2016 | Yao Noi Island, Thailand

Long-tailed macaque (Macaca fascicularis) nut cracking

It’s easy to see animals as an unchanging part of the landscape, a backdrop to the hectic pace of human development. This assumption, that animals and their behaviour are essentially the same throughout time, is paralleled by researchers who think that studying a given animal in a zoo is the same as studying the same species in their natural habitat. One animal is the same as any other.

Today’s post refutes that idea. It relates the story of the nut-cracking macaques of Ao Phang-Nga National Park in southern Thailand. These macaques were first seen using stone tools to open palm nuts just a few years ago, during surveys for the Primate Archaeology project that I led while I was at Oxford University. It’s a tale—still unfolding—of how a resourceful group of monkeys is using technology to adapt to a changing world.

Seas to trees

Following the destructive Indian Ocean tsunami of late 2004, island surveys along Thailand’s west coast found long-tailed macaques (Macaca fascicularis aurea) hard at work, using stone tools to break open shellfish. Primatologist Suchinda Malivijitnond and her Thai team reported their findings in 2007, and since then a long-term project led by Michael Gumert of Singapore’s Nanyang Technological University has delved into the details of this behaviour. (It turns out that this finding was actually a re-discovery, following a short report from 1887 on the same behaviour in Burmese waters by Alfred Carpenter, commander of the Indian Marine Survey Steamer Investigator.)

One distinctive pattern stood out almost immediately—the monkeys only seemed to use stone tools along the shoreline. They used small handheld tools to crack open oysters that were still attached to rocks in the intertidal zone (the part of the shore that is covered by water at high tide, and exposed to the air at low tide), while also collecting marine snails. The macaques took the snails to flat ‘anvil’ rocks, where they smashed open the prey to get at the mollusc meat inside.

Here’s a photo I took of a female adult long-tailed macaque breaking into oysters at low tide on Piak Nam Yai island, Thailand:

Even when later surveys by Gumert and our Oxford team surveyed for macaques opening sea almond (Terminalia catappa) seeds with heavy stone pounding tools, this activity was again restricted to the intertidal zone and adjacent forest border. The logical explanation was that monkey stone tool use was primarily for shellfish, with the shoreline nuts (and an occasional washed-up coconut) a secondary target when they overlapped with the molluscs.

I’ll go into much more detail on macaque shellfish processing another time. To keep following our main story we instead need to leave the west coast monkeys, and take a detour down the coast, around the corner, and up a steep hillside on the island of Yao Noi. This island is in Phang-Nga Bay, east of Phuket, a region of tourist resorts and crystal waters.

During field surveys in February and October 2016, Lydia Luncz (a chimpanzee expert and postdoc on my Primate Archaeology team) led an effort to explore deep into the forests of Yao Noi. Following straight lines as a guide through the dense trees and steep drops, the team came across an abandoned palm oil plantation. The local population of long-tailed macaques regularly passed through the plantation as part of their daily wanderings.

We were surprised to find abundant evidence of nut-processing, including hundreds of anvil and hammer stones, plus nut shell debris, throughout the plantation. Not only were the macaques making use of this human-provided resource, but they were adapting their tidal, animal-focused tool-use to a new setting and food. Further, the plantation was only set up post-2000, so this adaptation happened in just the last dozen years or so.

This is an image from our first report on this activity, showing a nut-cracking site, the palm nuts themselves (both fresh and dry), and some of the hammer stones used by the monkeys:

The median weight of the stone hammers was a few hundred grams, with a strong preference for the local limestone. Interestingly, every granite stone that our team found had been used by the monkeys, which suggests that they would have used more of this hard, dense material if they could get it.

Along with the archaeological survey of monkey tool-use sites, we set up motion-activated camera traps to record their behaviour. The macaques were not habituated to humans, and so they would flee the sites long before Lydia and our team had a chance to observe them in action.

The recorded footage showed that it was definitely the long-tailed macaques that were responsible for the nut-cracking sites, not local people or the pig-tailed macaques also roaming the island. There was purpose to their actions as well: they seemed to only visit the plantation when they were hungry for palm nuts. And they only opened the nuts with tools, never directly with their teeth or hands. They were clearly experienced in dealing with this hard, enclosed food.

A slippery comparison

Looking through the existing record of wild, tool-using primates, one obvious comparison with the Yao Noi macaques stands out. Western chimpanzees (Pan troglodytes verus) at the small Bossou site in Guinea also target oil palms, which in their case are a local species—the tree’s scientific name Elaeis guineensis literally means ‘Guinea oil’. Bossou’s chimpanzees use handheld hammerstones and place the nuts on movable stone anvils, a tool-using behaviour that primatologist Tanya Humle has described as:

probably the most sophisticated performed by any nonhuman animal as it requires the coordinated complementary use of both hands and the combination of three external objects – the nut and the hammer and anvil stones.

The Yao Noi macaques similarly use portable hammer, anvil and nut combinations (as do wild capuchins cracking cashews in northeast Brazil). Here’s an example from our camera trap footage:

Unlike other chimpanzee communities, however, Bossou chimpanzee nut cracking is specialised solely on the palm nut. Nuts opened by other west African chimpanzees are available at Bossou—for example those of the genus Parinari—but in decades of observation they have not been a target of tool use.

In this respect at least, the Thailand macaques show a more flexible approach to their stone tool use. Based on the similar shellfish tool use seen in other island macaques in Thailand and Myanmar, it is most probable that the Yao Noi group were already opening oysters and marine snails at the time that the palm plantation was established. Their extending this approach to another type of enclosed food, and in an unfamiliar tool-using environment deep in an upland forest, is a point of difference with the Bossou group that should caution us against making direct comparisons between the two primates.

Not only is research on wild macaque technology in its infancy compared to work on chimpanzees, but the recent adoption of palm nut processing shows that it’s an ongoing and evolving practice for the monkeys. Together, these facts suggest that even more diverse tool use will be found among wild long-tailed macaques in southeast Asia. And the future is not our only source of inspiration: stone tools are incredibly durable, making archaeological study of past non-human primate behaviour just as accessible as that of our own stone-tool-wielding ancestors. To borrow a phrase from Alfred Carpenter’s namesakes: we’ve only just begun.

Sources: Malaivijitnond, S. et al. (2007) Stone-Tool Usage by Thai Long-Tailed Macaques (Macaca fascicularis). American Journal of Primatology 69: 227–233. || Carpenter, A. (1887) Monkeys opening Oysters. Nature 36: 53. || Gumert, M. & S. Malaivijitnond (2012) Marine Prey Processed With Stone Tools by Burmese Long-Tailed Macaques (Macaca fascicularis aurea) in Intertidal Habitats. American Journal of Physical Anthropology 149: 447–457. || Falotico, T. et al. (2017) Analysis of sea almond (Terminalia catappa) cracking sites used by wild Burmese long-tailed macaques (Macaca fascicularis aurea). American Journal of Primatology 79: e22629. || Luncz, L. et al. (2017) Technological Response of Wild Macaques (Macaca fascicularis) to Anthropogenic Change. International Journal of Primatology 38: 872–880. || Proffitt, T. et al. (2018) Analysis of wild macaque stone tools used to crack oil palm nuts. Royal Society Open Science 5: 171904. || Humle, T. (2011) The Tool Repertoire of Bossou Chimpanzees. In Matsuzawa, T. et al. (eds) The Chimpanzees of Bossou and Nimba, pp. 61-71.

Main image credit: Michael Haslam, Primate Archaeology project, Ao Phang-Nga National Park, Thailand. || Second image credit: Michael Haslam, Piak Nam Yai, Laemson National Park, Thailand. || Third image credit: Luncz et al. (2017) Fig. 2 || Fourth image credit: Michael Haslam, Primate Archaeology project, Ao Phang-Nga National Park, Thailand.