For orangutans affected by El Niño, change unfolds over time

  • A long-term study in Kutai National Park in Indonesian Borneo has shown how weather caused by the El Niño Southern Oscillation cycle affects the behavior, habitat requirements, feeding ecology and birth intervals of the park’s orangutans.
  • The study increases conservationists’ understanding of how orangutans survive in difficult and variable climatic conditions — important information given the likely impact of climate change.
  • Understanding the influence of the ENSO cycle was only possible through a multi-year study, highlighting the value of long-term projects. But the current trend is for short-term studies, which are often more appealing to funders and researchers.

“It was very peculiar,” says Anne Russon, a primatologist and psychology professor from York University, Toronto, recalling her 2010 study of orangutans in Indonesia’s Kutai National Park.

Having previously worked only with apes being rehabilitated, Russon was pleasantly surprised by how easy it was to study wild orangutans in Kutai, in Indonesian Borneo’s East Kalimantan province. “These orangutans have not been studied for about 30 years and we walk in and it’s like, ‘Hi. No problem.’” The team were able to easily find and follow the orangutans, which appeared to be immediately at ease in their company — relaxed enough even to mate in front of them.

Then, after a few years, everything changed.

“We could not find our orangutans for love nor money,” Russon says. “We searched and searched but nobody [was] there.”

In the first few years, the team frequently found multiple orangutans feeding from the same tree. The orangutans didn’t seem to mind the researchers observing them, sometimes for days on end. But in the following years, few orangutans seemed to be left in the study area, and those the researchers could find were quick to evade observation.

So what had changed? After following Kutai’s orangutans for eight years, Russon concluded that the critical factor was the weather.

Over time, the team observed that dramatic weather patterns caused by the El Niño Southern Oscillation (ENSO) cycle have a profound effect on orangutan behavior. This finding not only deepens scientists’ understanding of a rare orangutan subspecies, it also highlights the critical importance of long-term ecological studies.

Map showing the location of Kutai National Park. Kutai is situated on Borneo’s eastern edge, where the effects of the ENSO cycle are at their strongest. Image by Orangutan Kutai Project.

Kutai’s isolated population of orangutans is part of the critically endangered and little-studied morio subspecies of Bornean orangutan (Pongo pygmaeus morio), found only in eastern Borneo. They were one of the first orangutan populations ever studied in the early 1970s. Massive fires devastated the park in 1982 and 1983, and the research was abandoned. In 2010, Russon began conducting a long-term ranging study of Kutai’s orangutans. “It’s like being a detective tracking humans,” she says. “You see what they do, who they hang out with and that gives you a pretty good idea of how they operate their lives.”

The team soon realized that the pattern they observed — periods where orangutans were relaxed and easy to find, and periods where they seemed to disappear — was linked to the ENSO cycle. In this part of Borneo, the ENSO cycle is marked by periods of drought interspersed by periods of normal rainfall and periods of unusually heavy rains.

Rainfall patterns affect vegetation growth and the risk of forest fires, and thus food availability for the orangutans. This is made more acute by mass fruiting events known as “mast” fruiting. Researchers have observed mast fruiting as a strategy in many tree species around the world. One hypothesis is that certain tree species have evolved to synchronize their fruiting into one mass event every few years, producing so much fruit, and therefore seeds, that predators cannot possibly eat it all, ensuring some survives.

In east Borneo, and to a lesser extent in central Borneo, the ENSO cycle causes a synchronized mast fruiting not only within tree species but across different species. According to one 13-year study published in Science, more than 50 tree species produced seeds only during a two-month period every three to four years linked to an ENSO event. The result is a period of intense glut for orangutans, followed by long periods in which fruit is less readily available and they must rely on less nutritious foods like leaves and bark.

Orangutans are known to be more socially tolerant of both humans and other orangutans when food is plentiful. This was exactly what Russon found when she first arrived in Kutai: the normally semi-solitary orangutans were willing to share the abundant fruit feast with others and tolerate their human observers. It was during the dry El Niño years, when conditions were tough, that the orangutans became much more elusive. “They’re very good at escaping if they want to,” Russon says.

A dead orangutan mother found in Kutai National Park. When orangutans are malnourished they are more susceptible to parasites and disease. Russon and her team were able find and rescue the infant orangutan, which is now in a rehabilitation program. Image by Orangutan Kutai Project.

But this wasn’t the only reason the orangutans were hard to find. After tracking individual orangutans through an entire ENSO cycle, Russon noticed that the orangutans moved out of the study area and used different areas of the park in different years. Orangutans aren’t normally considered a migratory species, but other instances of migration have been reported, predominantly in east Borneo. From her long-term observations, Russon now believes this is due to the influence of the ENSO cycle, since different habitat types will have varying food productivity at different points in the cycle.

The ENSO cycle influences the lives of Kutai’s orangutans in another surprising way. Much like in humans, orangutan females don’t ovulate when their bodies are malnourished, such as during an El Niño drought. When the drought ends, “Everyone eats a lot, the females eat a lot, they start to ovulate, everyone gets pregnant,” Russon says. In Kutai, 42 percent of the recorded orangutan births since 1970 have occurred around a year after the end of an El Niño event.

The team found further evidence of this in the time between orangutans’ pregnancies. El Niño events occur every six years on average; orangutans in Kutai had an average inter-birth interval of 6.1 years, “You couldn’t get much closer could you!” Russon says. Orangutans have one of the longest developmental periods of any animals; infants nurse for around six years and remain dependent on their mothers until around the age of 8 and often beyond. With Kutai’s orangutans reproducing on average every 6.1 years, compared to 8.75 in Sumatra and 7.7 years in central Borneo, orangutan mothers are still caring for their previous infant when the next one comes along. This phenomenon is known as infant stacking and is rarely recorded in orangutans.

A female orangutan with a juvenile and nursing infant. Infant stacking is rarely recorded in other orangutan populations, where the ENSO cycle has less influence. Image by Orangutan Program Kutai.

Kutai National Park is among the least productive rainforests where orangutans can be found; it is one of the driest regions on the island and the driest of all current orangutan research sites. Because the apes live in the most challenging and variable conditions of any orangutans, Russon’s findings can help conservationists understand how orangutans adapt to difficult climatic conditions — a potentially invaluable insight in the face of man-made climate change.

“It’s crucial to understand how orangutans survive in those areas where food availability really fluctuates between years,” says Serge Wich, a professor of primate biology at Liverpool John Moores University. Conservation managers need to understand how orangutans move around their habitat at different stages of the ENSO cycle to know how much of which areas of forest need to be conserved, he says.

With the potential impact of climate change, Russon’s data on which tree and plant species orangutans use during drought periods and which plant species survive could also prove particularly useful for conservationists working in forest rehabilitation. “If you’re thinking about reforestation, it could be very important to look at what species you’re going to plant,” she says. “It wouldn’t make much sense to put in plant species that are not going to be tolerant [to a changing climate] in the future.”

The importance of long-term studies

Scientists have long understood the importance of long-term studies such as the Kutai project. According to a recent article in Bio Science, long-term ecological studies are often valued higher than short-term studies by other researchers and policymakers. “Long-term studies have been essential for developing the foundational principles of ecology,” says Brent Hughes, lead author of the article and an assistant professor of biology at Sonoma State University.

We can only truly understand many aspects of an animal’s behavior through long-term studies, especially long-lived species like orangutans, says Wich. The Kutai project is a good example of this: the influence of the ENSO cycle on Kutai’s orangutans was only fully apparent through a multi-year study, and the team continue to learn more as they watch the behavior repeated through a second full ENSO cycle.

Even more importantly, it shows that short-term studies can offer an incomplete picture, Russon says. “For a student doing a master’s or a Ph.D., a year is a long time. So you come away and you think you understand it, and boy do you ever not.”

Russon points to her experience in Kutai. The behavior of the orangutans was so different at varying stages of the ENSO cycle that drawing conclusions from a study that only covered one stage in the cycle could be misleading.

Understanding the bigger picture is essential for conservationists trying to evaluate conservation performance, says Wich, “You might see differences that are nothing to do with improved conservation management — they’re just a function of how these animals move around in an area.” Wich recently co-authored a letter in Current Biology critiquing the Indonesian government’s claim that orangutan numbers have risen by 10 percent, a finding in stark contrast to recent reports showing a significant decline. The government’s claim was based on research from nine small sites over a two-year period. This point was emphasized in a recent study published in Bioscience that found that for 72 percent of species they looked at, it took at least 10 continuous years of study to detect a statistically significant change in population abundance.

Female and infant orangutan in Kutai National Park. The morio orangutans of east Borneo are usually much darker than their Sumatran cousins. Image by Orangutan Project Kutai.

But not everyone appreciates the clear value of long-term studies. One of the most important sources of ecological funding in the United States, the National Science Foundation, reduced its funding for long-term studies in favor of short-term studies over the last 10 years, Hughes found. He says he believes that the fast-paced world of research makes long-term studies seem a less attractive proposition for researchers and funders. “The culture in the scientific community and its funding agencies rewards researchers with high publication numbers, so there is incentive to publish shorter studies more frequently,” he says.

Russon was lucky enough to find long-term support from Indianapolis Zoo, which also supports a number of other long-term conservation projects. Importantly, she was also in a position where she was able to take the long view and make a long-term commitment to working in Kutai National Park. As she points out, setting up long-term studies is no easy task; but once up and running, they can act as an invaluable platform from which shorter-term studies can be conducted.

With the rapid changes to the climate and habitats caused by humans, Russon says she believes long-term studies are more vital now than ever. “What you see now might be true for another couple of years, but given the pace of destruction, it’s not going to be true for long.”

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