While gorillas are primarily terrestrial, like other primates they are capable of climbing. Surface locomotion is quadrupedal, the soles of the feet and the middle phalanges of the fingers being placed on the ground. Gorillas are reluctant to cross even the shallowest of streams. Platform nests are built for sleeping. They are diurnal, with most activity taking place between 6 a.m. and 6 p.m. Although they are nearly entirely vegetarian, some will accept meat or take insects. Gorillas are social, living in groups. When two groups meet they may ignore each other, temporarily associate, or express hostility. Groups can range from 2 to 30 in number and, if a group has more than one mature male, one will be dominant. Vocalizations consist of roars, barks, growls, grunts, purrs, croaks, hoots, squeaks and screeches. Flat or slightly cupped hands are beat in rapid alteration on the chest, abdomen, hips, tree trunks, the ground, or anything that is handy. This drumming sounds the alarm when danger threatens.