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1 June 2009 Owl Monkey Vocalizations at the Primate Research Institute, Inuyama
Sachi Sri Kantha, Hiroki Koda, Juri Suzuki
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Quantitative evaluations of vocalizations of nocturnally active owl monkeys (Aotus species) have been attempted only twice (Andrew, 1963; Moynihan, 1964). Andrew (1963) identified four call types (twitter-trill, sharp call, boom and squeak) in two captive individuals, and Moynihan (1964) identified six call types (low trill, gulp, grunt complex, moan, scream and hoot) in a quasi-wild simulation. These two reports record the study subjects as belonging to Aotus trivirgatus species and pre-date the current revision of Aotus genus into nine species (Hershkovitz, 1983; Ford, 1994). Their subjects would now be classified as A. lemurinus griseimembra, ranging from Colombia to east and extreme northeastern Venezuela, as described by Ford (1994). As such, they are representative of the gray-necked group, distributed in the regions north of Amazon River. It is unknown whether the other clade of owl monkeys, the red-necked species group also produces identical call types. The objective of this study was to identify the call types produced by members of captive A. azarae, a species that belongs to the red-necked group distributed in the regions south of Amazon River (Ford, 1994). Preliminary results of this study have appeared in abstract format (Sri Kantha et al., 2004).


Subjects and maintenance

The subjects of this study were 16 owl monkeys (12 A. azarae pure breds and 4 Aotus hybrids of red-necked types) reared at the Kyoto University's Primate Research Institute (PRI) facility. All, excluding one founder member from Bolivia, were captive born and have been studied since 2002 as previously reported (Sri Kantha and Suzuki, 2006; Suzuki and Sri Kantha, 2006; Sri Kantha et al., 2007). All experiments were carried out following approval from the Research Committee of the Institute, and according to the Primate Research Institute's Guidelines for the Care and Use of Laboratory Primates.

Acoustic Analysis of Vocalization Behavior

Under the premise that the vocalizations of dark-active owl monkeys have to be studied in the dark without distraction to the study subjects, we avoided using vision-enhancing goggles or other supporting aids to identify the individual monkeys eliciting the vocal notes. As such, the group vocalization behavior of 16 monkeys in the colony room was studied in two installments.

(A). Pilot observations: Pilot observations were made by one or two researchers, standing quietly in the colony room for 60–120 min during the dark phase for one session to, (a) distinguish the call type sounds, and (b) to allow the monkeys to become habituated to the researchers and the recording equipment. Two hundred and forty hours were spent on this installment.

(B). Sampling observations for acoustic analysis: Sampling observations were carried out in four sessions on separate days. The length of each session varied between 60 min and 240 min. Spontaneously elicited calls of the colony members as a group during dark cycle were recorded by digital audiotape recorder (Sony TCD-D100) connected to a hand-held microphone (Sony ECM-672) by ad libitum sampling with minimum interference. During recording, the two researchers and the recording equipment were stationary and the recording distance varied between 0.5 m (proximal cage) and 6.0 m (distal cages). Scream vocalizations were also elicited and recorded in one member, in response to the threat of trapping by net, during the light cycle (Moynihan, 1964). Sound spectrograms of vocal repertoires were analyzed by Multi Speech Signal Analysis Workstation Model 3700 ver. 2.21 (Kay Elemetrics Corp.), as described previously (Koda, 2004), to quantify frequency and duration of calls. On the basis of these two acoustic parameters, a cluster analysis was carried out to identify the different call types. For conformity, we adhered to the call type terminology introduced previously for owl monkeys by Moynihan (1964). Data collection began in October 2002 and ended in April 2004.

Table 1.

Categories and acoustic properties of Owl monkeys calls.



From the recorded and analyzed vocal repertoire of the owl monkey colony we distinguished six categories of acoustic call types. Table 1 shows the six categories of calls elicited during a 60 min recording session. Four call types, the sneeze grunt, low trill, gulp and moan (in the increasing order of mean duration, from 50 to 410 msec.) were recorded in the dark period spontaneously without any threat stimulus. Among these four call types, the moan call with the longest mean duration (410 msec.) had the lowest bandwidth range of 140–300 Hz (Fig. 1). The mean duration of the other three call types varied only by 4 msec; their bandwidths ranged between 1,580 and 5,800 Hz. During the recording period, low trill was the most frequent and sneeze grunt was the least frequent call. We also elicited two categories of scream call (short scream and long scream) from one Aotus hybrid female by threat induction due to net capture under light conditions. As shown in Fig. 1, the bandwidth range of short scream (980–3,300 Hz) and long scream (2,800–5,800 Hz) varied markedly, and the difference in mean duration between the two screams was only 70 msec.


Andrew (1963) identified four call types (twitter-trill, sharp call, boom and squeak) in two captive grey-necked owl monkeys. Moynihan (1964) identified six call types (low trill, gulp, grunt complex, moan, scream and hoot) in captive grey-necked owl monkeys. We were able to confirm the presence of five of the six specific call types, excluding hoot, reported by Moynihan (1964) in the 12 red-necked owl monkey subjects. Considering the restraints of captivity, we acknowledge that not all call types of red-necked owl monkeys may have been elicited during our recording. For example, we did not record the short sequence pure tone hoots, audible to humans in the night at a distance of 500 meters, as reported in the descriptive records of naturalists (Moynihan, 1964). The frequency range of nocturnally active Aotus monkey (140–5,800 Hz) vocalizations are narrow compared to the determined frequency range of vocalizations of other diurnally active, small-framed Platyrrhine monkeys such as Cebuella pygmaea (800–14,000 Hz), Callithrix spp. (500–14,800 Hz), Saguinus spp. (1,150–10,400 Hz) and Saimiri spp. (500–11,250 Hz), as tabulated by Hauser (1993). However, the frequency ranges of owl monkeys (this study, Moynihan 1964) are identical to that of diurnally active, large-framed Platyrrhini such as Cebus spp. (500–6,500 Hz) and Ateles spp. (200–3,800 Hz), as reviewed by Hauser (1993).

Figure 1.

Representative sound spectrograms of six call categories of Aotus azarae monkeys. In all six frames, X-axis indicates time (millisec.) and Y-axis indicates frequency (kHz).


In sum, with respect to frequency bandwidth and call duration, we have confirmed the six categories of calls reported for captive gray-necked Aotus recorded in Panama, in captive-born individuals of red-necked Aotus of Bolivian origin. The practical necessity of collecting acoustic data of owl monkeys in the dark somewhat hinders the precision of assembled data due to difficulty identifying the subjects generating the calls. As such, the reported vocalization data of owl monkeys in this study as well as earlier reports of Andrew (1963) and Moynihan (1964) need additional confirmation from carefully controlled playback studies (Byrne, 1982).



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Sachi Sri Kantha, Hiroki Koda, and Juri Suzuki "Owl Monkey Vocalizations at the Primate Research Institute, Inuyama," Neotropical Primates 16(1), 43-46, (1 June 2009).
Published: 1 June 2009
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