Researchers discover rat-eating monkeys acting as ‘pest control’ on Malaysia’s oil palm plantations

Macaques could reduce crop damage by rats from 10 per cent to less than 3 per cent, equivalent to a gain of around US$650 million per year, researchers said.
Anna Holzner

It seems like there’s monkey business afoot as scientists have discovered that some monkeys in Malaysia regularly kill and eat rats in the nation’s vast palm oil plantations.

A report released in Current Biology on Monday (Oct 21), said that researchers in Malaysia have discovered that southern pig-tailed macaques – previously thought to eat mainly fruit and, occasionally, lizards and birds – forage for rats on plantations.

Nadine Ruppert, co-author of the report and senior lecturer of zoology at University Sains Malaysia, said in a statement that she was “stunned” when she first observed the macaques feeding on plantation rats.

“I did not expect them to hunt these relatively large rodents or that they would even eat so much meat,” she said in a press statement.

Ruppert also said in a Facebook post that it was “quite gross” to see the primates feeding on the heads of the large rats, but added that it was fascinating to see how many rats the macaques hunted, and “in a very targeted and systematic way”.

“I was intrigued by the opportunity they provide for pest management and their own conservation, as they can become an important umbrella species if forest corridors and buffers are kept within the oil palm landscape, to allow them access to the plantations,” she wrote.

Originally thought as crop-raiders themselves, the report said that macaques only damage an average of 0.56 per cent of oil palm crops by eating the fruit. This is 17-fold less than the damage created by rats, which cause an average of 10 per cent crop damage.

According to the report, damage from rats can amount to monetary losses of US$930 million (RM3.9 billion) every year.

Researchers monitored two groups of pig-tailed macaques – each with around 44 monkeys – in plantations around Malaysia’s Segari Melintang forest reserve between January 2016 and September 2018.

They found that each group of macaques kills an average of 3,000 rats each year, which is a 75 per cent of reduction in rat population, the report said.

This means that macaques can help to reduce crop damage from 10 per cent to less than 3 per cent, equivalent to a gain of around US$650 million per year, it added.

Facebook / Nadine Ruppert

As such, the researchers urged plantation owners to protect macaques –  which are listed as vulnerable by the International Union for Conservation of Nature (IUCN) – as they are a form of biological pest control, and can contribute to greener practices in oil palms.

“We expect that our results will encourage both private and public plantation owners to consider the protection of these primates and their natural forest habitat in and around existing and newly established oil palm plantations,” said Anja Widdig, senior author of the study.

According to Widdig, the research will be used to work towards a plantation design that “maintains viable macaque populations and higher levels of biodiversity” while “increasing the plantations’ productivity and sustainability by effective and environmentally friendly pest control”.

“This ultimately can lead to a win-win situation for both biodiversity and the oil palm industry,” she added.

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