This brain controls body functions required for sustaining life, such as heart rate, breathing, balance and body temperature.
It is reliable, but tends to be cold, rigid and compulsive.
Behaviour relating to survival of the species, such as sexual behaviour, is instinctive and responses are automatic.
UPDATE: Thanks to Couves, commenting before and after me at Bernstein's, we learn that Donald Rumsfeld had a point-blank opportunity under direct questioning to deny that he is a lizard, and he wouldn't do it. This is around the 3 minute mark at the link. Make of it what you will. Of course, my mind was made up before I received this new gem of knowledge.
Reptilian vs Mamillian Brain
THE EVOLUTIONARY LAYERS OF THE HUMAN BRAIN