Darwin Online is an excellent resource, providing scanned text of his most important works as well as notebooks and letters that have been less known but are no less valuable in getting to grips with Darwin’s perspective. I will be using the website to find his references to primates.
Expression of the Emotions in Man and Animals (1890)
Darwin mentions apes in the chapter of Expression of Emotions entitled, “Means of Expression in Animals”. He notes the involuntary “erection of dermal appendages” in situations of fear or anger:
“The action serves to make the animal appear larger and more frightful to its enemies or rivals, and is generally accompanied by various voluntary movements adapted for the same purpose, and by the utterance of savage sounds.” (p. 100)
He continues with examples of this involuntary reaction in apes and monkeys:
“Mr. Sutton, the intelligent keeper in the Zoological Gardens, carefully observed for me the Chimpanzee and Orang; and he states that when they are suddenly frightened, as by a thunderstorm, or when they are made angry, as by being teased, their hair becomes erect. I saw a chimpanzee who was alarmed at the sight of a black coalheaver, and the hair rose all over his body; he made little starts forwards as if to attack the man without any real intention of doing so, but with the hope, as the keeper remarked, of frightening him. The Gorilla, when enraged, is described by Mr. Ford as having his crest of hair ‘erect and projecting forward, his nostrils dilated, and his underlip thrown down; at the same time uttering his characteristic yell, designed, it would seem, to terrify his antagonists.’ I saw the hair on the Anubis baboon, when angered, bristling along the back, from the neck to the loins, but not on the rump or other parts of the body. I took a stuffed snake into the monkey-house, and the hair on several of the species instantly became erect, especially on their tails, as I particularly noticed with Cercopithecus nictitans.” (p. 101-102)
In the following chapter, “Special Expressions”, Darwin returns to descriptions of monkeys and apes:
“The various species and genera of monkeys express their feelings in many different ways; and this fact is interesting, as in some degree bearing on the question, whether the so-called races of man should be ranked as distinct species or varieties [we can’t forget that Darwin was writing/researching in the midst of Victorian England, and this statement was a challenge to the beliefs held within that context]; for as we shall see in the following chapters, the different races of man express their emotions and sensations with remarkable uniformity throughout the world. Some of the expressive actions of monkeys are interesting in another way, namely for being closely analogous to those of man. As I have had no opportunity of observing any one species of the group under all circumstances, my miscellaneous remarks will be the best arranged under different states of mind.
Pleasure, joy, affection. – It is not possible to distinguish in monkeys, at least without more experience than I have had, the expression of pleasure or joy from that of affection. Young chimpanzees make a kind of barking noise, when pleased by the return of any one to whom they are attached. When this noise, which the keepers call a laugh, is uttered, the lips are protruded; but so they are under various other emotions. Nevertheless I could perceive that when they were pleased the form of the lips differed a little from that assumed when they were angered. If a young chimpanzee be tickled – and the armpits are particularly sensitive to tickling, as in the case of our children – a more decided chuckling or laughing sound is uttered: though the laughter is sometimes noiseless. The corners of the mouth are then drawn backwards; and this sometimes causes the lower eyelids to be slightly wrinkled. But this wrinkling, which is so characteristic of our own laughter, is more plainly seen in some other monkeys. The teeth in the upper jaw in the chimpanzee are not exposed when they utter their laughing noise, in which respect they differ from us. But their eyes sparkle and grow brighter, as Mr. W.L. Martin, who has particularly attended to their expression, states.” (pp. 139-140)