itch terribly as black hairs rapidly grow to cover most of your body. Smooth leathery patches grow and harden on your face chest and the soles of your paws.
You enjoy the power that your new size gives you, even if you are restrained. You look toward your cock and balls expecting a growth spurt instead they seem to be getting smaller!