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Orangutans as forest engineers

Orangutans quite literally are “persons of the forest,” at least according to their Malay name (orang means “person” and hutan is “forest”). But this is more than just a name. As well as their distinctively “human” qualities, these large charismatic fruit-eaters are also gardeners, forest engineers responsible for spreading and maintaining a wide array of tree species. In Borneo in particular, their role as ecosystem engineers is not simply aesthetic, they may be critical for mitigating global carbon emissions. But how exactly might orangutans do this? The answer is in their poo.

Orangutans are the largest tree-dwelling frugivores (fruit eaters) in the world. They eat leaves, flowers, some invertebrates, and may even scavenge meat on occasion, but they are primarily fruit eaters, being especially attracted to the energy-rich fruits of the biggest forest trees. While other large animals, such as elephants and rhino, are capable of spreading seeds from fallen fruits, orangutan are capable of spreading the seeds from the hanging fruits of the largest trees, making them critical for forest production and regeneration.

Image credit: Flanged male orangutan. Copyright Dr Esther Tarszisz.

In Central Kalimantan, orangutans are vital for the health and well-being of their tropical peatland homes, and not just for their own survival. Indonesia’s tropical peat swamps are massive carbon sinks, trapping up to 20% of the earth’s soil carbon. These swamps already face enormous challenges from clearing, peat drainage for agriculture and timber extraction, and subsequent fires. In 2015 alone, these activities released 0.89 – 1.29 gigatons of carbon dioxide equivalent from Indonesia’s peatlands. Crucially, collapse of these peatland ecosystems would see the release of gigatons of carbon to the atmosphere, comparable with thousands of years of this stored peat carbon being released in just decades; a Gaian “burp” of titanic proportion, and consequence.

To find out just how important orangutans are for these peatland forests, we collected over 200 individual poo samples from wild orangutans in Central Kalimantan. Poo samples were washed and the seeds collected and germinated in a local nursery, alongside those extracted by us from intact fallen fruits. We found 13 species of undamaged types of tree seeds in orangutan poo, ranging in length from <1cm to >2cm. Most of these germinated more successfully than the seeds we physically extracted from whole fruits. However, in addition to seeds from poo samples, we collected seeds that were spat to the ground by orangutans during feeding. Curiously, these spat out seeds had the highest germination success, especially for the very largest seeds (> 20 mm). The importance of the higher germination success of seeds spat by orangutan has hitherto been overlooked, but could be enormously important for the dispersal of the largest forest trees, as fruits can be carried considerable distance from a parent tree before the remnant seeds are ejected to the ground.

Image credit: Dr Esther Tarszisz planting seeds clean from orangutan poo. Image copyright Borneo Nature Foundation.

The movement of seeds from their parent tree is key to mapping seed dispersal patterns, but as well as noting seed spitting, we need to know when and where orangutans poo. To that end we followed wild orangutans habituated to humans over several seasons, noting where and when they fed, and more importantly, where they pooped! We combined this data with measures of the passage of artificial seeds (plastic beads) through captive orangutan at Australia’s Perth and Taronga zoos. We did this by training captive orangutans to swallow our seed mimics and then, you guessed it, we collected their poo. Overall, we found that plastic “seeds” took an average 70-90 hours to pass through the orangutan, and up to 120 hours to be completely eliminated. Thus, in terms of mapping the potential dispersal of seeds by orangutans, we showed that fecal deposition, or poo points, from where seeds were initially ingested lagged behind the actual animal movement by 3-5 days. This has several key implications for the forest structure and establishment of new plants. Female orangutans tended to be more centric in movements driven by food sources, but males roamed widely for food and mates. This suggests that seed dispersal implications of males and females may be very different, but critical for the maintenance of centrally located fruits as well as those spread more broadly to support wandering males.

After many hours following and watching wild orangutans it is hard to deny their human-like mannerisms. What is clear is that these “persons of the forest” may be vital for the preservation of Indonesia’s massive global carbon stores. But orangutans continue to be threatened by legal and illegal land clearing, as well as hunting. This needs global attention, and further declines, or worse extinction, of these charismatic forest gardeners may reflect a grim rehearsal of our own demise, unless we pay attention and take action now, before the poop really hits the fan!

Image credit: Mother and young orangutan. Copyright Dr Esther Tarszisz.


Featured image credit: Orangutan by ghatamos, CC BY 2.0 via Flickr.

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