Plants often need some assistance when it comes to moving their progeny away from home. Employing animals to disperse their seeds is a great way for plants to achieve this. Rodents and birds often come to mind, but would you think of orangutans? Yet, these remarkable great apes are actually the largest tree-dwelling fruit-eating animal in the world. Wondering how these charismatic creatures may impact the distribution of plant species in their habitat, Esther Tarszisz, a researcher with the Bornean Nature Foundation, supervised by Adam Munn from the University of New South Wales, Australia, in collaboration with a team of international researchers, set out on a mission to untangle the relationship between orangutans and their forests.
The first step was to time how long it takes a seed to travel through the entire digestive system of an orangutan and to show up in its poop. This is pretty important information if you want to know where seeds are likely to turn up after being eaten. The team chose different coloured beads ranging from 2 to 6 mm in diameter, similar to the size of seeds found in the faeces of wild orangutans, and added the seed mimics into the diets of six female and male orangutans in zoos in Australia. Tarszisz then had the job of spying on the orangutans throughout the day to catch them in the act of pooping, at which point she noted the time, collected the deposit and counted the plastic seeds that it contained. With these data, the team was able to calculate that all of the bead seeds took an average of 76 h to pass through the gut, regardless of their size.
Once the researchers had this figure, they ventured into the wilderness of peat swamp forests in Central Kalimantan, Indonesia, where they followed four females and three males around the forest. They documented the daily lives of these animals, including where they moved through the forest, why they moved, where they ate, how long they foraged and what they consumed. Orangutans definitely have a sweet tooth for fruit – this made up the chief component of their diet. The team also discovered that, perhaps not surprisingly, a day in the life of a female orangutan is quite different to that of their male counterparts. While females devoted their time to eating, displaying quite predictable movement patterns in their home range, males roamed much more haphazardly and over greater distances; they seemed to have other things on their minds than just eating, such as mating and fighting or avoiding confrontations.
Combining the information on seed transit time with details of the orangutans’ movements, Tarszisz and her colleagues realised that the orangutan population can influence the distribution of plant species in the peat swamp forest. It seems that within the 76 h that it takes for seeds to pass through the orangutan’s digestive system, the females have often returned to the core area of their home range, so the seeds that they deposit in their faeces will probably grow into trees not far from the parent plants upon which the females dined. In contrast, males would disperse seeds further and more randomly. This combination of movement patterns suggests that the gene flow of plant populations will occur over a wide area.
Tarszisz and her colleagues warn that any changes to the Indonesian environment are likely to cause orangutans to alter their behaviour, thus influencing their vital role as seed dispersers. Habitat loss in particular may cause populations of both orangutans and plant species to become genetically isolated, especially as there are no other dispersers of large seeds in this ecosystem. Genetic diversity is vital if orangutans and the forest are to survive.