Hi Ross, these are very lively little drawings. It took me a sec to get the "mouse" meaning. I was looking for them to look like mice LOL. Maybe it was too early in the morning.
I think the one on the right it the more successful of the two. The one on the left has some problem around the eyes and a little around the nose. Quick sketches are allowed to look sketchy, of course, but notice how the sketchiness enhances the feeling of the hair and works very well on the chin and jaw. On the eyes and nose the sketchiness seems to fight against the subject. It says "imagined" faces, so I must say, for working with no model, you have the proportions of these faces spot on!
I don't know what program you used, but if you have Gimp, PS or something with such comprehensive settings, why not experiment with a certain brush setting that lets the stroke "fade out". There are also settings that allow "pressure sensitivity", behaving like a real pencil. In this way you can have your thick and thin strokes transition naturally between each other. I'm sure stopping to switch between brush sizes jams up spontaneity a bit.
When I was in school one of our assignments was quick "gesture drawings" of this type. We were allowed 8 second or 15 seconds...The fun part is that we were to draw these with a twig and a bottle of ink. :-D I'd like to see you do lots more of these. You certainly have an eye and a sense. Give yourself the insane time limit.