Animals, too, have some fascinating adaptations to the dry forest's long rainless weeks and months. Foremost among these is "estivation," the summer-time equivalent of hibernation. Many frogs and insects simply burrow deep into damp mud, or their own excavated chambers, and go to sleep, reducing their metabolism (and thus their water needs). When the rains return, the increased moisture awakens these animals, and they return to the surface to breed. Other animals, such as birds and monkeys, show a remarkable degree of mobility during the dry season, retreating to damp areas such as stream beds, where year-round moisture enables them to survive. Howler monkeys, who eat nothing but green leaves and occasional fruits, have been known to cram into small stream side forests at amazingly high densities, yet they do not fight over territory as they would during the rest of the year.