Like their domestic relatives, guanacos are fast runners. They have more haemoglobin (oxygen-carrying pigment) in their red blood cells than any other mammal. This allows them to survive at altitude.
Guanacos mainly graze on grass, but they also pluck leaves from shrubs. They live in herds of about 15 individuals. Each herd is controlled by one adult male. Once a young guanaco reaches adulthood, it is chased away by the dominant male.