Climate and fossil records show that big swings in the Earth’s average temperature over eons radically changed the distribution and composition of life on the planet, wiping out entire groups of plants and animals in some areas, including large land mammals, while enabling others to thrive in unexpected places. The fossil record is filled with evidence of things like beech trees in Antarctica. 

But few species, if any, have survived a climate shift as sudden as the one humans have triggered with greenhouse gas emissions and global disruptions of forest, field and aquatic ecosystems. Fish are swarming poleward in both hemispheres, and trees are climbing higher up mountainsides to escape the rising heat. Some species will just run out of room as equatorial waters and many mountaintops grow too warm for them. 

Now, some of that migration appears to be downward. In new research published Oct. 11 in the Proceedings of the National Academy of Sciences, scientists showed that global warming and habitat degradation appear to be driving many arboreal primates down from the trees and on to the ground, where their survival is uncertain. 

Conservation scientists will have to develop and quickly implement effective strategies to protect them, said Timothy Eppley, lead author of the study and a wildlife researcher at the San Diego Zoo. Otherwise, “they’re just going to end up in these fragments [of habitat] and slowly die off because they have a restricted gene pool and aren’t able to disperse,” he said.

While shifting climates shaped primate evolution over geologic time scales, including leading some that lived in trees to move to the ground, Eppley said he doesn’t expect any of the 47 species his international team studied in the Americas and Madagascar to transition to a fully terrestrial lifestyle. There just isn’t enough time, he said.

“You know, the pace at which climate change and anthropogenic factors are degrading habitats, it’s just happening so fast,” he said.

The species that are less adaptable, including those that subsist on tree fruits, or those living in very small groups, are very vulnerable, and efforts to protect habitat for them is not keeping pace with global warming impacts, he added.

“In Madagascar, for example, there are multiple national parks that are on fire right now,” he said. “I think 40 percent of Baie de Baly National Park National Park in Northwest Madagascar has burned. Isalo National Park, which is one of the major tourist sites in Madagascar, is currently on fire, and this is something similar to what you see in Brazil, especially this time of the year.”

Long-term conservation plans like reforestation and the creation of migration corridors are important, “but I would also say that we need to put in efforts to simply protect the habitats that we have,” he said. Since some primates only reproduce once every few years, habitat disruptions can be “devastating.”

“We don’t have a huge opportunity when they’re faced with these issues,” he said. “If something disrupts that natural reproductive pattern, you may have 10 years between births, and that is a really dire situation.”

A Kid With a Notebook

Eppley and 117 co-authors from around the world analyzed 151,000 observation-hours on 47 arboreal primate species to identify behavioral traits linked with spending more time on the ground. They found that primates with less fruit in their diets, as well as those that live in large groups, in hot climates and where tree cover is decreasing, descended from trees more often, perhaps seeking to cool off, find water or seek alternate food sources. 

The species that are able to find relief on the ground are those that may have a better chance to adapt,” they concluded in the paper, “but less adaptable species may require prioritized conservation strategies.” And moving down to ground would also expose the primate to new predators, requiring additional adaptation, the authors noted.

Eppley said the data-gathering and analysis was a big effort, but it may have been a perfect challenge for him.

“I grew up in Michigan, and I spent a lot of time outdoors. All of our family trips revolved around visiting national parks,” he said. “I would keep a notebook, as a third- and fourth-grader, and I would follow groundhogs, taking notes about what they were up to, and what the neighborhood was up to. I always had this kind of extreme interest in behavioral observations of the wildlife around me.” 

He pursued that interest in college, taking his first paid research job with the Max Planck Institute in the Democratic Republic of Congo. “From that moment I was just kind of hooked with primates,” he said.

The group of 47 primates scrutinized in the new study was selected from the tree-dwelling species for which they could find good behavioral data, which provided a baseline that allowed them to measure the ecological pressures and anthropogenic pressures on the animals.

“I did a deep literature dive, and I emailed everybody that had done a behavioral study that was 10 months or longer,” he said. “And thankfully, there were a lot of people. We have a really remarkable set of data that covers over 150,000 observation hours. Each one of those observation hours is one of us following that primate in its natural habitat.”

He said field work during his Ph.D. research on southern bamboo lemurs offered some early clues about how climate and habitat change pressures primates to descend from trees. The species was “on all accounts, an arboreal species feeding mainly on bamboo,” he said. “But in the habitat where I was studying them, there was no bamboo. It was a fragmented and degraded ecosystem and I found that that species was spending a ton of time on the ground, for all of its activities, both feeding and for resting.”

The new study helps show similar patterns in similar species, “as these habitats are degraded, and the climate worsens, with hotter temperatures, we’re finding that these arboreal primates are more likely to shift to life on the ground,” he said. “For these species, these traits are kind of like a pre-adaptation that allow them to persist on the ground.” But for less adaptable species, he added,“It’s going to be really important that conservationists implement fast and effective conservation strategies in order to ensure their survival.”

‘Little Chance’ to Survive

Co-author Luca Santini, who studies large-scale ecological patterns at Sapienza University in Rome, said the combined results of the study tell an interesting story about how some primate species may be “capable of adapting to the rapid changes we are experiencing globally by modifying their behavior,” he said. “So while the study is not focused on global change specifically, it gives some good insights on species flexibility, which is something we rarely know. In climate change studies we often make simplistic assumptions that species will continue to behave the same in altered environments.”

That makes the research unique because most other studies tend to focus on “species distribution or population level responses,” he said. “Here we focus on behavior, which is something rarely studied at such scale, [or] for making inferences on climate change adaptability.”

Primates are critically important for forest ecosystems, Eppley added.

Many of them pollinate trees and disperse seeds, particularly in Madagascar, he said, where some trees are completely dependent on seeds spread and germinated by primates, he said.

“As numerous species potentially go extinct, then those trees will eventually go extinct as well. And it has this cascade effect on the whole ecosystem,” he said. ”Once you get rid of those trees, then there’s going to be other species that are going to go with those as well.”

Co-author Giuseppe Donati, a lecturer in primatology and biological anthropology at Oxford Brookes University, said the research supports an evolutionary biology theory that more flexible species can adapt quicker and more successfully to changes. 

“We expect this, but when we use field data and large-scale analyses we rarely find crystalline trends,” he said. “What we found here is that populations that have a more generalized diet, and so are more ecologically flexible, and live in larger groups, with a more developed sociality, are those more likely to descend from the trees and spend time on the ground,” he said.

“The more flexible of our cousins are already doing their best to cope with the changes we caused, while those who are more specialized are more likely to have troubles,” he said. But it’s also troubling to realize that “even the most flexible and adaptable of the animals, the primates, have little chance to survive the pace of human-induced modifications.” 

Eppley put the findings squarely in the context of the current human-driven mass extinction.

“There was a study earlier this week about how more than half of the world’s bird species have populations that are declining,” he said. “And tropical birds are also responsible for seed dispersal. So, we have these two major groups that are largely responsible for forest regeneration. And if we lose them, God help us.”