The elephant

makes a vocal sound of a windlike sort by the mouth
alone, unaided by the trunk, just like the sound of a man panting or
sighing; but, if it employ the trunk as well, the sound produced is
like that of a hoarse trumpet. With regard to the sleeping and waking of animals, all creatures
that are red-blooded and provided with legs give sensible proof that
they go to sleep and that they waken up from sleep; for, as a matter
of fact, all animals that are furnished with eyelids shut them up when
they go to sleep. Furthermore, it would appear that not only do men
dream, but horses also, and dogs, and oxen; aye, and sheep, and goats,
and all viviparous quadrupeds; and dogs show their dreaming by barking
in their sleep. With regard to oviparous animals we cannot be sure
that they dream, but most undoubtedly they sleep. And the same may
be said of water animals, such as fishes, molluscs, crustaceans, to
wit crawfish and the like. These animals sleep without doubt, although
their sleep is of very short duration. The proof of their sleeping
cannot be got from the condition of their eyes-for none of these
creatures are furnished with eyelids-but can be obtained only from
their motionless repose.