Donnie experimented on his mind. He did it a lot more than he probably should. One day he couldn't fathom how idiotic his brothers were so he decided to put himself in a little test...

You tried going into his lab but it was locked. A holographic screen popped up and Donnie was on it.

"Greetings. It appears that you are trying to locate me. I am currently engaged in a experiment, albeit rest assured, it is entirely benign. I have implemented ten contingency measures. I have made a serum designed to slowly diminish my intellectual capacity to a level of 68, facilitating a gradual process of cerebral refinement. I have multiple inventions to bring me sustenance for the entirety of the experiment. You will be given instructions on how to get me back to normal if somehow, all the other 9 fail. There will be an observatory box next to my bedroom door. My starting IQ is 195 Signing off, this is Donatello Hamato."

You immediately go into the box and inspect. And there he is. Donnie. He's acting the same but as the recording said, he'll slowly but surely get dumber.

"Day one. No noticable differences." He says to the camera in the room. "I still have all of my intelligence."