De-evolution
It was a tiny change at first.
A warm sort of sensation he got in the lower part of his stomach, when he gazed at the, quite frankly excessive, cleavage belonging to Miss Satchi the headmistress of Jugoria Institute for Gifted Cavortiens. Cavortiens being the natives of the planet he and Donna had decided to land on.
He gazed a bit longer than normal actually; he didn't remember them being so interesting, or bouncy. His head snapped up at the sound of Donnas voice and the thought got tucked to the back of his mind. It was then further tucked away when he found himself having to rescue the young Cavortiens from having their brains sucked out their ears.
The whole thing was comfortably forgotten until a few days later, when he awoke with a start from one of his rare naps, with a strange yet curiously pleasant dream fading into obscurity and something else being decidedly not obscure at all. He stared down at it in chock, eyes widening more and more as he realized what it was. He turned his stare to the ceiling, partly to think and partly out of a self-conscious embarrassment. It wasn't normal, or well… not normal to him. Humans, Gyratans even the Tunguars if you weren't too fuzzy about the distinctions. Lots of races had this particular anatomic function, but not Time Lords, not any more at least.
He looked back down and sighed in relief at lack of any further… functioning. Maybe it was just a freak occurrence, a chemical miss-fire in his brain in response to a subconscious conception of unrelated fantasies.
He could almost hear the screech as his mind stalled in an attempt to come to terms with the wording of that particular thought.
Bad broth, he nodded to himself, definitely that dodgy looking broth he had admonished Donna for not daring to try at the Occarin space-station yesterday. Some foods could seriously muck up perfectly good brain chemistry after all.
In the following weeks he ignored how lewd propositions from a 1600th century Italian working girl, made his mouth go suddenly dry. He explained away the tingly feeling he sometimes got when thinking of Rose, as old age sentimentality. He outright refused to admit to himself, or Donna, that he had been "checking out" Lady Carrington's bum.
But when the sight of Donna, bent over, erecting herself after yet another rough landing, brought a very un-gentlemanly idea to the forefront of his mind, he could take it no more.
"I've gone mad." He mumbled.
"Whot?" Donna looked up and saw the Doctor facing the time-rotor, wide, nearly panicky, eyes staring into space and a magnificent blush creeping up his face.
He stood like that for a while, lost in thought until Donna got tired of waiting for him to answer.
"Oi! What is it?" His eyes snapped to Donna and his face fell as the receding blush made a near instant comeback.
"Er... Nothing! Nothing at all. So, where would you like to go next?" He asked while running laps around the console, pressing buttons as he went. "I was thinking Paris. Early spring your time. Bit o' shopping, bit o' sightseeing. I bet you always wanted to see the Louvre? Eh? Am I right?" His smile faltered as Donnas incredulous stare bore into him.
"What?" He squeaked.
"You really are bonkers! We just landed, what is it with you?"
"Nothing, I just... I just thought you might like to see Paris. And then I could stay in the TARDIS and fix that... that temporal under-link connection." He swallowed once, and tried his very best to look innocent.
"Oh. Alright, well I suppose we could do that." She mumbled, refusing to look him in the eye. The Doctor steeled himself against the guilt and put the TARDIS into motion, He had bigger concerns.
Donna lasted a full three minutes before she cracked.
"Am I bothering you? Is that it?"
"What?"
"One second it's all 'oh! Let's go to Gost and ride giant rabbits!'"
"Hamsters."
"Wha'ever! And the next second you want 'o dump me off in Paris!"
"I didn't mean it like..."
"If I'm annoying you so much then just tell me! I don't mind giving you a few hours to yourself." Her throat clenched painfully and her eyes went misty. The Doctor sighed and let the TARDIS glide around in the vortex for a while.
"You are not annoying me Donna, I swear." He said earnestly, giving her his full attention.
"Then what is it? What are you not tellin' me?" His eyes shifted away for a second as he rubbed the back of his neck, looking quite awkward.
"Is it... something embarrassing?" She asked, humour sneaking its way into her voice.
"No." He was blushing again, he just knew it. "It happens to be a perfectly normal biological reaction, except..." She urged him on with nods and a hand movement, "Except I'm not supposed to have them. Sort of." Donna thought it over for a second.
"You're not making sense." He sighed and wished to be anywhere in the universe but here.
"I'm getting these... My brain is releasing these weird chemicals or combinations of chemicals really. And it shouldn't, it hasn't, ever!" He started pacing and waving his arms about, so Donna sat down and got comfortable, knowing full well that a rant was in progress. "Now humans, humans get them all the time, your species need them. If you didn't you'd die out. But time lords don't, not for millennia now; in the end it just wasn't necessary. So it was weeded out." Donnas brow furrowed as the meaning of his words slowly dawned on her. "But now here I am, my brain's going wonky and I'm having thoughts Donna! Rude and... And... Really rude thoughts! It's not funny!" Donna shook her head in compliance and bit down on the inside of her cheek. After a few deep breaths she managed to keep the glee down to a minimum.
"So what you're saying...?" Was how far she got before she had to fight off another bout of the giggles. "What you're saying is that something has... oh... has turned on your libido?"
"...you could say that." Donna fell to her side on the bench, arms wrapped around her stomach and shaking with laughter.
"It's not funny Donna! My brain is literally regressing!" His only response was a strained intake of air, followed by more laughter. The Doctor huffed and went to fiddle with one of the monitors, enveloped in an air of wounded pride.
A few minutes later, Donna had managed to pull herself together and even succeeded in feeling a little guilty.
"I'm sorry. But you have to admit, it's a little funny."
"Donna I'm turning primitive! How is that funny?" If hadn't been for the completely pathetic look in his eyes, she would have started laughing all over again.
"Sorry. But why's it happening?"
"I'm not sure, maybe a poison or an infection except I already checked for those." Donna looked thoughtful for a moment.
"You know how a dog tends to become more and more like its owner?" She asked.
The Doctor really hadn't but he thought he got the gist of what she meant, so he just nodded.
"It could be something like that, I mean how long have you been around humans anyway?"
"Oh about three, four hundred years, give or take."
"Well then we're bound to rub off on you right? Hang around someone long enough and you start to think like them." In a way it sort of made sense actually.
"Hold on. Are you calling me a dog?"
"You do remind me of this black lab I had once..." She said and followed it up with a smirk. Having had quite enough mockery for one day, thank you, the Doctor decided not to take the bait this time.
"Anyway, it can't be that, the chemical changes are too extensive to have purely environmental causes." Donna rubbed her eyes and sighed.
"Is it dangerous, whatever it is?"
"Oh I doubt it, 's been happening for weeks now."
"Then can we just go ride the giant rabbits-"
"Hamsters."
"Wha'ever. And just worry about it later?"
"Sounds like a plan." He beamed as he moved to the console, more than happy to let the whole embarrassing matter lie for a while.
"But just so you know spaceman, I catch you ogling my arse, I'll smack you."
"Got it." A beat passed. "What about the cleavage?"