The past couple of months have been excellent for our data collection, as we’ve encountered a number of parties of orangutans. This is a more common occurrence in the high productivity forests of Sumatra, where we're working, than on Borneo, where animals tend to be much more dispersed due to limitations in food availability. For us researchers, it’s great! It’s much easier to follow a number of animals and watching their interactions is fascinating, not to mention entertaining! One such group we encountered comprised Irma (our focal animal), her infant Irfan, Suci, the injured female described in my previous post, and her infant Siboy. This, we figured, would be a nice group to follow, as the two infants could play while the mothers ate, and for a while at least it looked like we would be correct, as the pair grabbed, playfully bit and wrestled with one another.

Why is play important? Despite its ubiquity, to date it’s unclear what the function of social play is. Yet as an energetically costly and indeed risky behavior it should have some adaptive value in order to be maintained across such a wide range of species. Accordingly, a number of hypotheses have been suggested including improved motor, sensory, social and cognitive skills, with play potentially playing a key role in the evolution of intelligence and behavioral flexibility.

For orangutans, I think it’s likely that these social encounters do provide the benefit of helping to develop motor skills in a younger infant like Siboy. He will learn to move more rapidly through the canopy as he learns exactly what movements his body is capable of performing through these interactions, which require anatomical manipulations very different to those he makes while travelling and eating. This has an add-on benefit for Siboy’s mother Suci, who won’t have to carry him as frequently or bridge gaps between trees while waiting (impatiently) for Siboy to cross. Thus, in addition to improving the young one’s motor skills, play could reduce mother-infant conflict.

For an adolescent like Irvan, I think these social interactions provide an important opportunity to develop affinitive relationships with other orangutans, a vital consideration for when he sets off and needs to survive on his own. Indeed, a positive relationship with Suci might lead to mating opportunities in the future.

While the two infants played, the females didn’t interact. Instead, Irma watched from a distance while Suci maintained a much closer position, ready to jump in and prevent Siboy from being hurt by his much larger playmate. This is pretty typical behavior for orangutans, though females with really young infants will often join in playtime when bigger orangutans are involved, demonstrating to the infant how he or she should play, while also keeping them safe from harm.

The Siboy and Irvan happily wrestled until Irvan got a little too rough. Quickly, Suci collected her crying son and swung a little distance away. Irvan took this as part of the game and began grabbing at Siboy over his mother’s shoulders.

For the next couple of hours, the Irvan chased poor Suci around the clearing, grabbing and slapping at Siboy whenever he got close enough. Suci did not respond aggressively. Perhaps because we’ve followed her more than any other orangutan at Sikundur research station, or perhaps because she saw it as a viable tactic, instead of running from Irvan she would move closer and closer to us, which kept the less habituated male at a bit of a distance.

Irma didn’t offer any respite from this harassment, maintaining her position sitting and watching warily from above. From an evolutionary standpoint, it would be indirectly advantageous to her fitness for her infant to develop his “play” skills in this way, since chasing and harassing females is the principal way unflanged males obtain copulations, rather than calling and waiting like their flanged counterparts, whom females will actively seek to encounter. Additionally, a more direct benefit to Irma’s fitness, given Suci’s injuries, is that it would increase her chances of obtaining access to food and territory if Irvan was to hurt Suci further.

Eventually, Suci decided she’d had enough and used an interesting tactic to escape her adolescent tormenter, climbing down the tree and walking away on the ground.

Sumatran orangutans typically do not spend much time on the ground, unlike their Bornean equivalents (especially in the east of the island), supposedly because of predation by tigers. However, at Sikundur, we often see a number of our focal animals across all sex-age classes come down to the ground to walk for periods, drink from streams and eat mineral rich soil. Perhaps, this is because we don’t have any tigers at the site, or because the ground is extremely dry here compared to the swamps at Suaq where much of the Sumatran research has taken place. I think it’s actually partly because of habituation since it is only the well-habituated individuals, like Suci, whom we see coming to the floor. We haven’t followed Irvan as much, so while he came close to the ground, he stayed a few meters up and quickly lost sight of Suci (as did we).

It will be fascinating to see how encounters within this group develop in the future, along with all of the other social interactions within our orangutan population. Taken together over the long-term data set we’re collecting, observing the different relationships and encounters within the population and how they change will allow us to develop our understanding of orangutan social structure at Sikundur, and contribute to our knowledge on the evolution of play.