Rat-hunting macaques: a greener solution for rodent pest control in Malaysian oil palm plantations?

In the lush landscapes of Southeast Asia, oil palm plantations stretch across vast land, producing palm oil, one of the world’s most versatile and economically significant commodities. However, the cultivation of oil palm has long been accompanied by environmental concerns about deforestation, habitat and biodiversity loss, and the use of chemical fertilisers and harmful pesticides. In the quest for more sustainable practices, an unlikely ally has emerged from the depths of the rainforest: southern pig-tailed macaques.

Traditionally rather seen as pests because they feed on the oil palm fruits, these intelligent primates are now being recognized for their ability to serve as biological pest control agents, offering a natural and sustainable solution to mitigate the use of harmful rodent poison. To learn more, we spent two days with experts of the Macaca Nemestrina Project in a mosaic landscape of jungle and oil palm plantation, studying habituated pig-tailed macaques to learn how they adapt to their changing environment.

It’s not the oil, it’s the practice!

Since 2010, primatologist Dr. Nadine Ruppert has been studying Southern pig-tailed macaques in Malaysia’s oil palm plantations. “The oil palm industry in Malaysia is the second largest in the world, reaching about 30% of the world production,” Nadine explains. The cultivated African oil palm species Elaeis guineensis in fact comes from West Africa and until the mid-19th century, all of it was produced by local communities on a small scale and closely linked with local livelihoods for thousands of years. But in South-East Asia, palm oil has a colonial history.

Indonesia’s first oil palm plantation was established by Adrien Hallet, a Belgian entrepreneur in 1911, after which, in 1917, Malaysia’s first commercial plantation was started in Selangor, when the country was a British colony. Foreign investors took advantage of the “open-door” policy and by 1936, Sumatra had surpassed Nigeria in palm oil exports. After WWI, the demand for natural rubber declined, which was the biggest plantation crop in the region by then. Subsequently, several plantation companies repurposed their existing rubber plantations and switched to oil palm cultivation. These were the very companies that started intensifying oil palm agriculture in colonial territories. By 1939, when WWII broke out, Indonesia and Malaysia had over 100.000 hectares of plantations, and even the palm-specific pollinator insects had to be imported from Africa. 

Since then, the industry expanded, and Nadine tells us that today, the size of one industrial plantation can be up to 500.000 hectares or more. Nowadays, many people, especially in the west, know that unsustainable oil palm farming leads to habitat destruction and species decline. But it is not such common knowledge that oil palm is by far the most yielding oil crop. “Oil palm bears fruit three years after planting and can be harvested twice per month for at least 20 years,” Nadine tells us. In fact, oil palm is 8-20 times higher in yield than any other oil crop, like sunflower, soy or rapeseed. Therefore, palm oil requires only 7% of the land that other crops would require, according to the Sustainable Palm Oil Movement, a project by the Malaysian Primatological Society.

This means that if palm oil was to be boycotted, planting of alternative crops would have to be intensified using up many times more land and resources. “Certified sustainable palm oil production is the answer, also as a solution to protect Malaysian wildlife and habitats,” explains Nadine, which is why with her colleagues and students, she aims to understand what kind of threats different primate species face in the country. And as palm oil is here to stay, Nadine set out to study what impacts the conventional palm oil industry has on southern pig-tailed macaques – an endangered primate species commonly observed visiting plantations – and vice versa. 

Macaques: a surprising pest control

We are headed to Teluk Senangin to meet Dr. Nurul Iza, the field manager of the Macaca Nemestrina Project – named after the scientific name of the southern pig-tailed macaques. “We have a team of researchers and volunteers here, who follow the macaques in the forest and oil palm plantations to collect daily behavioural data,” Iza shows us around the field base. Here we meet Cedric and Maelle, two student internship volunteers from France who have been conducting behavioural observations for several months. 

The field site is located on the border of an oil palm plantation and the Segari Melintang Forest Reserve. Many oil palm farmers across Malaysia and Indonesia consider southern pig-tailed macaques as crop pests, for coming out from the forest in search of palm fruits. In response, plantation workers often use dogs or firecrackers to chase away macaques, and sometimes put out traps across the plantation. However, when the Macaca Nemestrina Project team looked into the impact of macaques on the yield of oil palm plantations, they came to a surprising realisation. 

The endangered southern pig-tailed macaques live in large groups. They are mainly terrestrial, but also very skilled climbers.

These agile and resourceful primates possess a natural instinct for foraging and hunting rodents, making them great at finding and consuming rats, the most destructive pest of plantations. The team’s study shows that a single macaque group can devour around 3,000 rats per year, which reduced rat numbers by 75% in their foraging area. In the meantime, they only take less than 0.6% of the overall oil palm fruits, all in all reducing the losses caused by rats (>10%) and increasing yield. By strategically allowing macaques into oil palm plantations from nearby forests in mixed landscapes, plantation owners can harness their innate pest-control abilities to help manage pest populations naturally, in a sustainable and eco-friendly manner, without the conventional use of harmful rat poison in oil palm plantations.

Pesticides may harm macaques

After proving that instead of chemical rat poison, southern pig-tailed macaques can be effective and sustainable pest control agents, the Macaca Nemestrina Project team became curious about how pesticides commonly used in plantations, like weed killers, impact these primates in return. Iza tells us about their findings: “Between 2014 and 2023, over half of the infants born in this forest-oil palm mixed landscape died within the first year.” she explains. “This exceeds the commonly known mortality rates of all other wild primates. Pesticides are conventionally used in plantations, usually applied on the ground, and therefore they get into puddles and other water bodies macaques drink from. We suspect that after such pesticides accumulate in the mother’s body, they potentially can get transferred to the infant during pregnancy and with breast milk.” 

That’s why the team now focuses their study to assess whether high mortality is really caused by pesticides. The impact of pesticides on wildlife and in particular primates is still understudied, while the agricultural sector is growing and expanding everywhere, and its often unregulated or poorly regulated pesticide use can have dire, long-standing impacts on wildlife. 

Where macaques go, researchers follow

Early in the morning, we are heading out with Iza, Maelle and Cedric to find the study group in the dense jungle. We find the group shortly before sunrise at one of their numerous sleeping sites, and follow them as they venture deeper into the forest. It seems like this morning, they are not in the mood for the plantation, because they lead us through a particularly swampy area of their habitat. There is no time to collect observation data so far, because in this dense, thorny and swampy area, we can barely keep up with these agile primates. As the macaque group disperses, Iza decides we should also split up – in this way we can locate the group more quickly. 

Through the thick and dense forest and the thorny swamp we march, trying to keep up with the macaques. They are way faster than us.

After some time of battling with the elements, we meet each other again at an opening, where finally the macaque group also decides to stop. It’s time to record their behaviour. While Maelle makes sure to take some water samples for a quick dip-stick test to assess pollution from nearby puddles, Cedric is particularly interested in the mother-infant bond. “Their social needs and social repertoire is not fulfilled in a plantation. In plantations, mothers keep their infants under much closer surveillance than in the forest, and infants seem to spend more time directly with their mothers,” Cedric notes. 

After we finally catch up with the group, they decide to stay in one place for a while, which means that Maelle, Iza and Cedric can record behavioural data and take samples.

In a recent study the team observed several differences in the macaques’ social behaviour between the forest and plantation, and showed that while macaques spend their time in plantations primarily on feeding, they are also exposed to higher risk there. At the same time, grooming and playing, which are behaviours that strengthen social bonds, occur mostly in the safer forest and at the plantation edge. 

Sustainable and wildlife-friendly agriculture

The research of the Macaca Nemestrina Project clearly shows the importance of enhancing biological pest control, a concept widely realised by introducing barn owls in oil palm and rice agriculture in Malaysia. In the context of sustainable oil palm plantations, the integration of biological rat hunters, like barn owls and macaques into pest management strategies offers several compelling advantages. Firstly, it presents a natural and non-invasive approach to pest control, minimising the ecological footprint and health risk for wildlife and humans, that is otherwise associated with chemical agents. Unlike rat poison, which can harm non-target species and persist in the environment long after application, macaques leave no lasting traces of contamination, preserving the health of soil, water, and biodiversity within plantation ecosystems.

Furthermore, facilitating access to plantations from nearby forest for macaques aligns with broader conservation goals for this endangered species and native biodiversity. Experiencing shelter and safety in a nearby forest supports wildlife populations, and by incentivizing the permanent protection of forested areas adjacent to plantations, plantation owners can create corridors for wildlife movement and maintain vital ecosystem services, such as pollination and seed dispersal.

“The goal of the research is to influence policy. We want to achieve pesticide-free plantations that contribute to the resilience of the whole landscape while facilitating forest protection near plantations. It is important to understand that unlike barn owls, macaques cannot be translocated into plantations as they will not be able to thrive in these monocultures, as our studies have clearly shown. They need intact forests for survival. Thus, protecting forests near plantations will benefit biodiversity and farmers alike, a win-win situation for all.”

Dr. Nadine Ruppert, founder of the Macaca Nemestrina Project

Critics may raise concerns about the feasibility and scalability of this vision, however, proponents argue that when integrated thoughtfully with other sustainable practices, such as agroforestry and organic farming, macaque-based pest control can serve as a valuable component of a holistic approach to greener palm oil production. 

As the world strives towards a more sustainable future, embracing innovative solutions like macaque-based pest control underscores the importance of harmonising agricultural production with ecological stewardship.