How Monkeys Travel Through The Treetops

Scientists are combining new technologies to reveal how the animals use tree canopies.
Spider monkey

Black-handed spider monkeys (Ateles geoffroyi) are found throughout Central America. 

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Emily DeMarco, Staff Writer

(Inside Science) -- Sure of foot, hand and tail, black-handed spider monkeys flash through forest canopies with enough speed and security to make Olympic gymnasts envious. And good luck to any primatologist trying to keep up on foot while pushing through the wild tropical vegetation.

Recent advances in tracking technology are dramatically improving scientists' ability to describe such animal movements. But they have grappled with "seeing" the dense, lofty treetops as the creatures that live there do, unable to make sense of how the animals travel through the canopies of entire forests.

Now, a new technique that combines laser-scanning technology called LIDAR and animal movement data collected from GPS receivers is poised to help remove those blind spots. The system could help researchers develop more effective conservation plans as well as piece together how fruit-eating animals disperse the seeds vital to the regrowth of forests generation after generation.

"I'm really excited about the ability for scientists to start doing better work on canopy mammals," said Meg Lowman, an ecologist at the California Academy of Sciences, who has studied the ecology of forest canopies for over 30 years and wasn't involved in the research. "That's always been a tricky subject."

Lowman, who is also known as "Canopy Meg," was one of the first to venture into that overhanging firmament in the name of science. She and her colleagues used an array of tools such as canopy walkways and hot air balloons to help access the treetops to study plants and insects.

Getting their feet off the ground helped scientists understand that basic forest structure -- tree height, thickness or thinness of crowns, gaps between trees -- affects how the animals that live there move around. But measuring those attributes across entire forests and learning to tell good habitat from bad from the point of view of highly mobile and acrobatic animals like monkeys, for example, has been difficult.

In the new study, Kevin McLean, a doctoral candidate in ecology at Yale University, in New Haven, Connecticut, and his colleagues analyzed LIDAR data covering nearly all of Barro Colorado Island, a six-square-mile biological reserve in the Panama Canal. From an aircraft known as the Carnegie Airborne Observatory, the high-resolution LIDAR sends laser pulses downward that then reflect back to a sensor, measuring the distance between the instrument and the vegetation and allowing researchers to create 3-D images of ecosystems.

Using the LIDAR data, the researchers zoomed in on properties of forests thought to affect the movement of (non-flying) mammals -- how closely the trees are woven together, how mature the forest is, and how dense or thick tree crowns are. Next, they paired that information with data collected from high-resolution GPS devices for three primate species -- white-faced capuchin, mantled howler and black-handed spider monkeys.

Then, they created models relating the structure of the forest to the primates' movements. That allowed the researchers to see the canopy in 3-D in the exact locations that they knew the animals were moving through, revealing how the monkeys use the forest in much more detail than ever before.

The technique mainly confirmed what primatologists expected of the monkeys -- all three avoided gaps and the capuchin and spider monkeys preferred to move through higher, more mature canopy -- based on previous small-scale observational studies. More importantly, the study, published in Landscape Ecology last month, illuminated the system’s potential for larger, landscape-scale research on the ecology and habitat requirements of the many different species that live up in the trees. 

"The approach and the promise for more work in this area are pretty exciting," said Roland Kays, a zoologist at North Carolina State University in Raleigh who has also studied primates and other mammals on the island. "This [study] is the tip of the iceberg."

Those new insights into the types of tree canopies various species need to survive can't come too soon, scientists say. That’s because forests, and tropical ones in particular, are facing a host of threats including deforestation, forest fragmentation, and climate and land use change. While current management and conservation plans typically address whether or not forest cover exists, they don’t take into account the quality of that cover for animals that live entirely off the ground, said McLean. As any black-handed spider monkey might tell you, not all canopies are created equal.

And that's not all. Animals are vital to keeping forests connected and growing. Studying how arboreal mammals use the forest could shed light on which species are the most important for spreading seeds around after they eat fruit, said McLean. "It’s important to remember that these animals aren't just ornaments on the forest."

With the new technique, scientists can finally start to take off the blinders imposed from being mammals that walk around on two feet, said Lowman. "The canopy walkways, the hot air balloons, the cranes and the ropes helped," she said. "But this [technique] will give a perspective of the forest that even those could never provide."


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Emily DeMarco is a former Inside Science staff writer. Follow her on Twitter: @emily_p_demarco