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The calls of the squirrel monkey (Saimiri sciucreus) were grouped according to structural similarities and transitional frequencies of occurrence, that is the relative frequency with which two call types follow each other in spontaneous vocal sequences, into five classes. Within each class, the calls were grouped according to the aversiveness of the emotional state underlying these calls. The aversiveness of a call was determined by eliciting this call by electrical brain stimulation and giving the animal the opportunity in a self-stimulation procedure to switch the stimulation on and off by itself. Altogether 251 intracranial electrode positions yielding 47 different call types were tested in 38 squirrel monkeys. The results are as follows : all five call classes contain representatives expressing highly aversive, slightly and neutral emotional states; one call class, in addition contains calls indicating a (positively reinforcing) emotional state. The aversiveness of a call is positively correlated with its total frequency range. In high-pitched harmonic calls, the aversiveness increases with pitch of fundamental frequency and irregularity of frequency course. Short calls with a pronounced upward shift of frequency express a less aversive state than calls with the same average pitch but constant or predominantly downward shift of frequency. The five call classes can be characterized as follows : 1. Purring-growling-spitting group. The calls of this group consist of short ([30 msec), non-harmonic, click-like elements which are repeated in a 50 ± 20-Hz rhythm. They express an emotional state corresponding to self-assertiveness. The wider the frequency range of a call, the more intense is its threatening character, that is the probability that it will be accompanied by an aggressive action. 2. Groaning-cawing-shrieking group. This group consists of non-rhythmic, harmonic or noise-like calls with a duration of 30 to 800 msec and major acoustic energy between o and 3 kHz. All calls have a protest character. Calls with a small frequency range express slight uneasiness, calls with a wide frequency range express defensive threat, that is a high probability of being followed by either aggressive or submissive actions. 3. Clutcking-yapping-alarm-peep group. The calls of this group are short and loud and are all characterized by a steep fall of main energy from higher to lower frequencies. The calls of this group have a more-or-less pronounced warning character. Low-pitched variants are used intra-specifically and express annoyance with a lack of aggressive tendency. High-pitched variants serve as warning calls against potential predators. 4. Chirping-peeping-squealing group. The calls of this group have a fundamental above 2 kHz and are non-rhythmic; their frequency modulation is on average smaller than in the foregoing group. They all express the desire for social contact. A short ascending frequency course is used to draw another group member's attention to the vocalizer in non-agonistic situations, long drawn-out calls serve as long-distance contact-calls, calls with a very irregular frequency course indicate submission or social frustration. 5. Twittering-chattering-cackling group. The calls of this group consist of elements of different shape which are repeated in a 14 ± 3-Hz rhythm. They serve to confirm social bonds. Twitter-like calls are used to announce pleasurable events, cackling-like calls announce intra-specific mobbing., The calls of the squirrel monkey (Saimiri sciucreus) were grouped according to structural similarities and transitional frequencies of occurrence, that is the relative frequency with which two call types follow each other in spontaneous vocal sequences, into five classes. Within each class, the calls were grouped according to the aversiveness of the emotional state underlying these calls. The aversiveness of a call was determined by eliciting this call by electrical brain stimulation and giving the animal the opportunity in a self-stimulation procedure to switch the stimulation on and off by itself. Altogether 251 intracranial electrode positions yielding 47 different call types were tested in 38 squirrel monkeys. The results are as follows : all five call classes contain representatives expressing highly aversive, slightly and neutral emotional states; one call class, in addition contains calls indicating a (positively reinforcing) emotional state. The aversiveness of a call is positively correlated with its total frequency range. In high-pitched harmonic calls, the aversiveness increases with pitch of fundamental frequency and irregularity of frequency course. Short calls with a pronounced upward shift of frequency express a less aversive state than calls with the same average pitch but constant or predominantly downward shift of frequency. The five call classes can be characterized as follows : 1. Purring-growling-spitting group. The calls of this group consist of short ([30 msec), non-harmonic, click-like elements which are repeated in a 50 ± 20-Hz rhythm. They express an emotional state corresponding to self-assertiveness. The wider the frequency range of a call, the more intense is its threatening character, that is the probability that it will be accompanied by an aggressive action. 2. Groaning-cawing-shrieking group. This group consists of non-rhythmic, harmonic or noise-like calls with a duration of 30 to 800 msec and major acoustic energy between o and 3 kHz. All calls have a protest character. Calls with a small frequency range express slight uneasiness, calls with a wide frequency range express defensive threat, that is a high probability of being followed by either aggressive or submissive actions. 3. Clutcking-yapping-alarm-peep group. The calls of this group are short and loud and are all characterized by a steep fall of main energy from higher to lower frequencies. The calls of this group have a more-or-less pronounced warning character. Low-pitched variants are used intra-specifically and express annoyance with a lack of aggressive tendency. High-pitched variants serve as warning calls against potential predators. 4. Chirping-peeping-squealing group. The calls of this group have a fundamental above 2 kHz and are non-rhythmic; their frequency modulation is on average smaller than in the foregoing group. They all express the desire for social contact. A short ascending frequency course is used to draw another group member's attention to the vocalizer in non-agonistic situations, long drawn-out calls serve as long-distance contact-calls, calls with a very irregular frequency course indicate submission or social frustration. 5. Twittering-chattering-cackling group. The calls of this group consist of elements of different shape which are repeated in a 14 ± 3-Hz rhythm. They serve to confirm social bonds. Twitter-like calls are used to announce pleasurable events, cackling-like calls announce intra-specific mobbing.
Uwe Jürgens (Mon,) studied this question.