**Inspired by lovely art by IvoryLungs (which you can find on my AO3 account of this story).

Sherlock leaned carefully around his microscope and squeezed out .03 microliters of solution into the petri dish. It would vaporize over the next six hours, and hopefully that would do it. He had faith in his research, and his calculations had been meticulously checked and double checked. He cast a quick eye over the cell recombination presently occurring under the 120x magnification lens of the microscope. Perfect.

He stood and stretched, feeling four lumbar vertebrae pop back into alignment as he twisted his back. He'd been at this impromptu kitchen version of a lab bench for... hmmm. Had it really been 39 hours? Suddenly concerned, he whirled around and darted back into his darkened bedroom, leaving the door widely ajar to catch the kitchen light.

The wire cage on his dresser still contained its tiny captive, properly upside down, hanging from thankfully living feet. Nyctalus noctula, more commonly known as the Greater Noctule, the largest species of bat to be found in London. Even so, it was smaller than the palm of his hand, a miniature creature, weighing only 32.64g after it's last meal of moths (18 hours earlier), with a wingspan of 35.46cm. He was an agreeable fellow, still groggy with semi-hibernation. It had been a simple matter to wander Regent's Park, find an appropriate tree, and pluck the specimen from among his sleeping companions in an old woodpecker hole. Really, it'd been ridiculously easy: Sherlock would have gone to much more trouble and expense.

The DNA Sherlock was using for his recombinant experiments was from a recently deceased army officer. The military had DNA on file for all their soldiers, but Sherlock thought there'd be less fuss if he 'borrowed' from someone who couldn't complain about it afterward. Of course, since he had taken a sample of the stored DNA without anyone's knowledge or consent, that was unlikely to happen in any case.

He returned to the kitchen to get the petri dish, settled it on a heating pad that maintained a consistent 21.5C (which would adequately vaporize his precious solution), balanced the wire mouse cage over it (litter tray insert removed, so the gasses could get through), and covered the entire thing with a glass bell. He made sure the seal was tight, and then went to prepare himself a much needed cup of tea. Six hours. Enough oxygen to maintain the hibernating bat, so he wouldn't have to worry about that. He sat at the foot of his bed, drew his knees up into the circle of his arms, and rested his chin on them. His notebook was ready at his side. Now, nothing to do but wait and observe.

There was no noticeable difference after the six hours. Sherlock was neither surprised nor disappointed. Cell metamorphosis took time. This was only the catalyst. Hopefully, change would coincide with spring, when the mostly sleeping bat would liven up. He stroked a finger thoughtfully across the fragile thing, feeling the aggressive heat it generated, the soft bristle of fur, the insubstantial bones and velvety wings. "I'll call you John," he said. Naming it after its DNA donor, Capt. John Watson of the RAMC, seemed appropriate.

Over the next month, the thatch of hair on the crest of the bat's head lightened, until it was an ashy brown, almost blond. Thirty eight more days passed and Sherlock recorded the regression and flattening of its snout, noted sharp teeth filling in and becoming more blunted. The facial area lost hair, revealed skin smooth and golden. Teeny, pinprick black eyes widened, changed to deep blue, and migrated closer to the now distinctly human nose (which, although perhaps a bit large, Sherlock thought unique and personable). The single digit the bat had originally begun with, a "thumb" at the final joint of his wing, now had been joined by delicate clawed fingers, forming a tiny brown hand which was quite dexterous.

Sherlock's fascinating new chimera would stir every few days, and he was scrupulous about keeping a good supply of moths available. However, as its features shifted, so did its diet. Sherlock had to provide a variety, not quite sure what would take. Fruit, rice, leftover takeaway, even bacon all seemed to be acceptable to the changing bat. "John," Sherlock reminded himself, finishing up with his notes for the day. "John."

John looked up from the corner where he crouched, eyes bright and ears twitching. He stuffed a last piece of curry into his mouth and crawled to the front of the cage, climbing swiftly up the bars to look at Sherlock.

"You know me, don't you, John," Sherlock said smugly. Part of his protocol was to handle his experiment frequently, sometimes as often as once per hour, to encourage familiarity. Since it was still rather sluggish, he had no qualms about leaving the cage door down when the bedroom was secure, as it was now. John pulled himself through the opening and stared solemnly back at Sherlock.

Sherlock held up his finger in what he hoped was an inviting fashion. "Come on, then John. Don't delay. Come investigate if you're curious."

John gave him a very human look, and in a flurry, launched himself across the room. He was clinging to Sherlock's finger in less time than it takes to describe it. His little palms were hot; his whole body radiated heat, and Sherlock questioned the efficiency of a metabolism that expelled that much energy. He brought his arm slowly up in front of his face, until it was close enough that each exhale ruffled the blond hairs on John's head.

John remained relaxed. He released Sherlock's finger with one dainty paw and groomed around his mouth, hair and ears until he felt satisfied that he was clean. His eyes only left Sherlock's when obscured by a passing forearm. Sherlock used his other hand to gently pet along the bat's back. "You're so strong, John," he said. He'd become accustomed to speaking aloud over the past few months, keeping the bat company and noting its progress.

"You look so delicate. Well, you are delicate: your bones are like toothpicks. But you've a perfect design. You're fast, your hearing is among the most sensitive in the animal kingdom, you can shift momentum with no delay, fly against a strong wind... A flying mammal. You're evolutionary perfection, is what you are. Or, at least... now you will be." Sherlock indulged in no false modesty about his own skills as a researcher and scientist. "A flying mammal with a human brain." John emitted one of the high trilling noises that was part of his repertoire. It sounded almost like birdsong; and although Sherlock's face remained expressionless, his eyes reflected a smile.

"Go explore, then. As you seem to be waking up for the spring." He gave his finger a courteous shake and John took off, careering erratically around the room. Sherlock snapped off the desk lamp and lay back against his pillows. Now the room was dark, except for a dim sliver of light through the crack in his curtains. He couldn't see anything, but John was clearly doing just fine.

The soft fluttering continued to sound from rapidly changing points, and Sherlock could just barely discern the high pitched squeaks as John mapped the room with echolocation. Sherlock lowered the pillow and curled on his side, making mental notes, since he could neither see to observe nor to write. After an hour or so, the flapping paused, and faint scratching noises indicated that John was on the blanket near Sherlock's elbow. The trill came again, and Sherlock huffed.

"I'm glad you're happy, John," he murmured, a very deep, vast counterpoint to the bat's thin soprano. Little pinpricks heralded John's arrival on his dressing gown sleeve, and the miniature beast crawled up his arm to his shoulder, trilling and clicking, feather-light. Sherlock actually laughed, when it reached his hair. "It is a good thing I do not subscribe to those old wives' tales regarding bats and hair," he whispered. John emitted a series of short, staccato cries, and nipped sharply at Sherlock's ear.

Sherlock grimaced and gently cupped his hand around the tiny body. It stilled, but didn't struggle, and allowed him to fold it in his fingers and carefully pull it out of his hair, stretching curls straight before they bounced softly back. A lilliputian heart trembled under Sherlock's fingertips, and he brushed the contented little bat against his lips before carefully releasing it onto the pillow next to his face.

John crept forward and stretched up to rub his nose against the philtrum under Sherlock's own, paws pricking on the bow of his lip. Then he settled down on the pillow near that mouth, cozy and snug in the flow of sultry breath and soothed by the company. After all, he was accustomed to having a clan tightly packed around him. Sherlock stroked him, comforting and calm, and they both drifted off.