Guanacos live in either family groups, including one adult male, several adult females, and youngsters under one year of age; male, or bachelor, bands; or are solitary males. The all-boy bands are made up of young males that were kicked out of the herd by the dominant male when they reached sexual maturity at about one year old.
The young males stick together for protection, and they also practice fighting skills through play fights. The solitary males tend to be mature males looking for females or a herd to take over so they can start their own families. A male often picks a territory that has high-quality vegetation to help him attract the females.
Females wait to become pregnant until environmental conditions seem right. They give birth every other year to a single calf during the summer months, which are December to February in South America. Mountain lions are the guanaco's main predator but can only carry off one or two young. For this reason, many females give birth at about the same time so the babies have a greater chance of survival.
Newborns can stand five minutes after birth and begin to follow their mother immediately. It is a rough life for a baby guanaco, though. Predators, lack of food, bad weather, and accidents can mean death to the little ones—only 30 percent of guanaco babies born in the wild live long enough to become adults.