Session 9: The Orangutans

1.  Key Concepts:

Orangutans represent the extremes of many parameters.  They are the largest of the primarily arboreal mammals, they have the longest period between births (up to 9 years), are the most solitary of the diurnal primates, and they are the most sexually dimorphic.  This variety in orangutans has proven very difficult for researchers to study.

2.  Terms & Definitions:

Bimaturism

The presence of two distinct morphological forms, present in adulthood.

3.  Orangutan Taxonomy:

The orangutan has been historically classified as two subspecies.  However, recent investigations of the orangutan clade, based on molecular, morphological, and some behavioral characters, have resulted in a reclassification of the orangutan into two distinct species.  Interestingly, adult males exhibit bimaturism which includes a flanged and unflanged morphology.  This bimaturism does not correspond with the subdivision of species; instead, it is exhibited in males of both species.

• Pongo abelii: endemic to the Indonesian island of Sumatra.
• Pongo pygmaeus:  endemic to the Indonesian island of Borneo.

Although historically, the orangutans have had  a wide range—extending into China, Thailand, Vietnam, throughout much of Indonesia, and possibly Burma and northeastern India—ecological and anthropogenic factors contributed to the collapse of the orangutan, and now populations are restricted to Borneo and Sumatra.  The Bornean species has recently been further broken down into three subspecies.  This is based largely on genetic and morphological variation, with most evidence supporting rivers as the main geophysical barriers.

• P. pygmaeus pygmaeus: found in Sarawak and northwestern Kalimatan.
• P.p. wurmbii: found in southwestern and central Kalimantan.
• P.p. morio: found in Sarawak and east Kalimantan.
  

Photo by Schalk Lubbe. Some rights reserved.

4.  Ecology & Social Organization:

Ecology:

The orangutans are largely frugivorous.  However, the Asian rain forest in which they live is dominated by dipterocarps.  The trees of the dipterocarp family periodically experience mast fruiting.  This periodic mast fruiting heavily influences the behavioral ecology of the orangutan.

To deal with mast fruiting events, and the overall lower availability of food resources throughout the rest of the year, orangutans spend their days feeding and resting. 

Daily Activity Patterns:

• Feeding - 50%.
• Resting - 34.4%.
• Traveling - 12.9%.
• Nest Building - 1.3%.
• Socializing, mating, grooming - 1.3%.

Social Organization:

Most orangutan ranging behavior is poorly understood because of their exceedingly large ranges.  Adult females minimum range size averages 850 ha while adult males (both flanged and unflanged) minimum range size averages 2500 ha.  Orangutans, unlike other apes, tend to be female philopatric.  Females tend to have overlapping ranges, and true territoriality is not often exhibited.

Grouping Patterns and Social Organization:

Predominantly solitary, the orangutan has a dispersed social organization. Can be involved in three types of groupings: Travel bands—where individuals co-feed and travel together during high food availability. Temporary aggregations—where individuals feed together but travel independently during food scarcity. Consortships—where receptive females travel with either flanged or unflanged males for hours, days, or weeks. Feeding competition tends to occur around the patchily distributed food sources. Orangutans have an unusually low level of grooming among individuals.  This has been linked to either a smaller need to participate in the social function of grooming OR a lower ectoparasite load. Dominance patterns between flanged and unflanged males exist, as evidence of displacement at mast fruiting events, but is not fully understood.

Photo by Bob Rockefeller. Some rights reserved.

5.  Sexual Dynamics:

The male great call has been documented as being heard up to 800 m away.  It is used to define territories between flanged males, to coordinate group movements and cohesiveness, and to attract females.  Only flanged males call; unflanged males, which are lower in status than flanged males, may be tolerated by flanged males in their territories.

• http://www.orangutan.com/orangutans_facts.html
  

Photo by Tim Williams. Some rights reserved.

 
Mating behavior:

Due to the size differential between males (80 kg, with indeterminate growth capabilities) and females (39 kg) and to their dispersed social organization, mating behavior occurs in the context of consortships.  Flanged and unflanged males take different strategies with flanged males being preferentially sought out by receptive females.  However, the the dispersed nature of orangutan society prevents the dominant, flanged male from preventing subordinate, unflanged males from mating.  Flanged males frequently participate in coerced copulation.

6.  Additional Material:

Primates in Perspective.  2007.  C. J. Campbell.  A. Fuentes.  K. C. MacKinnon.  M. Panger.  S. K. Bearder.  Oxford University Press.

Chapter 17:  Orangutans in Perspective: Forced Copulations and Female Mating Resistance - Knott & Kahlenberg