How to show authors short username with git log?

I try to get the username of the last commit of a repository using
git log -n 1 --pretty="%an"

Some entries in
git log --pretty="%an" | uniq
shows a full username not the short login name of a commiter. I expect the git/github username.

Does there exist a way to force git log to show short author names and not a a maped full name?

Git Baby is a git and github fan, let's start git clone.