Why do some angry animals lay their ears back—like this jaguar—while others do not? Survival advantage goes to those who fold, Darwin argues: Any animal that fights with its teeth will be wise to get those delicate ears out of harm’s way.
The ears through their movements are highly expressive in many animals; but in some, such as man, the higher apes, and many ruminants, they fail in this respect.
A slight difference in position serves to express in the plainest manner a different state of mind, as we may daily see in the dog; but we are here concerned only with the ears being drawn closely backwards and pressed to the head. A savage frame of mind is thus shown, but only in the case of those animals which fight with their teeth; and the care which they take to prevent their ears being seized by their antagonists accounts for this position. Consequently, through habit and association, whenever they feel slightly savage or pretend in their play to be savage, their ears are drawn back.
That this is the true explanation may be inferred from the relation which exists in very many animals between their manner of fighting and the retraction of their ears. All the Carnivora fight with their canine teeth, and all, as far as I have observed, draw their ears back when feeling savage.... The retraction of the ears may likewise be seen in kittens fighting together in their play and in full-grown cats when really savage. Although their ears are thus to a large extent protected, yet they often get much torn in old male cats during their mutual battles.
Even one of the Eared Seals, the Otaria pusilla, which has very small ears, draws them backwards, when it makes a savage rush at the legs of its keeper. When horses fight together they use their incisors for biting and their forelegs for striking much more than they do their hind legs for kicking backwards. This has been observed when stallions have broken loose and have fought together and may likewise be inferred from the kinds of wounds which they inflict on each other. Everyone recognizes the vicious appearance which the drawing back of the ears gives to a horse.
that the chief expressive actions exhibited by man and by the lower animals are now innate or inherited—that is, have not been learnt by the individual—is admitted by everyone.
No doubt as long as man and all other animals are viewed as independent creations, an effectual stop is put to our natural desire to investigate as far as possible the causes of Expression. By this doctrine, anything and everything can be equally well explained, and it has proved as pernicious with respect to Expression as to every other branch of natural history. With mankind some expressions, such as the bristling of the hair under the influence of extreme terror or the uncovering of the teeth under that of furious rage, can hardly be understood, except on the belief that man once existed in a much lower and animal-like condition.
The community of certain expressions in distinct though allied species, as in the movements of the same facial muscles during laughter by man and by various monkeys, is rendered somewhat more intelligible if we believe in their descent from a common progenitor. He who admits on general grounds that the structure and habits of all animals have been gradually evolved will look at the whole subject of Expression in a new and interesting light.