Rat-eating monkeys hold promise for sustainable agriculture

By Elena Bruess
Medill Reports

A macaque sits patiently on the forest floor, waiting by an oil palm tree.

There’s a rustling and a thump as two more macaques gripping a nearby trunk remove some of the tree’s leafy base. A large rat falls from its hiding place. It attempts to run, but the macaque is faster. A scuffle, a jump, a chomp. The pest hangs limp from the macaque’s mouth, a tasty snack. The monkeys move on to the next tree.

On the oil palm plantations of Malaysia, the southern pig-tailed macaque – a primate primarily found in Southeast Asia – loves to feast on rats. In a recent study, researchers spent three years following two groups of these monkeys around forests and plantations, monitoring their daily activities. The findings showed that, not only do macaques go out of their way to eat an outstanding number of rats, but their eating preference also a proved to be a great benefit to the oil palm plantations by ridding the workers of their greatest pest. According to the research, the rat-eating monkeys are astonishing and make a pretty good case for wildlife preservation and reconnection.

“There is a perception that the macaques are crop raiders and they eat a lot of the oil palm fruit,” said Anna Holzner, the leading author of the study. Holzner is a Ph.D. student in primate behavioral ecology at the University of Leipzig in Germany and researcher at the Max Planck Institute for Evolutionary Anthropology. She worked along with behavioral ecologist Anja Widdig, also of the university and the Max Planck Institute, and partnered with Nadine Ruppert out of Universiti Sains Malaysia to conduct the study.

“We looked at the amount of fruit the macaques actually eat, and it is really low compared to what the main pest – the rats – eat. It’s less than 1% per macaque group, compared to 10% of damage by the rats.”

Macaques will frequent oil palm plantations, sometimes in search of rats. These primates were studied to eat less oil palm fruit than previously understood. (Courtesy of Nadine Ruppert)

The researchers were able to determine, through trapping rats and behavioral surveillance of the two different macaque groups, that the monkeys ate about 3,135 rats per group. Holzner and the others then equated these numbers with the amount of oil palm fruit consumed by each macaque study group. The result dropped the rate of crop damage to just under 3%. Regular visits of the pig-tailed macaques could actually benefit the plantation owners, the study concluded.

Yet, Malaysian oil palm farmers try to keep the macaques away.

Along with the misconception that these primates eat most of the fruit, it is also the misidentification of primate species that causes alarm on the plantations and other orchards. Some monkeys, such as the long-tailed macaque that are often found in agricultural lands as well, can give the beneficial rat-eating monkeys a bad reputation with nuisance behavior. The farmers will often not distinguish between the various kinds of monkeys and actively scare them away with loud fire crackers or gun shots, or in rare cases trap or kill any monkey they see.

Oil palm plantations are a primary source of income for millions and an incredibly important cash crop for Southeast Asian countries such as Indonesia, Malaysia and Borneo. This dependence has often fueled conflict between the natural wildlife and humans – such as what is also seen between macaques and farmers.

This is in part due to the fragmentation within these areas.

“The forest area, which is the macaques’ main habitat and quite crucial for them to live, is often surrounded by oil palm plantations and urban settlements. In our study site, they are living on a forest island more or less,” said Ruppert, the head of Macaca Nemestrina Project, a program that tackles these issues with relation to macaques. “What is happening everywhere when forests and habitats are being fragmented, is that animals try to cross areas and they come in contact with humans. Many farmers don’t know how to exactly manage these encounters. We need to reconnect wildlife habitats and use forests corridors to effectively enhance connectivity, so populations can disperse and the diversity can be preserved.”

Macaques stalking though a Malaysian oil palm plantation. (Courtesy of Anna Holzner)

Oil palm plantations in Southeastern Asia have been facing sustainability challenges for years as Malaysia and Indonesia make up for 85% of the world’s oil palm production. According to the World Wildlife Foundation, the cultivation and production of palm oil has potential to cause serious environmental issues such as deforestation to build the plantations, land degradation, major human/wildlife conflicts as well as air and water pollution.

One pollutant comes from Warfarin – an anti-coagulant in a poisonous form of a poison most conventional plantations use to kill the rats and other pests. The rat must ingest the poison more than once to die. According to Ruppert, this poses a great danger to other nontarget species too.

“[The poison] accumulates in the body and will cause the rats to internally bleed,” said Ruppert. “They will have it in their system for a while before they die and this can cause a lot of secondary poisoning when other animals go for the bait or for the rat that ingested the bait as well.”

Macaques are an umbrella species, so keeping their habitat alive and healthy could positively impact the rest of the forest-dwellers as well. If farmers worked alongside the primates, it could be a win/win for both sides of the conflict. A kind of “you scratch my back and keep my habitat intact, and I’ll eat your rats.” Or a biological pest-control to highlight of the benefit of biodiversity. This does not mean moving macaques to plantations, but more so pursuing a valued relationship between the two.

A group of macaques finding some fruit on the plantation. (Courtesy of Viola Eckerle)

One way this could work, as the study suggested, is investing in wildlife corridors in and around plantations, green hallways between fragmented forest areas. According to the researchers, these corridors could give space for animals to walk along the plantations or safely through them. A benefit to both the animals and the farmers. Experiments in this variation of oil palm sustainability are currently being attempted with elephants due to human/elephant conflicts on plantations, even as recent as last month in Kota Kinabalu, Malaysia. The wildlife corridor pilot is the first of its kind, and depending how this program works out, it could give way to other corridors throughout the country.

Big oil palm sustainable change in Malaysia could be difficult, but according to Widdig, studies like this one could push the country in the right direction.

“First we produce the results, then we try to get the politicians doing something about it,” said Widdig. “This is not in our hands right now. It’s important that things change in these plantations to become more biodiverse. The macaques are just one example of how we can make these places more sustainable.”

Photo at top: A Southern pig-tailed macaque finishing up an oil palm plantation rat, one of Malaysian farmers greatest pests. (Courtesy of Liu Kunpeng)