Drop Zone

An amazing rain forest discovery, and how a UTMB researcher stumbled onto it

By Tom Curtis

Insect ecologist Stephen P. Yanoviak was deep in the Amazon River basin, in the rain forest near Iquitos, Peru. He had climbed about a hundred feet up into the forest canopy and was using his bare arms as bait to collect mosquitoes for a UTMB project looking at the effects of deforestation on the ecology of disease transmission

As he settled in, Yanoviak noticed a parade of nearly half-inch-long black ants with a striking Darth Vader-esque appearance—female worker ants of the species Cephalotes atratus—meandering along a branch. He casually brushed the critters off—and then watched in amazement as they glided back to the trunk of the same tree in a J-shaped cascade. Since that experience last year, understanding gliding behavior in ants has occupied much of Yanoviak’s free time.

A number of tree-dwelling animals—from so-called flying squirrels to lizards, frogs, and even snakes—either glide through the forest air or parachute downwards in what scientists call controlled descent

But before Yanoviak spotted this phenomenon in tree-dwelling ants, no other scientist had documented controlled descent among wingless insects. Moreover, nobody previously had observed and reported on any animal but microscopic ones that intentionally glide backwards, as these ants do.

Yanoviak discussed what he had seen with colleagues Michael Kaspari, an ant ecologist with the Smithsonian Tropical Research Institute in Panama and the University of Oklahoma at Norman (where Yanoviak was awarded his Ph.D.), and Robert Dudley, a University of California at Berkeley and Smithsonian Tropical Research Institute biologist who specializes in flying and gliding animals. Together, the three reported their findings in an article titled “Directed aerial descent in canopy ants,” published February 10, 2005, in the journal
Nature, with Yanoviak as the lead author.

The discovery has provided Yanoviak and his colleagues with well over the fifteen minutes of fame that the late artist Andy Warhol predicted everyone in the future would experience: media outlets ranging from the BBC, the Canadian Broadcasting Corporation and Discovery Magazine to New Scientist, Smithsonian Magazine, MSNBC.com, Highlights for Children, and many others beat a path to their doors—or at least to their telephones—to find out about the intriguing news.

The trio reported that the ants descend in three distinct phases, starting with a free fall lasting two to three yards, followed by a quick turn re-orienting themselves toward the tree, and concluding with a sharp but purposeful glide to the tree trunk.

The first phase for the falling ants, called uncontrolled parachuting, involves splaying their legs, which has the effect of slowing their rate of descent by increasing their drag in the air. This is followed by spectacular 180-degree turns, in which the falling ants apparently shift their center of gravity and ultimately sail backwards with their abdomens and hind feet pointing towards the trunk so that their flight path redirects them to their tree. Often they just bounce off the trunk, only to execute successive hairpin turns that effectively carry them back to the tree.

In order to videotape these very black ants plummeting toward the dark rain forest floor, Yanoviak painted them with white nail polish.

Despite plunging downwards in steep glide trajectories at velocities averaging eight feet per second, about 85 percent of the thousands of ants they dropped in field experiments successfully used directed aerial descent to return to their home tree trunk, the trio reported.

In yet another field experiment, Yanoviak—a former Fulbright Scholar and Smithsonian Fellow who, in addition to his paid UTMB postdoctoral researcher job, also holds an unpaid position as courtesy assistant professor at the University of Florida’s Medical Entomology Laboratory in Vero Beach—blinded some of the ants by painting white nail polish over their eyes. The blinded ants’ rates of return to the tree trunks ran about 8 percent—just about what would be expected by chance. The researchers concluded that the non-blinded ants clearly “use visual cues to locate tree trunks before they hit the forest floor.”

It’s unclear just how the ants latch onto the tree trunk. It may be the insect world’s equivalent of a fighter plane landing on an aircraft carrier, with the claws on the ants’ back legs serving as grappling hooks to secure them to the tree. Once the ants have landed on their tree of origin, Yanoviak calculated that within ten minutes or so they climb as much as eighty feet back to where they came from in the relative safety of their arboreal home.

Why do these tree-dwelling ants glide?

“To avoid getting lost,” Yanoviak speculates. He notes that the ability helps them to survive when they are brushed off branches by passing monkeys or leap off to escape from predators such as lizards and birds during their day jobs of gathering resources from the lowland rainforest canopy and bringing them to the nest.

While ants and other small creatures generally survive the impact of a fall, directed aerial descent spares these ants from the perils of dropping onto treacherous and unfamiliar vegetation on the rain forest understory or leaf litter on the forest floor. Yanoviak explains:

“Leaf litter is particularly complex foreign terrain—and thus very hazardous—for a small animal that is adapted for following chemical trails along sunlit branches.”

Or as Yanoviak’s co-author Michael Kaspari put it somewhat more poetically to reporter Robin Lloyd of the Web-based publication LiveScience: “An ant falling to the forest floor enters a dark world of mold and decomposition, of predators and scavengers, where the return trip is through a convoluted jungle of dead, accumulated leaves.” He continued: “Gliding is definitely the way to go, and we won’t be surprised if we find more examples of this behavior among wingless canopy insects.”

Gliding also saves those ants that land on the forest floor from succumbing to nearly certain death during the half year when the Amazon basin is flooded and teeming with carnivorous fish.

Cephalotes atratus lives in relatively small colonies that often inhabit a single tree. A foraging ant’s death or disappearance in such a little community “represents a net loss to the colony,” Yanoviak observes. Natural selection, he notes,“should favor adaptations” or innate abilities “that prevent worker attrition.” Gliding in ants seems to represent one such evolutionary example.