In my textbook it says that a high respiratory quotient often means the organism is respiring anaerobically as well as aerobically. Surely this only applies to organisms such as yeast and not to mammals because mammals neither consume O2 nor produce CO2 in anaerobic respiration? So does any anaerobic respiration not affect the RQ of mammals?

Someone may correct me but I think your logic is sound. CO2 (and ethanol) is an endproduct of anaerobic respiration in plants and many microorganisms. In mammals, lactic acid is produced.