Morning call of siamang (Symphalangus syndactyllus) in Subayang River, Riau – Indonesia

Morning call in siamang is very important for the life of siamang. This behavior is important to mark its territory, communicate with neighboring groups, etc. The this research aims to determine the time of vocalization, the position of trees and the strata canopy used by gibbons in vocalization. This research was conducted in WWF Indonesia Central Sumatra Research Station Subayang Camp at Rimbang Baling Wildlife Reserve for 2 months from July until August 2017 using Visual Encounter Survey (VES) method. From this research we knew that morning call in siamang usually occurs from 08.00 am – 10.00 am in the morning. Siamang appears to choose emergent tree in the forest ridge or emergent tree on the hill slope to do vocalization. During vocalization, siamang tends to use the 3 and the 4 strata or upper canopy layer and emergent layer which are higher than 20 m from the forest surface. This height facilitates the siamang to maximize transmission of the vocalization to the surrounding area and communicate with other groups.


Introduction
Siamang (Symphalangus syndactylus) is an arboreal animal, so it uses more sound in communication compared to body movements (Liebal et al. 2004). Several sound studies on this species have been carried out. Most of the gibbons speak at 08.00-11.00 but rarely speak after 2:00 p.m. (Fox 1972). During sounding, the gibbons use the distal part of their hands to modulate the sound, called hand modulation. Hand modulation vocalizations distinguish siamang from other species in Hylobatidae (Badraun 1980). Morning call is an important voice for siamang (Chivers, 1980). Morning call is a loud voice and is mutually friendly in the morning and is a voice (vocalization) that is unique to siamang (Supriatna and Wahyono 2000). This behavior serves to tell its existence, mark the territory of a group to another group (Chiver 2001), and to look for partners (Cowlishaw, 1992). Siamang performs a morning call while braking on the tree. The choice of trees to make a morning call is important so that the morning call function is beneficial for individuals who do it. At present, the existence of siamang is only in conservation areas (Nijman and Geissman 2008), including in the Bukit Rimbang Wildlife Reserve in Bukit Baling (BRBB).
This study aims to determine the voice time (morning call), the position of the tree when speaking of topography, and determine the canopy strata used by gibbons in conducting vocalizations.

Materials and Methods
The research was conducted from July to August 2017 at the Central Sumatra World Wide Fund

Result and Discussion
During the 15 days of observation, there were 10 times a morning call (Figure 1).
Siamang does not make morning calls when it rains in the morning. There are 2 allegations related to this. First, the rain in the morning causes the room temperature to be lower so that it increases energy requirements in conducting morning calls (Cheyne, 2008). If the morning call has passed because of the rainy day, the gibbons choose to look for food rather than the morning call. Second, the sound of rain in the morning disguised the morning call so that his voice would not be heard by other groups. As a result the morning call function is not achieved (Raemaekers et al, 1984).

Figure. 1 Time and duration of siamang morning calls
Siamang performs a morning call starting at 08:00 -10:00 WIB with a voiced duration of 5-18 minutes with an average of 11.8 ± 4.3 minutes (Fig. 1). In general, gibbons perform vocalizations for 17 minutes (Geissmann, 1999). And according to Badraun (1998)  Morning call trees are very important, meaning that for gibbons associated with the success of the morning call function. The characteristic of voiced trees used by siamang is the tallest tree, with no branches on the stem, and trees located on the back or middle of the slope ( Figure 2.) The tallest tree without lianas, chosen by siamang because at that position, the danger of predation for gibbons is reduced. Besides that, the tall tree located on the back or the middle of the slope is thought to allow the sound to be heard further so that the function of the morning call as a tertiary marker is reached (Chivers, 1980). Reicard (1998) states that gibbons tend to use tall trees, without lianas and are located on steep slopes. However, sometimes gibbons also use trees surrounded by other trees to allow for escape from all directions (Fan, 2009).
The selection of the same morning call tree characters was also confirmed by  is Altingia excelsa. In this study, the morning call tree could not be identified because of the topography that was not possible.

Figure. 2: A) Position of vocalisation tree, B) Tree without liana
Morning call on gibbons is carried out while brachiating from one branch to another on the sounding tree. In morning call activity, brachiation is observed in strata 3 and 4 with a height of <20 m (Figure 3). This strata is in the canopy of the middle and top of the tree so that the sound issued by the gibbon is able to be heard at a greater distance (Chivers, 1980). In addition, the selection of canopy strata aims to avoid predation (Reicard, 1998). The form of branching in the 3 and 4 canopy strata that is tight makes it easier for gibbons to brakiate during morning calls.
The same thing is done by Javan gibbons (H. molloch), which morning calls while brachiating using middle and upper canopy strata with altitudes above 20 m (Oktaviani, 2009).