It's because they think it's another animal so they kinda go alpha-alpha to their reflection, especially those pack leaders of those pack oriented species like the gorilla.
Love the video tho, Most of them got surprised at first when they walked towards the mirror. Haha, the Elephant just looked and walked away. He didn't give a crap lol. On the otherhand, I feel like the humans our primate buds were kinda curious since they were like sitting in front of the mirror looking at it which shows a bit of consciousness right there which is not surprising since they're our distance cousins.
===
I wonder when will I grow a full beard. My facial hair is growing again but it's patchy and all over the place. Last time it grew I was 15-16.