There were just some things they didn't talk about in front of April.
Or Casey; though Raphael had regaled him with enough reptilian gross-out stories that he knew most of it by now. But there were things they didn't discuss openly with any mammal except Splinter.
Take basking, for instance. They'd actually discussed basking in front of April once, innocently going back and forth on optimal temperature and relative decrease in aggression and typical length of bask. And out of the corners of their eyes they'd seen her face morph from friendly insider to bewildered outsider, and they had changed the subject. That look had been on her face often enough when she'd first met them; they didn't want to see it there so far into their friendship.
After that they had, by unspoken agreement, never brought up anything else to do with being a turtle. It was fine that she never asked about the dark, irregular patches on their shells, because they didn't want to discuss shell rot with her anyway. And they tried to avoid crowding her on the couch during movies, even though she was the warmest thing in the lair besides Splinter, who didn't mind, and the sun lamp, see above. They couldn't help her finding out about their superhuman (but rather sub-turtle) lung capacity eventually, but they tried not to bring it up much.
Casey thought the whole thing was hilarious. He took as many opportunities as he could get to say just the wrong thing in front of April, something they endured with less than perfect grace. But they never did forgive him for the time he brought up churring.
It wasn't something normal turtles did. They'd all asked Donatello this, quietly and on their own. He'd had no answers for them, though not for lack of trying. But come spring, every year since they were eleven or so, something about the season tore loose a small part of their reason and they all went a little crazy.
They were better about it now. They recognized that the season had come upon them and made a special effort not to be snappish with each other. Splinter quietly adjusted the training regimen to include far less sparring during this season than he normally did, and they coped. Their first mating season with April in their lives, though, was almost worse than that horrific first one when they'd thought they were all going crazy. Because this year, April was not only a part of their lives, she was, at the moment, living with them.
They would never begrudge her the space. She was like family to them, and they were kind of responsible for her place being out of commission at the moment. Besides, they were men now, and they could handle themselves just fine with a woman around, even during spring. What they weren't sure they could handle was Casey Jones.
The first time it happened none of them were sure what had happened. They had been sitting around watching some horror flick during movie night, when right in the middle of an especially tense part came a sound: a sound so hardwired into their brains that they were out of their seats and glancing wildly around the room before they'd had time to register what had happened. Then came another sound, much less pleasant: Casey's loud, uncontrollable laughter as he threw himself off the couch and onto the floor, howling.
April had gotten up and gone into the kitchen then, and if her look told them they were all crazy, well, at least Casey had been included in that look. Raph had punched Casey in the head and told him never to do that again, but Casey was still laughing, and it took the four of them to haul him into the bathroom and hold him under the shower until his howling changed its note to one of indignation. Then, in the relative privacy of that room, they had all joined Raph in handing out death threats to any black-haired, hockey stick-wielding vigilante who dared try that stunt again. Casey had promised, on his Scout's honor, looking offended at their skepticism that he had ever been a Boy Scout, though they'd gotten him to admit that he'd only joined for the camping and had been kicked out after a month.
Still, they weren't surprised the next time they heard the noise. Or the next. Or the next. The fact that Casey only did it when April was around made it that much worse; they couldn't wale on him too hard because then April would want to know why. She still thought it was just an annoying noise, and they wanted to keep it that way.
"How does he even know about that?" Donatello asked Raph one day when the four of them were out, far away from any human likely to embarrass them either on purpose or simply by being there. Raph looked away.
"I… told him once. That bonehead actually did some research about turtles and he wanted to know if spring really was… Anyway, we… may have been a little wasted at the time."
They rolled their eyes. It was one of the few things they kept from Splinter, Raph's late-night carousing with Casey, partly because taking care of drunks was one area in which Casey was actually responsible and partly because Raph was, inexplicably, a pretty docile drunk. They had agreed to keep Raph's secret on the condition that Casey pass along any funny stories, which actually served to keep the drinking to a minimum because in addition to being a docile drunk, Raph was also a very hilarious drunk. Hilarious to his siblings, anyway.
"That doesn't explain how he can imitate it so well," Mikey pointed out. His brothers swung around and stared at Raph, who glowered and swung at the wall. "Raaaaph—" Mikey began.
"He bribed me, okay?" Raph exploded, his face bright. "He wouldn't effing leave it alone until I did it. Just the noise!" he added, whirling around and jabbing a finger in the direction of his brothers, who had definitely not been about to say anything. Nope. Not a word. Raph relaxed marginally, withdrawing the finger.
"What did he bribe you with?" Mikey asked innocently. Raph smacked him, which meant Mikey pestered him about it for the rest of the night. (He eventually revealed that it had been more alcohol, which had lowered his little brother's esteem of his intelligence, earning said little brother another smack upside the head.)
Things came to a head eventually. April finally turned to Casey after another noise right in the middle of a movie and demanded an explanation.
"It's churring," Casey announced, looking pleased with himself. April lifted an eyebrow.
"Yeah, churring. It's, like, their mating call." A flick of his hand to indicate the turtles, who had frozen in place.
April's eyebrow was as high as it could go. Casey was having a hard time keeping a straight face, especially in light of the murderous glares he was getting from four parts of the room.
"A mating call."
"A mutant turtle mating call."
"And that's the sound you've been making for the past three weeks."
She regarded him a moment longer, and then smiled a thin, dangerous smile.
"Wow, Casey, I didn't know you felt that way."
Casey turned bright red and scratched the back of his head, grinning idiotically.
"Ehehe, well, you know, I mean…"
"But, now that I know, I won't get in your way." She turned to walk out of the lair. "Be gentle with him, boys."
"Oh, we will, April," Raph assured her, advancing menacingly on the now horrified man. "We will."
Yes, there were some things they didn't talk about in front of April. But there were some things that just didn't need to be said.
AN: I'm not an expert, but I'm pretty sure normal turtles don't churr. If I'm wrong, please let me know, but for now I'm just going to point and laugh at all the smutfic with churring in it.
Concept shamelessly stolen from Amicitia's fic Labyrinth, chapter 4.