If you don’t have expressive lips, I guess perky ears are a viable alternative.
This is a serious study.
Don’t you wonder how it compares with dog-ear-smile studies? For surely those are out there, too.
This is the tickling technique. We are left to assume rats like to be tickled.
And the alternative tickling technique. (What happened to the “b” that should be in the corner of that photo?)
For those of you fascinated with this subject, click on the following link to view the study.