and new members migrating in. In the study done at Lope, gorillas harvest most of their food arboreally, but less than half of their night nests are built in trees. They are often found on the ground and are made up of up to 30 gorillas. Western lowland gorillas live in the smallest family groups of all gorillas, with an average of four to eight members in each. The leader (the silverback) organizes group activities, like eating, nesting and travelling in their home range. Those who challenge this alpha male are apt to be cowed by impressive shows of physical power. He may stand upright, throw things, make aggressive charges, and pound his huge chest with open or cupped hands while barking out powerful hoots or unleashing a frightening roar. Despite these displays and the animals' obvious physical power, gorillas are generally calm and nonaggressive unless they are disturbed. Young gorillas, from three to six years old, remind human observers of children. Much of their day is spent in play, climbing trees, chasing one another and swinging from branches. Reproduction Female western lowland gorillas do not produce many offspring due to the fact that they do not reach sexual maturity until the age of 8 or 9. Female gorillas give birth to one infant after a gestation period of nearly nine months. Female gorillas do not show signs of pregnancy. Unlike their powerful parents, newborns are tiny, weighing , but are able to cling to their mothers' fur. These infants ride on their mothers' backs from the age of four months through the first two or three years of their lives. Infants can be dependent on their mother for up to five years. A study of over 300 births to captive female gorillas revealed that older females tend to give birth to more male offspring as opposed to females under 8 years old. This pattern is likely to result from selective pressures on females to have males at a time when they can provision them most effectively, as male reproductive success probably varies more than that of females and depends more on the maternal role. Female western lowland gorillas living in a group led by a single male have been observed to display sexual behavior during all stages of their reproductive cycle and during times of non-fertility. Three out of four females have been observed to engaged in sexual behavior