The sexual development, mating habits and social hierarchy of the orangutans are more heavily dependent on their environment than had previously been assumed: where the rain forest supplies more food, the influence of the dominant male increases. In order to escape his attention, many other males remain small. This is the conclusion of a study supported by the Swiss National Science Foundation (SNSF).
In Malay, the word "orangutan" means man of the woods. In fact, however, these rain forest dwellers clad in a reddish-brown coat are our most distant relatives within the great ape family. The orangutan differs from all of the others because the male can go through two different phases of development. It is for this reason that there are two types of sexually mature males, the smaller appearing externally like the female and the larger developing secondary sexual characteristics such as cheek pads and throat pouches.
Certain small males may remain at an arrested stage of development for years or even throughout their lives without the final spurt of growth ever arriving. As Lynda Dunkel, holder of a Marie Heim-Vögtlin scholarship of the SNSF, and her colleagues at the Anthropological Institute and Museum of Zurich University have now shown (*), this developmental arrest occurs more frequently on Sumatra than on Borneo, the other south-east Asian island which is home to the orangutans.
On Sumatra, the researchers observed twice as many small males as adults with cheek pads. During a five-year period of observation in the rain forest, only a single male was seen to develop secondary sexual characteristics. By way of contrast, on the island of Borneo, there are twice as many males with cheek pads as without.
These males engage in disputes for the favour of fertile females much more often than those on Sumatra where, within the area under observation, a single dominant male monopolises sexual relations with the females. As there is more food available in the jungle on Sumatra than in the forests of Borneo, the dominant male has sufficient time to keep a close watch over the females in his environment and he drives out any other males with cheek pads before they can mate with a female.
However, smaller males without secondary sexual characteristics are less conspicuous. On Sumatra, this makes it easier for them to copulate with a female, even though the females put up resistance in 60% of the cases observed. Forced copulation also takes place on Borneo. There, however, as the males are constantly engaged in fighting in which the smaller ones never prevail, the advantages of developmental arrest disappear.
The fact that food supply in the forest has such a strong impact on the mating behaviour of the orangutan came as a surprise to Dunkel. "It goes to show," she says, "that the organisation of these great apes – and perhaps that of our ancestors as well – is more variable than we had hitherto assumed. Apparently, natural selection not only moulds appearance but also adapts social behaviour to the conditions of the local environment."
(*) Lynda P. Dunkel, Natasha Arora, Maria A. van Nordwijk, Sri Suci Utami Atmoko, Angga Prathama Putra, Michael Krützen and Carel P. van Schaik (2013). Variation in developmental arrest among male orangutans: a comparison between a Sumatran and a Bornean population.
(Manuscript available from the SNSF; e-mail: email@example.com)
Anthropologisches Institut und Museum
Tel.: +352 27 29 08 43
Abteilung Kommunikation | Source: Schweizerischer Nationalfonds SN
Further information: www.snsf.ch/
More articles from Studies and Analyses:
Footwear’s (carbon) footprint
23.05.2013 | Massachusetts Institute of Technology
Rate of bicycle-related fatalities significantly lower in states with helmet laws
23.05.2013 | Boston Children's Hospital
This morning at 05:45 CEST, the earth trembled beneath the Okhotsk Sea in the Pacific Northwest. The quake, with a magnitude of 8.2, took place at an exceptional depth of 605 kilometers.
Because of the great depth of the earthquake a tsunami is not expected and there should also be no major damage due to shaking.
Professor Frederik Tilmann of the GFZ German Research Centre for Geosciences: "The epicenter is exceptionally deep, far below the earth's crust in the mantle. Such strong ...
The Ring Nebula's distinctive shape makes it a popular illustration for astronomy books. But new observations by NASA's Hubble Space Telescope of the glowing gas shroud around an old, dying, sun-like star reveal a new twist.
"The nebula is not like a bagel, but rather, it's like a jelly doughnut, because it's filled with material in the middle," said C. Robert O'Dell of Vanderbilt University in Nashville, Tenn.
He leads a research team that used Hubble and several ground-based telescopes to obtain the best view yet of ...
New indicator molecules visualise the activation of auto-aggressive T cells in the body as never before
Biological processes are generally based on events at the molecular and cellular level. To understand what happens in the course of infections, diseases or normal bodily functions, scientists would need to examine individual cells and their activity directly in the tissue.
The development of new microscopes and fluorescent dyes in ...
A fried breakfast food popular in Spain provided the inspiration for the development of doughnut-shaped droplets that may provide scientists with a new approach for studying fundamental issues in physics, mathematics and materials.
The doughnut-shaped droplets, a shape known as toroidal, are formed from two dissimilar liquids using a simple rotating stage and an injection needle. About a millimeter in overall size, the droplets are produced individually, their shapes maintained by a surrounding springy material made of polymers.
Droplets in this toroidal shape made ...
Frauhofer FEP will present a novel roll-to-roll manufacturing process for high-barriers and functional films for flexible displays at the SID DisplayWeek 2013 in Vancouver – the International showcase for the Display Industry.
Displays that are flexible and paper thin at the same time?! What might still seem like science fiction will be a major topic at the SID Display Week 2013 that currently takes place in Vancouver in Canada.
High manufacturing cost and a short lifetime are still a major obstacle on ...
24.05.2013 | Life Sciences
24.05.2013 | Ecology, The Environment and Conservation
24.05.2013 | Physics and Astronomy
17.05.2013 | Event News
15.05.2013 | Event News
08.05.2013 | Event News