Turns out, he bites his arm when he's excited too. For more than a year now he's been taking out his emotions, mostly anger, on his right arm. When he gets mad he bites it. If Violet steals one of his toys and makes a run for it, which she does at least five times a day, instead of trying to take her down like any respectable brother, he vents his frustration by chomping down on his own arm.
I've advised her to enjoy her superior strength while she can. Before long he'll be using farts as weapons and pinning her to the ground, holding her nose until she gasps for air and then spitting in her mouth.
What? Isn't that what brothers do?