Derrick focused on the slick black cunt and reached out to slide his fingers into it. The bear reacted with a grunt and lay down with him still upside-down in her powerful forelegs. He gasped as a rough tongue started licking his genitals, but reached deep in her crotch to continue teasing her sex. Her cunt got slicker, but then his hand started to feel oddly swollen. He pulled it back and stared at the massive brown paw that was now his. It was slick with her fluid, and he found himself licking it and smearing the fluids across his face. There was pressure in his nose and jaws, then a growing mass between his eyes as his lower face pushed out into a muzzle. He was coming erect as the female continued to lick his groin, and the organs there felt quite different.
The female released him, and he twisted to look at himself - a doglike sheathed cock sat between furry, thick thighs. His legs were changing as well, and his belly was pushing out. The female grunted and moved to all fours, facing away from him, and he saw her welcoming cunt glistening. He was pounding erect, and decided he would take full advantage of this first sexual experience. Happily, he knew what to do - not so much from anything he had read or seen in the past, but from a more basic instinct that grew stronger. He was becoming a mature male bear, and that change was brining with it the beast's mentality.
He moved to cover the female, still too small for proper mating, but managed to get the tip of his cock in the right opening. She grunted and shifted back, and between the two of them, they managed to start coupling. Derrick's body swelled out and he rapidly became more than a match for the female, though any sense of transformation was overwhelmed by the primal need to mate. By the time he felt himself building to climax, Derrick had completed the change to a male bear, and he bellowed with a fully animal roar when his seed jetted into the female.
They stayed coupled for a short time, and he humped her mindlessly until they finally separated and lay beside each other, panting. They licked each other instinctively, and then slept.
Derrick woke and stretched out, grunting in satisfaction. It took a while for him to remember what had happened, and he sat up to find he was still a massive male bear. The female was gone - did they stay together? Maybe not. Still, he had a strong sense of satisfaction and somehow knew he had impregnated the female. What now? He rose and shook himself, then wandered out to find food. Berries were good, as were acorns he found on the ground. A creek provided cold water. The bear wandered the forest, occasionally catching other scents, but not encountering any other large creatures. No one wanted to mess with a bear. It wasn't until the sun was starting to set that Derrick realized he had spent the whole day foraging and wandering without any human thoughts at all! It was sorta cool in a way, just being a bear. But maybe he did not want to spend his whole vacation eating berries and scratching at fleas.