As these calls are amplified in one direction, other orangutans can use that information to meet up with, or avoid, the caller. So, at the end of the day, do the male orangutans end up more or less where they said they would be? The following graphic illustrates two possible expected outcomes. If orangutans travel independently of the direction in which they’re shouting, the average deviation between the two would be 90° (left). If, on the other hand, the travel direction was not random, there would be considerably less than 90° between long call and travel direction (right).
When the data was analyzed, the researchers found that the long call direction did more or less predict the subsequent travel direction. At least, the deviation between those two directions was significantly less than 90°. In particular, males began their day by moving in the direction they had announced before retiring the previous night. At the very least, this indicates some ability to remember what they had planned to do the night before.
The graph depicts the median of deviation angles between travel direction and the direction of the last spontaneous long call of the previous day, given shortly before nesting or from the nest. Time is clock time during the day following the long call.
The authors conclude from this that the orangutans are indeed planning for the future. They decide on a direction of travel, announce where they intend to go, and then go there.
One counterargument is that the apes have to face in some direction during the long call, and they simply continue traveling in the direction they’re already facing. In other words, they aren’t really planning ahead, it’s just coincidence that the call direction and travel direction match up. However, keep in mind that we’re not talking about striding along a road. It’s impossible for a large animal to travel in a straight line through the trees of Sumatra. It’s not even clear what clues the animals could be using to navigate through the canopy. Yet, despite following a convoluted, meandering path, the orangutans still end up where they said they would be.Carel P. van Schaik, Laura Damerius, & Karin Isler (2013). Wild Orangutan Males Plan and Communicate Their Travel Direction One Day in Advance PloS ONE, 8 (9) doi:10.1371/journal.pone.0074896.g005.