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Siamang

 

Overview

Siamangs (Symphalangus syndactylus) belong to the family of gibbons (Hylobatidae) or small apes and represent one out of four genera within this group. They represent the most ancient branch within the Hominoidea, since they split approximately 18 years ago from the line leading to the great apes.

The division and phylogeny of the different species of small apes is still discussed, but to date there are at least 12 different species all restricted to the tropical rainforest of Southeast Asia.

Siamangs are the largest species of small apes and are the only representatives of this group that live sympatrically with other gibbon species (Hylobates agilis, Hylobates lar) throughout their whole range of distribution. With an approximate weight between 9–12 kg there are twice as heavy as other gibbon species. Both sexes are black and have a laryngeal air sac that is inflated during the production of their species-specific, complex duet songs.

Although many gibbon species are close to extinction, siamangs seem to represent a stable population, with an estimate of approximately 360,000 individuals (MacKinnon, 1987). However, there are no recent surveys on the siamang population, with the exception of some studies restricted to certain areas such as the Bukit Barisan Selatan National Park (O'Brian et al., 2004).

 

Distribution and ecology

Siamangs are restricted to the tropical mountain forest on the island of Sumatra and the peninsular Malaysia preferring altitudes between 700-900 m. Due to their arboreal lifestyle, siamangs inhabit the canopy between 20 - 40 meters and almost never enter the ground. Siamangs spent 55% of their daily activity with foraging, 29% with resting and 16% with traveling. They are often considered as a folivorious species because their diet consists of a higher proportion of leaves in comparison to other gibbon species.

 

Social structure and behaviour

Siamangs live in monogamous social groups consisting of an adult pair and its one to three offspring born with an interval between two and three years. There is virtually no sexual dimorphism and no differentiated “role behavior” between the adult male and female. The group life is characterized by a strong bonding between the family members that spend most of their time in close proximity.

Each siamang family inhabits a particular home range with a size between 15 and 35 ha, whereof about 75% are defended as territory against other siamang groups. A probable function in this regard is attributed to the complex duett songs of siamangs that are mostly uttered in the early morning hours. Maturing offspring is forced to emigrate and form a new group after the corresponding pair partner is found.

 

Projects

  • Use of gestures and facial expressions in siamangs and great apes
  • “Towards a ‘grammar’ of gesture”: Comparison of gestural communication of nonhuman and human primates — in collaboration with Cornelia Müller, Ellen Fricke (European University Viadrina, Frankfurt/Oder) and Hedda Lausberg
    • Linear and simultaneous structures in gesture use of great apes
    • Ontogeny of gestural communication in great apes
  • Socio-cognitive skills of gibbons — in collaboration with Juliane Kaminski, Max Planck Institute for Evolutionary Anthropology

 

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