Why Some Tree-Dwelling Primates Are Moving to the Ground

Climate change and deforestation are prompting some primates to leave their tree homes and spend more time on the ground.

A new study of nearly 50 species of monkeys and lemurs has looked at the reason these arboreal animals have moved to terrestrial habitats. The longer they are on the ground, they are more likely to have a difficult time finding food and shelter. Out of the trees, they’re also more likely to have negative run-ins with people and domestic animals.

The study was led by Timothy Eppley, Ph.D., a postdoctoral associate at San Diego Zoo Wildlife Alliance. About 15 years ago, Eppley began working with southern bamboo lemurs (Hapalemur meridonalis) in southeast Madagascar.

At the time, he says, no one had studied that species, but other bamboo lemurs were known to spend most of their days in the trees, feeding on bamboo leaves and stalks. The forest where he researched, however, was degraded and didn’t have much bamboo.

“I was surprised to find that these arboreal lemurs were spending a significant portion of their day on the ground, so I began to wonder whether habitat degradation and limited food resource availability may be driving this species to the ground,” Eppley tells Treehugger.

“Over time I found that this population had an incredibly diverse diet, but would also spend time on the ground engaging in all of their activities, even sleeping!”

Eppley discussed his findings with colleagues and heard similar observations of tree-dwelling primates descending to the ground, so he was inspired to research it further. He reached out to every researcher he could find that may have significant data on primates from Madagascar or the Americas, the two places with almost all tree-dwelling primates. He wanted to uncover what leads them to move to the ground.

Large Groups and Diet

With his research, Eppley examined more than 150,000 hours of data observed on 32 monkey species and 15 lemur species at 68 sites.

He found that they spent an average of only 2.5% of their time on the ground each month. But the findings showed some interesting differences.

“As habitats are degraded and the climate worsens, (i.e., hotter temperatures), our results suggest that arboreal primates that feed on less fruit and that live in larger groups may be more likely to shift to life on the ground,” Eppley says. “However, for species that are less adaptable, we will still need to implement fast and effective conservation strategies to ensure their survival.” 

When forest canopy cover was sparse and temperatures increased, some species were more likely to increase the time they spent out of the trees. Some tendencies in diet and group living also had an impact.

“Specifically, having a more mixed diet (or perhaps a more diverse diet) may allow a species to be better adapted to cope with anthropogenic and/or ecological pressures, allowing them to feed on whatever resources may be available to them at any given time,” Eppley says.

“As for living in larger groups, descending to the ground is a risk-sensitive situation that may expose arboreal primates to novel predators, thus having more individuals in a group to be vigilant affords a degree of protection.”

The results were published in the journal Proceedings of the National Academy of Sciences.

Dangers on the Ground

Although moving to the ground has some advantages in certain situations, primates can face problems when they leave the trees. They can become victims of new predators, such as native carnivores, feral dogs and cats, and zoonotic diseases.

Sometimes they may go to the ground when they’re looking for other habitats in order to seek food or mates. In a fragmented environment, they need to cross open areas with little canopy.

“In those cases, they could be exposed to raptors and even humans, as bushmeat hunting is, unfortunately, a very real threat to primates in many countries,” Eppley says. “Beyond predation, they will be faced with finding appropriate food resources on the ground, thus having a broad or flexible diet is important.” 

Transitions from the trees to the ground have happened many times throughout primate evolution. Researchers say that although certain traits and conditions might have influenced these earlier shifts, this time they’re concerned about the speed of deforestation and climate change that could threaten primates.

“The pace at which everything is happening in today’s world is much faster than the presumed slow evolutionary processes that likely occurred with ancestral primates,” Eppley says.  

That’s why they believe these findings are concerning.

Those that are flexible enough to adapt because of their diverse diets or large groups may be OK for a while, but it’s extremely dangerous for tree-dwelling primates to move to the ground because of predators and diseases creating a “dire situation” for arboreal species.

Other species that rely on fruit diets or live in small groups are less adaptable and are less willing to descend. Losing any of these primate species will have negative impacts on the forest ecosystem.

“Primates are important seed dispersers and pollinators within their forest habitats and are critical for the germination of some tree species. The loss of primates would lead to a cascade of negative effects within the ecosystem,” Eppley says.

“Regardless of whether an arboreal primate species has specific traits that may act as a ‘pre-adaptation’ for terrestriality, we will still need to implement fast and effective conservation strategies to ensure their survival under the current changing conditions.”