Killer Rat-Eating Monkeys Leave Scientists "Stunned"

An adult male pig-tailed macaque consuming a rat at an oil palm plantation. Anna Holzner

Rat-eating macaques found in the rainforests of Malaysia have left researchers “stunned” thanks to their unpleasant dining preference and the impacts it could have. Despite their pesky reputation, new research suggests that these cunning predators may aid in the production of palm oil and could relieve some of its environmental impacts.

Researchers studied the ecology and behavior of pig-tailed macaques by collecting ranging and behavioral data between January 2016 and September 2018. They found that the primates ate fewer oil palm fruits than expected. While one group ate more than 12 tons of oil palm fruits in one year, that equates to just 0.5 percent of overall palm oil production in the area. What they ate more of were rats, which is a good thing for palm oil plantations. Annually, rats can reduce overall production resulting in a roughly 10 percent loss of yield.

"I was stunned when I first observed that macaques feed on rats in plantations," said study co-author Nadine Ruppert. "I did not expect them to hunt these relatively large rodents or that they would even eat so much meat. They are widely known to be frugivorous primates who only occasionally feast on small birds or lizards."

Macaques have the potential to act as “biological pest control” agents by feeding on rats, write the researchers in Current Biology. Each group is capable of reducing rat populations by about 3,000 individuals every year, which saves farmers about $112 per hectare.

"By uncovering cavities in oil palm trunks where rats seek shelter during the day, one group of pig-tailed macaques can catch more than 3,000 rats per year," said lead author Anna Holzner in a statement. Macaques are largely effective due to their foraging behavior and can reduce rat numbers by more than 75 percent. Estimates suggest that macaques can reduce yield loss from 10 percent to less than 3 percent in a plantation of just over 400,000 hectares (1 million acres) in size, which equates to about $650,000.  

This picture shows an adult male (left) and an adult female pig-tailed macaque foraging for rats under boots of oil palm trees. Anna Holzner

Oil from palm trees is used in everything from cooking oil and cosmetics to biofuel, according to the World Wildlife Fund. The oil palm tree is very productive and grows only in the tropics, currently covering 18.7 million hectares (46 million acres) of land worldwide. As international demand increases, so does deforestation, human-wildlife conflicts, ecological impacts, and biodiversity loss that threatens Sumatran tigers and orangutans, among other species.

Rodenticides, which are poisons used to kill rodents, are not only expensive and largely inefficient but can harm other wildlife, previous research has found. The authors of the new study say this highlights the “global importance of improving sustainable palm oil production, including the use of efficient environmentally friendly pest control.” Alternative production methods could reduce the impact of palm oil plantations on nearby habitats, and the authors hope that their findings might encourage plantation owners to protect forest habitat found within and around new and existing plantations.

"In collaboration with local palm oil companies and NGOs, we will work towards the realization of a plantation design that maintains viable macaque populations and higher levels of biodiversity via wildlife corridors while increasing the plantations' productivity and sustainability by effective and environmentally friendly pest control,” said senior author Anja Widdig.

“This ultimately can lead to a win-win situation for both biodiversity and the oil palm industry."

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