[Kromm]

The reason why small animals seem to evade well is because, usually, we're trying to grab them. SM applies as usual when grappling; if you try to grapple a mouse, you suffer its SM on your attack roll just as you would on any attack roll, and you grapple at -9 or so. Even if you succeed, the mouse may dodge. In plain English, the mouse has "evaded" you if you fail. However, it hasn't evaded you in GURPS terms -- you just missed, or it dodged. The mouse would only have evaded you in game terms if it had tried to run over your body or between your legs and you had failed to swat it back.