From the Sketchbook- Amphibian Rhapsody Sketch

The gritted teeth seemed kind of odd especially with all of them doing it at the same time and then looking up that way... as if "nature called" and they were defying that call. The Ninja Turtles were usually gritting their chompers, especially in the first few issues, but for this particular piece that just wouldn't do.  Hence, the mouths were closed.