- As oil palm plantations encroach on rainforests, wild primates increasingly enter them to forage, where they face the threat of being eaten by feral dogs, killed for raiding crops, or caught by traffickers for the pet trade.
- A new study from Peninsular Malaysia finds that exposure to oil plantations also significantly increases the risk of death among infant southern pig-tailed macaques.
- In addition to known threats, researchers speculate common pesticides used in oil palm plantations might play a role in the increased death risks for infant macaques, but their study stops short of providing direct evidence implicating any specific toxic chemical in these deaths.
- Conservationists call for using environmentally safe and wildlife-friendly agricultural practices in oil plantations to minimize risks and establishing wildlife corridors and tree islands so that endangered primates, like southern pig-tailed macaques, can move freely without being exposed to threats.
For 12 years, primatologist Nadine Ruppert and her colleagues have had one recurring task on their calendar: tagging along with a group of southern pig-tailed macaques in Segari, Peninsular Malaysia, as these primates hop between native rainforests and the neighboring oil palm plantations. Over the years, the researchers successfully habituated the group: they named every individual, discerned their life histories, and watched their behavior to understand how these monkeys adapt to human-modified landscapes such as agricultural plantations.
Southern pig-tailed macaques (Macaca nemestrina), an endangered species native to the tropical rainforests of Southeast Asia, live in groups of up to 60 individuals and feed primarily on fruits. With oil palm plantations encroaching on their natural habitats, these primates increasingly venture into plantations to feed on palm fruits and rats. In the process, they risk being eaten by feral dogs, hunted by humans, or caught by traffickers for the pet trade.
When Ruppert and her colleagues noticed that in 10 years, more than half of the baby pig-tailed macaques born in the group died before they turned a year old, they knew something was off.
“When we looked into birth rates and survival rates of these infants, in some years, there were shockingly not a single individual baby surviving,” Ruppert told Mongabay.
After analyzing the behavior and movements of the macaques for a study published in the journal Current Biology, the researchers identified the culprit: exposure to oil palm plantations.
Between January 2014 and February 2023, the researchers noticed 52 of the 92 infants disappeared from the two groups of macaques they studied. Statistical analysis revealed that when infant macaques spent more than three hours each day in the plantations, they were three times more likely to die. The whopping 57% death rate, much higher than is common among wild primates, can jeopardize the already-endangered macaque’s future, they say.
“In one of our study groups, we observed a 50% decline in group size in only 10 years, which is a clear sign that this high infant mortality is really critical with regard to the long-term survival of the population,” said co-author Anna Holzer from the Max Planck Institute for Evolutionary Anthropology in Germany.
“The importance of this study is that it is looking at the next generation,” said conservationist Felicity Oram, director of the Malaysian NGO Orang Juga, which focuses on human-primate coexistence in the country. “We need more studies like this because there’s not a lot of studies about macaques, especially in natural environments.” Oram wasn’t involved in the study.
Agricultural pesticides to blame?
While the current study didn’t find any direct evidence to implicate a specific toxin in the deaths of infant monkeys, the researchers observed telltale signs of pesticide-related impacts.
The high death rate among infants correlated with the time they spent in oil palm plantations, where they not only face threats from predators and humans but are also exposed to toxic pesticides like rat poison and the herbicides glyphosate and paraquat. Previous studies in humans and other mammals have shown these chemicals reduce fertility in females, lead to the accumulation of toxins in the food chain, wreck children’s health, and are possibly fatal.
Researchers also noticed that baby macaques born to first-time mothers and those giving birth after a long period were more likely to die than those born to mothers with a short interval between births. A one-year increase in the birth interval led to a twofold increase in the likelihood of the infant dying. This is counterintuitive because longer birth intervals generally lead to healthier mothers giving birth to healthier babies.
“That’s the complete opposite in our study,” Ruppert said, adding that while threats from predators and humans are higher in oil palm plantations, “that doesn’t explain how infants with moms that have shorter birth intervals [are] proportionately more likely to die in the first year.”
The researchers point out this trend could be because when females are exposed to pesticides, these fat-soluble chemicals accumulate over time in the body. The longer the exposure, the higher the amounts of toxins. Since females can only expel these toxins by passing them to their infants during pregnancy or through breastfeeding, moms with longer birth intervals pass on higher amounts of toxins that could be fatal to their infants.
The researchers also observed other indirect signs that tied to pesticides. Some infants were born weak and underweight, and a few had facial malformations, like an open nose, previously seen among chimpanzees and baboons exposed to pesticides. The group also had infertile females and those with cancer-like abscesses on their genitals. In humans, studies have linked pesticide exposure to many forms of cancer.
These indirect observations led to researchers concluding pesticides may be behind infant deaths in the pig-tailed macaques. “That’s a big speculation,” Ruppert said, adding that five of the retrieved corpses indicated the babies were underweight and not fully developed. Since primate mothers carry dead infants for days before discarding them, the researchers could not perform autopsies to confirm their hypothesis.
“This is an important study as our understanding of the health impacts of chemicals used in agriculture on wildlife and especially on non-human primates is relatively limited,” Tatyana Humle, coordinator of the ARRC Task Force of the Primate Specialist Group at the IUCN, the global wildlife conservation authority, told Mongabay in an email. “The impact of the use of chemicals on wildlife is a critical topic, and agribusiness companies need to consider, monitor and mitigate and ideally avoid such impacts on wildlife.” Humle wasn’t involved in the study.
The researchers call for using environment-friendly agricultural practices that don’t involve harmful chemicals, such as removing weeds manually and using biological pest control with barn owls, leopard cats — or even rat-eating pig-tailed macaques. They urge plantation managers to establish wildlife corridors and tree islands that connect fragmented forests and promote gene flow between populations.
Having forest patches intermixed with oil plantations also benefits farmers, Ruppert said: “The biodiversity that comes from those forests into the plantation benefits people by [providing] pest control services.”
Banner image: A southern pig-tailed macaques infant. Image by Andrea Schieber via Flickr (CC BY-NC-ND 2.0).
Oil palms may be magnet for macaques, boars, at expense of other biodiversity
Citations:
Holzner, A., Rameli, N. I., Ruppert, N., & Widdig, A. (2024). Agricultural habitat use affects infant survivorship in an endangered macaque species. Current Biology, 34(2), 410-416. doi:10.1016/j.cub.2023.12.002
Sun, Y.-L., Wang, X.-L., Yang, L.-L., Ge, Z.-J., Zhao, Y., Luo, S.-M., … Yin, S. (2021). Paraquat reduces the female fertility by impairing the oocyte maturation in mice. Frontiers in Cell and Developmental Biology, 8. doi:10.3389/fcell.2020.631104
Eskenazi, B., Gunier, R. B., Rauch, S., Kogut, K., Perito, E. R., Mendez, X., … Mora, A. M. (2023). Association of lifetime exposure to glyphosate and aminomethylphosphonic acid (AMPA) with liver inflammation and metabolic syndrome at young adulthood: Findings from the CHAMACOS study. Environmental Health Perspectives, 131(3). doi:10.1289/EHP11721
Beswick, E., & Millo, J. (2011). Fatal poisoning with glyphosate-surfactant herbicide. Journal of the Intensive Care Society, 12(1), 37-39. doi:10.1177/175114371101200109
Krief, S., Berny, P., Gumisiriza, F., Gross, R., Demeneix, B., Fini, J. B., … Wasswa, J. (2017). Agricultural expansion as risk to endangered wildlife: Pesticide exposure in wild chimpanzees and baboons displaying facial dysplasia. Science of The Total Environment, 598, 647-656. doi:10.1016/j.scitotenv.2017.04.113
Holzner, A., Ruppert, N., Swat, F., Schmidt, M., Weiß, B. M., Villa, G., … Widdig, A. (2019). Macaques can contribute to greener practices in oil palm plantations when used as biological pest control. Current Biology, 29(20), R1066-R1067. doi:10.1016/j.cub.2019.09.011
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