Garrus sizes Kaidan up, trying to get a read off him. They could have this standoff all night and it wouldn't do them any good. More than that, Kaidan is a biotic. He seems serious about this weird feeling of his, and if Shepard trusted him-- that's enough for Garrus.

"Alright, if you think that's best, we can do that. I don't know the first thing about earth-fauna, so I'll defer to you, Commander." The last bit is said with emphasis-- Garrus is just being a bitch though. He has a terrible sense of humor.