The mouth of a mature Sumatran male orang-utan, Pongo abelii, has side flanges that can open up to for a handy megaphone effect. Carel P.van Haik and his colleagues from the University of Zurich decided to see if this "long call" megaphone effect can be used to show up how the male moves around his forest. Luckily, there are still 7 orang-utans/square km in the Suaq Balimbing peat swamp forest in Gunang Leuser National Park, so this study could check extensive communication activity there.

The benefits of planning to animals may be arguable, but humans gain so much from looking one day ahead, that orang-utans must gain the same kind of benefits over a short period. The tiny subset of animals that use planning would have changeable target locations and an essential daily return, for, in this case, foraging the forest.

The brain power needed would be offset by the increased returns but would only be found in large-brained animals, for example. An episodic memory to recall specific events is present in humans and probably, chimpanzees. Even birds such as 2 species of jays have recently proved to plan their storage of food. Next, rodents such as rats seem destined to prove their genius in the same area.

To separate associative learning from this very real planning of routes, we have to know that orang-utans communicate their ever-changing travel patterns to other individuals. Just hours before they carry them out, this actually makes it non-associative. Every day they traverse an average of 1000m within the 2,000 hectare home range. The sounds made in the long call last from 80 seconds to 4 minutes and are audible for 1000m on about 4 occasions every day. They have been shown to provide the caller's identity and also indicate they are not territorial calls.

These are caused by the presence of other "flanged males." The effect is interesting. While low-ranking, and often unflanged males are repelled, both mature males and females are attracted. For example, females with offspring can stay within earshot as this may reduce harassment by other males. The paper's figure shows this well:

Orang-utan long call; PLOS ONE Credit: © Tomer Czaczkes

To reach the calling male or remain within earshot of him, female orangutans should move in the direction of the flanged male if they hear his long call only faintly. Due to the large cheek flanges that probably act as a megaphone, to female A the call is not as loud as to female B, even if they are at the same distance from the calling male, and A should move in the direction of the male whereas B need not. However, this system works best if the male actually travels in the same direction as he is calling.

Because the male faces in one direction during the long call, it can be considered faint by female A in the cartoon. Female B on the other hand would be able to understand the direction he is travelling is towards her and this could promote mating or simply help her to prevent any other male from approaching.

To test the direction of the call and the travel, observations were made on new calls, to see if their direction implied a new travel direction. Evening calls may be cheaper on the telephone, but these orang-utan calls could also show how the next day's travel would proceed, even if the male didn't want to travel until after he had made his nest and slept!

Audience is very important to a performer and the flanged males' females had to be checked to see in what direction they set off on the following morning. What happened was that females stayed within earshot during the call. The next morning, when there was no further communication from males before 0800 hours, the ladies set off in directions significantly less than 90 degrees from the call direction. This was at a constant distance from the male, while males retreated from the sound and moved at totally unconnected angles to that of the call.

This is probably the first time that great apes have been able to demonstrate this episodic memory. The natural context is perfect for demonstrating that the last long-call gave a better than random prediction of a male orang's plans till 1600 hours the next day. (ie. 22 hours later) The listeners would also have episodic memory as they delayed their response the next morning before responding with a travel direction more or less connected to the call direction.

Benefits of this behaviour are legion. Mating is the greatest, and protection from other, even unflanged, males is another. There is also back-up for the evidence. It's a detour that males tend to make in the early morning before resuming their long-term direction of travel. Instead of returning to their nest site, they forage and eat a little and then immediately set off in the specified direction.

Carel P. van Schaik, Laura Damerius, Karin Isler publish their paper in the journal PLOS ONE.