Giraffes are social animals, living in loose, open, unstable herds varying from 10 to 20 individuals, although herds of up to 70 have been observed. Individual giraffes join and leave the herd at will. Herds can include all female, all male, female with young calves, or mixed genders and ages. Female giraffes are more social than male giraffes. Isolated individuals can also be encountered in the wild. They spend the day browsing but at night retire to an open area where their great height and excellent eyesight make it difficult for a predator to approach unobserved. They take turns sleeping, with one or two always on guard. They pull leaves off twigs with their long prehensile tongues, which are blue.