The origin and evolution of the calls and facial expressions of the primates.

1. It is not possible to treat mammalian vocalisations and facial expressions in terms of a drive model. They can be best described as basically dependent on "stimulus contrast" : that is, on those features of a stimulus which serve to arouse attention. As the contrast increases so does the intensity of the call. 2. "Protective responses" are given by mammals to startling or unpleasant stimuli (i.e. stimulation with high contrast) ; in such responses the mouth corners are drawn back. the lips retracted, the head shaken from side to side, the eyes closed, the eye-brows lowered and the glottis lips closed. Independently- in the Artiodactyls, Perissodactyls, Carnivores and the Insectivore-Primate stock, (and perhaps several times within each group), these originally protective reflexes appear to have given rise to greeting displays. Vocalisations seem to have originated from interaction between the glottis closure of this group of responses and the violent expiration which often follows it. The grin may have also originated from a protective response; it is at least associated extremely widely with intense vocalisation. During the evolution of vocalisations the expiratory reflex of m. orbicularir oris contraction has also been incorporated into facial expressions. Snarls may have derived independently from intention movements of threat. 3. A systematic account of the calls and facial expressions of the primates follows, illustrated by spectograms. The shrews perhaps best represent the condition of the ancestral form, with twitters (short calls with a high chevron-shaped fundamental), and series of clicks given throughout the year to stimulus contrast, and, in the breeding season, in greeting to potential mates. Violent expirations are given in threat. 4. Galago crassicaudatus (Lorisoidea) may be taken as typical of a relatively primitive primate. The infant gives clicks with grins to visual stimulus contrast, and as a result of loss of contact with the mother. Males click when attempting to reach a female in oestrus, as do females when attempting to reach their infant. Galago also gives a series of soft clicks which pass into grunts by fusion together. At higher intensities grunts lengthen into moans and then, with rise in pitch, become wails and shrieks. In relatively solitary Lorisoids such as crassicaudatus, there has been little facilitation of vocalisations, and as a result these calls are elicited only by intense stimulation from fellows and occur chiefly in threat (defensive or offensive) or in courtship. In the Lemuroidea there has been progressive facilitation of vocalisation from the solitary Microcebus to the very social Lemur fulvus. Absence of fellows becomes very effective in eliciting calls. Click-series and grunts come to be very readily evoked in greeting. The grin remains very difficult to elicit in fulvus, but is given in association with wails by Lemur catta, in which facilitation of changes in facial expression is beginning. Barks are given to startling objects. The cough of fulvus appears to represent a trill (fused twitter), and is used in contact like the trills of Cercopithecus. 5. The Platyrrhines have developed the twitter as well as clicks for use in greeting equals or frightening superiors. A full group of protective responses is given in greeting by Cebus and Lagothrix (and probably by other advanced genera). It is as difficult to elicit changes in facial expressions in such a form as Aotes, as in a Lorisoid, but twitters are given in greeting very readily; it is clear that facilitation of vocalisation has preceded facilitation of facial expression, just as in the Lemuroidea. 6. The Old World monkeys appear originally to have given twitters as well as clicks in greeting. Cercopithecus retains twitters and also trills (which are fused twitters). The 'arrr' calls of Cercopithecus and Macaca represent modified click series which, like those of Lemur fulvus are given in greeting or submission. They are given more faintly in threat with much superimposed noise, as is true of the greeting calls of most primates. Wails like those of Lemur are given by Macaca. 7. In threat in the Lemuroidea and in Old World monkeys the month is opened and m. orbicularir oris contracted enough to cover the teeth. This probably represents the expiratory reflex contraction. Grins are given most readily in greeting a frightening superior. They show progressive facilitation until they finally come to be given in friendly greeting. 8. In Papio, and (presumably independently) in apes and man an exaggerated snarl (m. naso labialis contraction) has evolved in threat, probably from an intention movement of biting. The apes and man approximate the eyebrows at the same time (an intense gaze), whilst Cercopithecids raise them. 9. Man shows marked platysma contraction in crying and certain other loud vocalisations. This may derive from platysma contraction used to expel air from ventricular air sacs. 10. Papio and the Pongidae and man show a marked drop in pitch of vocalisations so that, for example, 'arrr'-Iike calls become segmented grunts. This appears to have occurred in order to bring most vocalisations into the pitch range in which the buccal cavity is efficient as a resonator. Thus changes in facial expressions come to alter the quality of calls, so that calls carry more specific information. (Some resonance effects on calls occur throughout the primates, and involve not only the exaggeration of m. orbicularis oris contraction but the participation of air sacs.) II . In man and the chimpanzee the basic system of calls consists of segmented grunts, long grunts, wails and shrieks. The evocation of laughter in man by humorous situations appear to be best interpreted as the evocation of vocalisation by stimulus contrast. 12. The evolution of human language and problems of affect are considered.