Chapter Index

Acoustic communication in Northern Ceylonese slender lorises (Loris tardigradus nordicus )
and some information about vocalization by other forms or species of Lorisidae

Based on a chapter from: Schulze, H.; Meier, B., 1995: Behavior of captive Loris tardigradus nordicus: a qualitative description, including some information about morphological bases of behavior. Pp. 221-249 in: Creatures of the Dark, Alterman, L.; Doyle, G.; Izard, M. K. (eds.), Plenum Press, New York; slow loris information updated, improved.

See also ethogram with information about vocalization in the Husbandry Manual for Asian Lorisines by Helena Fitch-Snyder et al.

Descriptions of vocalizations do not always allow a clear identification. Therefore, sonograms from L. t. nordicus from the breeding colony at Ruhr-University Bochum have been included in this chapter with kind permission of H. Helmich (1987, unpublished). The table below gives a review of synonyms of Loris vocalizations used in literature. Some vocalizations of Nycticebus coucang have been included for comparison because acoustic communication in this species has been studied in detail (Zimmermann 1985, 1989). Vocalizations in slow lorises, however, apparently include more and different calls than Loris vocalizations, and similar calls may be uttered in different contexts than in Loris.

Table 1: loris vocalizations (German synonyms in parentheses). A question mark behind a synonym indicates that context and description are similar, but vocalizations might be not identical. Similarly described slow loris calls, included for comparison, partly occur in a different context.
Slender loris vocalization
Slow loris vocalization (under construction, incomplete)
Names used at Ruhr-University
(in parentheses: German)
Description, sample Names* used by other authors 
(in parentheses: German)
Possible meaning, context Names* Description Possible meaning, context
Territorial whistle  (Pfiff) Loud, sharp whistle, usually mono-, seldom trisyllabic 
Hear it (423 KB WAV-File)
Whistle 2, 3; (Pfeifen)4 Aggressive threat, territorial defense Whistle, 
II.1-II.3 5
  Does not seem to be developed until weaning 6
Low whistle, squeak (Kontakt-Pfiff) Short low, soft whistle or squeak, monosyllabic  - Probably calling for a conspecific, affiliative Contact call (Stimmfuehlungslaut) 7  Short: "eeh!"  
        Isolation sound 7 shrill 7 By animals kept isolated; neighbouring conspecifics answer with kecker calls 7
Chitter  (Meckern) Rhythmic low to medium-intense vocalisation 
Hear it (24 KB WAV-File)
Chitter 2; Distress sound, screeching? 3;  (Pfeifen) 4 Mild defensive vocalisation, for instance used when rejecting a conspecific during estrus or too rough play long kecker, 
II.5 5
Fundamental frequency 0.09-0.15 kHz, dominant frequency band 10.5-12.5 kHz, duration of single call 10.5-12.5 ms, often given in conjunction 6 From birth on. Infants: emitted during vigorous tactile contact with conspecific (being handled, groomed, social play, genital licking, adult stealing food from the youngster) or when the youngster is not allowed to suckle) 6
Screeching  (Zirpen) Rhythmic loud chirping 
Hear it (910 KB WAV-File)
Chitter 2; screeching? 3;  (Pfeifen) 4 Defensive vocalization (high intensity chitter, indicating stress)      
        Aek calls 6   In youngsters: in same context as "long kecker", but appear to indicate a higher level of disturbance 6
        Threat call (Drohlaut) 7 Series of quick rhythmic sounds 7  
        Quarrel (Streit)  Series of quick rhythmic sounds 7  
        Low excitement (Leichte Erregung) 7  "hr-hr-wooh" 7  
        Rage (Wutlaut)   "a-a-ooeeae" 7  
Monosyllabic chitter (Abwehr-Laut) Low short clicking or smacking sound, monosyllabic  - Defensive vocalisation, most often by females rejecting a sexually interested male  Short kecker? 6   Mothers respond with short kecker when hearing infant zek calls 6
        Defense call (Verteidigungslaut) 7  Short: "uck!" 7  
Zic call; infant zic  (Zick-Ruf) Short sharp monosyllabic click 
Hear it (1145 KB WAV-File)
Infant click 2 In babies: call indicating unwellbeing or fear, for instance after falling down from the mother, sometimes when a baby dislikes to be groomed Zek-call 5, 6 Infants: very short broadband calls, usually in trains of up to six calls in long sequences (= short kecker of adults) Often the whole group is alerted by this call. Reactions of the mother: "short kecker", shows phonotactic approach, retrieval and brood care behaviour 6
Krik call 1 ("Kcht") Low hissing, with higher intensity tonal call, may be repeated 
Hear it (143 KB WAV-File)
(Schniefen) 4 Appeasing call. Often by males when following a female in estrus; by animals threatened or rejected by a conspecific Krik call 2
ek-call: II.4? 5
        Play call (Spielkeckern) 7  "Kee-kee kae" 7  
 Play sounds (Spiel-Laute) Low, inconspicuous non-vocal "sniffing" sounds heard during playwrestling          
Growl  (Fauchen) Repeated loud growling or hissing interrupted by inhaling 
Hear it (Growling and fear cries190 KB WAV-File)
Pant-growl 2; growl 3; (Fauchen) 4 Defensive threat, often in connection with sham attacks; sign of excitement Snarl: II.7 5
grunt: II.8 5?
  Snarl does not seem to be developed until weaning 6
. . . . Infant "chro"-call (= adult grunt) 6 0.78-6.8 kHz, duration 71.6-200 ms 6 From birth on. Emitted during vigorous tactile contact with conspecific (being handled, groomed, social play, genital licking, adult stealing food from the youngster) or when the youngster is not allowed to suckle) 6
Scream (Angstgeschrei) Loud, long, repeated sceams interrupted by inhaling 
Hear it (Growling and fear cries190 KB WAV-File)
Scream 2; S.O.S. 3; (Schreien) 4 For instance heard when shy lorises are seized by humans Scream 5
Infants: fundamental frequency 0.9-1.65 kHz, duration 12.8-268.6 ms 6 In infants: heard only once from an undernourished baby when separated from the mother  6
?     Warning sound 3 scream 5, II.6 5    
Ultrasound vocalization ? Hopeful information in the future     Ultrasound 6 Frequency range 17-160 kHz, duration 0.15-3 ms 6 From birth to the age of 120 to 150 days; occurs when a baby is visually isolated from the mother, has fallen down, has not been suckled for a longer time or is otherwise in danger 6

* Based on literature (descriptions and context). Since usually no sound samples were available, this may not always be the same vocalisation
1 Designation "krik call" adopted from Rasmussen (1986), who used it for a comparable Nycticebus call emitted in a similar context (see below). Names / synonyms used by other authors: 2  Rasmussen, 1986; 4  Subramoniam, 1957, 4  Helmich (unpubl.); 5  Zimmermann 1985; 6  Zimmermann 1989; 7 Zimmermann et al. 1979;

The Whistle is a loud, usually monosyllabic, occasionally di- and trisyllabic vocalization, each syllable ending with a decrease of frequency. The duration ranges from short low squeaks to sequences of loud, clear whistles lasting more than 5 seconds. Frequencies up to 14 kHz, with a maximum intensity between 6 and 9 kHz, have been recorded (Helmich, unpublished). Loud, sharp whistles are uttered in aggressive excitement, for instance if an adult unrelated conspecific of the same sex approaches the cage of the animal. They may be emitted by both sexes, but are much more common in males. If quarreling occurs within a family group, one dominant animal may whistle, often in connection with noisy locomotor  display (see below), while inferior group members show signs of social stress and avoidance behaviour. Whistling may cause immediate aggression in neighbouring groups; animals in other cages may also begin to whistle. Petter and Hladik (1970) observed that whistles of wild lorises were emitted two or three times per night and could be heard over a distance of 100 m by humans.  Hear it (423 KB WAV-File)
In the breeding colony at Ruhr-University Bochum, a similar, but lower, shorter and less sharp-sounding whistle is sometimes heard which somewhat resembles the territorial whistle, but rather seems to have an affiliative meaning. It is for instance heard when lorises prepare for sleeping in the end of the night, maybe serving as a contact call for joining in sleeping communities, and never caused aggressive reactions during observation.
In Nycticebus coucang, whistling in connection with sexual interest or as a contact seeking call has been described (Rasmussen, 1986; Zimmermann, 1985). In our adult slender lorises, no whistling in connection with estrus, for instance by a sexually interested male following the female, was heard. In N. pygmaeus, whistling during estrous has been reported. In this context, no aggressive behaviour has been noted. (R. Lippe, pers. comm.). Whistling might be a contact or affiliative vocalization in this species.
Low whistle-like squeaks by abandoned slender loris infants (in connection with zic calls, see below) might be a similar vocalization, but have been heard only rarely from highly excited infants after having fallen down from the mother, they seem to be no usual infant vocalization.

Figure: vocalization. a: duration of mono-, di- and trisyllabic whistles. b: facial expression during whistling. c-k: sonograms and facial expression during vocalization: c, d: chitter. e: infant zic call. f, g: growl. h, i:krik call. j, k: scream.
Sonagrams from H. Helmich 1987 (unpublished).


Chitter consists of a series of short rhythmic clicks which may last up to 4 minutes. With increasing intensity, chitter turns to voiced, cricket-like screeching. Durations of 50 ms of the single clicks, with breaks of 12,5-30 ms, and frequencies up to 20 kHz have been recorded (Helmich, unpublished). Chitter is an intraspecific defensive vocalization, often connected with defensive staring (turning of the face towards the conspecific) or pushing away of an opponent with one or both hands. Chitter as a low intensity threat is frequent in females; males may screech during severe quarreling, but rarely chitter. Screeching occurs in connection with abrupt movements, sometimes urination and defecation indicate considerable excitement.According to Subramoniam (1957), chitter and a "warning sound" (repeated "krichit-krichit" in quick succession) were uttered when lorises were seized by a human or threatened by a dog. In our animals chitter directed toward humans occurs rarely. No other rhythmic vocalization in the described context was ever heard. Hear it (Higher intensity, aggressive, 910 KB WAV-File) Hear it (Lower intensity, not too serious, 24 KB WAV-File)

Monosyllabic Chitter is a single sharp smacking or crunching noise uttered by females when approached by a male. It is possibly identical with the higher, but otherwise very similar monosyllabic zic call of Loris infants (see below). The male usually reacts with retreat, turning away of the head or with an appeasing krik call (see below).

The Zic Call is a high, sharp, usually very short monosyllabic call uttered by infants. With increasing intensity, it turns to voiced disyllabic and trill-like calls of longer duration. In rare cases a single squeak, resembling a low short whistle, may be emitted between two zic calls. Zimmermann (1989), who described "zek-calls" in N. coucang infants, also recorded pure ultrasound calls, "presumably being high frequency variants zek calls". In Loris , no study of the emission of ultrasounds has been made. The zic call obviously signals unwellness, for instance when an infant dislikes grooming or has been separated from the mother; high intensity zic calls occur when an infant is abandoned for a longer time or hurt. The mother usually reacts by approaching and picking up the infant, or she may wait until it has mounted her on its own. She may, in addition, try to appease the infant by krik calls. Other group members, too, may react by approaching and handling the infant. Continuous high intensity zic calls cause considerable excitement, not only in the mother who may start to growl (in one case, even sham-attacks against the infant were observed), but also in animals in neighbouring cages. Hear it (Infant Zic Call, 1145 KB WAV-File)

The Krik Call is a low short vocalization, single or repeated at irregular intervals, which sounds like a combination of a crackling noise and a hiss. With increasing intensity it turns to a voiced, whining or squeaking sound which resembles a human baby´s voice. Durations of 50-75 ms and frequencies up to 4-9 kHz have been recorded (Helmich, unpublished). We adopted the designation for this call from Rasmussen (1986) who used it for a "string of short soft clicks" by N. coucang observed in a similar context, but did not hear it in Loris. Sonograms of similar N. coucang calls ("calls type II.4", "ek-calls") published by Zimmermann (1985) resemble the Loris sonogram in Fig. 5h.
The krik call is an appeasing vocalization regularly heard from males when they approach a female for sniffing. During estrus, males frequently emit krik calls while pursuing females; in a few cases male and female were heard duetting. A blind old male, attacked by a female he approached during estrus, uttered loud voiced krik calls and high calls closely resembling infant zic calls; the attack was immediately stopped. When group members quarrel, infants may become so excited that they threaten or attack their mother. In such cases, females sometimes use the krik call, combined with allogrooming, for appeasing the infant. Hear it (Male, 143 KB WAV-File)
In rare cases, a short, monosyllabic krik-like call has been heard from sick old lorises before accepting a titbit from the keeper. It seems possible that these animals, suffering from renal disease, were in a conflict situation, interested in the food, but at the same time afraid to eat it. The call is therefore interpreted as a sign of stress. Hear it (9 KB WAV-File)

Growl is an unvoiced expiratory vocalization, resembling the spit of a cat. It may include voiced sounds. Growl may be repeated with every exhalation, alternating with a voiced sound during inspiration, as long as a perceived threat continues. Loudness varies with the intensity of the stimulus. Growl is mainly a defensive threat against predators; in intraspecific encounters it is used only occasionally when the animal is very excited. Growling is often connected with bipedal defensive postures, attacks or mock attacks or abrupt lateral swaying of the body. Fierce biting or emission of an odor from brachial gland exudate may occur. Still (1905), who kept both slender lorises and cobras in captivity, observed that a loris threatened by a cat gave a "perfect imitation" of cobra threat behavior, including the "regular breathing sound", now and then a "cobra´s hiss", and an erect bipedal posture in connection with lateral swaying. Hear it (Growling and fear cries190 KB WAV-File)

Screaming (fear cries): with increasing intensity of fear, growl may suddenly turn to long, loud screaming like the screams of a cat. Screams are an expiratory vocalization and, like growling, may be repeated as long as the perceived threat continues, only interrupted by short periods of inhaling. Duration of screams was always more than 0.5 seconds; frequencies up to 18 kHz were recorded (Helmich, unpublished). Screaming occurred in rare cases when an animal was caught and handled; in one case a female began to scream when observing that other group members were caught. Screaming is usually accompanied by the emission of odor from brachial gland exudate. Screaming during intraspecific attack fights, as reported by Rasmussen (1986), was never heard in our animals. Hear it (Growling and fear cries,190 KB WAV-File)

Rattling with twigs and other non-vocal noise: besides vocal communication, rattling of twigs is apparently used as an acoustic signal. If quarreling occurs in a group, dominant aggressive animals show a locomotor display in the course of which they produce a continuous rattling of twigs. Most of this noise is caused by rapid locomotion with conspicuous, jerky movements which looks clearly different from the usual silent and flowing movements. Several other techniques are employed to produce noise, for instance brushing against twigs with the body during rapid locomotion or pulling along twigs with the feet which, when finally released, spring back. During running, the animals may repeatedly "fall" down from the branch by releasing the grip of the forelimbs while both feet remain firmly attached, both body and branch abruptly swinging down. This behaviour looks unintended, but it has repeatedly been observed in this context and does not occur during rapid locomotion in a different context. An aggressive animal has been observed standing, firmly holding a twig with both hands and shaking it by rapid flexion and extension of both arms; in this case intention was evident. A noisy locomotor display may also be shown in playful mood, and some animals regularly cause considerable noise with twigs or dry leaves when impatiently waiting for food or tidbits. One exciteable young male developed the habit to bite into the rim of a plastic food bowl when waiting impatiently for mealworms. The bite was well audible; after biting the male looked out for the success and often repeated the behaviour until unsuccessful. Rattling of twigs, caused by the keeper, causes flight reactions or signs of stress. When animals are frightened by some sudden stimulus, quick flight upwards often causes an initial rattling of twigs which might possibly be a substitute for vocal alarm calls.


Daschbach, N. J.; Schein, M. W.; Haines, D. E., 1981: Vocalizations of the slow loris, Nycticebus coucang (Primates, Lorisidae). Internatl. J. Primatol. 2: 71-80. (German)

Helmich, H., 1987 (unpublished): Das Lautrepertoire adulter Schlankloris (Loris tardigradus) [The vocal repertoire of adult slender lorises]. Staatsexamensarbeit, Fakultät für Biologie, Ruhr-Universität Bochum (German)

Petter, J.J.; Hladik, C.M., 1970: Observations sur le domaine vital et la densité de population de Loris tardigradus dans les forêts de Ceylan.Mammalia 34 (3): 394-409

Rasmussen, D.T., 1986: Life history and behavior of slow lorises and slender lorises: implications for the lorisine-galagine divergence. Dissertation, Duke University.

Still, J., 1905: Notes: On the loris in captivity. Spolia Zeylanica 3: 155-157.

Subramoniam, S., 1957: Some observations on the habits of the slender loris, Loris tardigradus (Linné). Journal of the Bombay Natural History Society 54 : 387-398.

Zimmermann, E., 1985: Vocalizations and associated behaviours in adult slow loris (Nycticebus coucang ). Folia Primatologica 44: 52-64.

Zimmermann, E., 1989: Reproduction, physical growth and behavioral development in slow loris (Nycticebus coucang , Lorisidae). Human Evolution 4 (2-3): 171-179.

Zimmermann, E.; Zimmermann, P.; Zimmermann, A., 1979: Soziale Kommunikation bei Plumploris (Nycticebus coucang). [Social communication in the slow loris]. Z. Kölner Zoo 22: 25-36 (German).

Information on the Web:

Flannery, S., 1999-2002: Primate behaviour - Nycticebus coucang. http://members.tripod.com/uakari/nycticebus_coucang.html

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Population database - behaviour  Last amendment: 10 September 2003


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