Sumatran orangutan (Pongo abelii) seed extracting

Some plants really like you. They package their seeds in delicious, soft, colourful treats that primates like us can’t get enough of, safe in the knowledge that we’ll redistribute their seeds for them (either before or after digestion). Grapes, oranges, apples, watermelons, tomatoes and the like deserve our appreciation for being such pleasant playmates.

Other plants, though, not so much. These misers pull every trick in the evolutionary book to hide and protect their seeds. They use tough outer shells (looking at you macadamias and coconuts), or add a bitter tasting or even noxious coating (thanks cashews). And then there’s the Neesia tree, found in the rainforests of southeast Asia. As Indonesian orangutans have found, a dense shell is just the start of Neesia’s defence. Given a chance, this plant will try to stab you.

Close, but no cigar

Growing up to 40 metres tall and over a metre in diameter, Neesia is a close relative of another well-known Asian fruiting tree, the durian. The latter gives its name to the plant tribe that contains both it and Neesia, the Durioneae. If you’re ever forced to differentiate them, just open up the fruit (if you can)—inside its thick, woody shell the Neesia fruit is lined with a myriad of fine, small, incredibly sharp hairs. Likened to fibreglass, they appear brownish while still in the plant, and whitish once they’re released into the light. If you look closely you’ll see how they form fuzzy margins around the inner seeds:

The fruit is initially a closed whole, which some orangutans in both Sumatra and Borneo—those with the strength to do so—tear open with their teeth and feet. But once it begins to ripen, the ca. 15-20cm long fruit gradually splits itself into the five-lobed structure seen above. (The Neesia tree also has five-petaled flowers, one of those evolutionary quirks.) That’s when the Sumatran orangutans at the Suaq Balimbing site get interested.

The orangutan detaches the opening fruit from its branch, typically after making a short tool from a nearby twig, or having arrived already carrying a tool. That thin twig, less than a centimetre across, is broken off, stripped of its bark and trimmed to around six inches long. Then the orangutan takes both the fruit and its tool to a comfortable position on a solid branch. The target seeds, roughly the size of lima beans and high in nutritious fats, are deep within the crevices between the splitting fruit valves. The seeds actually may be within reach of a long, red-haired finger, but those irritating spikes warn against such a rash move.

The first western scientific report on orangutan Neesia tool use was made by primatologist Carel van Schaik, then of Duke University and now of the University of Zürich, with colleagues from the US and Indonesia. This is how they reported the ape behaviour in a 1996 paper:

Holding the tool in their mouth, they inserted it into the cracks between two valves, scraped out the hairs by moving it toward the fruit's apex, and removed the accumulated hairs from the tool by blowing and wiping with a finger nail. Subsequently, the seeds would be pushed toward the apex of the fruit and scooped out with one finger or with the (hand-held) tool.

You can watch this activity in the video below. It’s actually a 50 minute documentary on orangutans, but the Neesia tools and their use are in a two minute section from around 31 to 33 minutes if you want to skip ahead:

[Note: although I can’t embed it here, you can see the same footage in higher resolution at the PBS Learning Media site.]

There are a few noteworthy parts to this behaviour. The first is that orangutans hold the tool in their mouth, and the fruit in their hands, while wedging out the seeds. That isn’t how you or I would likely approach the task, but orangutans routinely make use of their flexible, sensitive and reasonably strong lips to manipulate (mouthipulate?) objects. The Suaq Balimbing apes similarly hold stick tools in their mouth to extract insects from tree hollows. It makes sense when a primary concern is using the hands and feet to avoid falling out of a tree, but orangutans seem to have adopted this strategy much more readily than the other great apes such as chimpanzees and gorillas. Humans do make use of our teeth and mouths when our hands are full, for example while scraping hides or sewing, or to temporarily hold an object, but our tendency is to go in hands first.

It also means that between uses, the apes can resemble someone sitting contentedly with a cigar:

A second thing to note is that the fruit is moved up and down to wheedle the seeds from their spiky nest—it’s not all about moving the stick tool. So why use a tool at all, and not just scratch the partly-open fruit on a nearby exposed twig, staying as far as possible from the snow of irritating hairs? The advantage is that the mouth-hold means the orangutan maintains a close eye on proceedings. This permits fine control, as well as allowing the apes to target individual seeds that can be pushed up and out of the fruit, each to be nabbed by those flexible lips. Remember that all this is happening perhaps tens of metres up in a tree canopy—any seeds or their oily attachments that pop out unexpectedly are lost to the forest floor below (which may be a win from the plant’s perspective, but not the ape’s).

A final point is that the orangutans don’t avoid the Neesia hairs altogether. But they primarily end up in clumps on the end of the stick tool, from whence the tool users flick them away with a fingernail. The result is a blizzard of tiny, invidious spears floating downwards and away from the seated ape, as in the image below. It does create quite a hazard for any human observers watching from beneath, though, as keen as they are to document this unusual behaviour. You really don’t want a bunch of Neesia hairs in your eye.

Cultured eating

Prof. van Schaik and his team started work at Suaq Balimbing in August 1993, and by 2001 they had racked up over 17,000 hours following orangutans through the site’s swamp forest. They’d also collected a lot of data on other orangutan sites, including Gunung Palung on Borneo (with over 25,000 hours of observation), and more swampy sites in Sumatra that had both orangutans and Neesia, such as Ie Mdamai, Muara Singgersing and Krueng Seumayam. These surveys allowed the scientists to compare tool use behaviour between islands and sites.

They found that tool use was present at most—but not all—of the Sumatran swamp sites, and absent in the Borneo observations. This geographical pattern suggested that it was not merely a subspecies difference driving the tool use. Instead, the Sumatran tool-using orangutans were more likely than their Borneo relatives to hang out with their ape neighbours, meaning that once invented, Neesia tool use could be copied and spread locally. In their 2001 report, the researchers pointed out that chimpanzees and even we humans behave similarly—most of us copy things others have figured out rather than inventing new things independently:

the implication is that great apes are better copiers than inventors, and human cultural history is consistent with this idea as well.

These comparisons were taken even further with a detailed genetic study the team released in 2011. That work showed that even if genetic differences and diverse environments were taken into account, there was still variation that couldn’t be explained. The conclusion is clear: the tool-use shown by the Suaq Balimbing orangutans can best be described as cultural. That is, their behaviour is learned and cultivated within groups thanks to social interaction. Neesia tool use shares the same foundations as differences we find between human groups that differ in how they eat a particular food, depending on where and by whom they’re raised.

Think of rice, eaten by various cultures with forks, spoons, chopsticks or directly with the hands. It’s like that, if the rice was actively trying to stab us at the same time.

Sources: Milton, K. (2006) Diet and Primate Evolution. Scientific American; https://www.scientificamerican.com/article/diet-and-primate-evolution-2006-06/ || Soepadmo (1960) A monograph of the genus Neesia Blume (Bombacaceae). Reinwardtia 5:481-508. || van Schaik, C. et al. (1996) Manufacture and use of tools in wild Sumatran orangutans. Naturwissenschaften 8:186-188. || Fox, E. et al. (1999) Intelligent tool use in wild Sumatran orangutans. In S. Parker et al. (eds) The Mentalities of Gorillas and Orangutans. Cambridge University Press, Cambridge. || van Schaik, C. (2006) Why are some animals so smart? Scientific American 294(4):64-71. || van Schaik, C. & C. Knott (2001) Geographic variation in tool use on Neesia fruits in orangutans. American Journal of Physical Anthropology 114:331-342. || Ktutzen, M. et al. (2011) Culture and geographic variation in orangutan behavior. Current Biology 21:1808-1812.

Main image credit: PBS Learning Media; https://www.pbslearningmedia.org/resource/nat15.sci.lisci.tool/orangutans-tool-use/ || Second image credit: @lithosgraphics via Twitter; https://twitter.com/lithosgraphics/status/1076825307582091264 || Video credit: https://www.youtube.com/watch?v=grRiDgONILk || Third, fourth and fifth images credits: PBS Learning Media; https://www.pbslearningmedia.org/resource/nat15.sci.lisci.tool/orangutans-tool-use/