Research - Gliding Ants - FAQ
Some species of ants that live in the tropical rain forest canopy use a form of gliding (or "directed aerial descent") to return to their home tree trunk when they fall from branches. When a gliding ant falls, jumps or is brushed off of a tree branch (e.g., by a passing monkey), it drops straight down for a few meters, then makes a rapid adjustment in orientation such that its abdomen is pointed toward the tree trunk. This alignment consistently directs the path of the falling ant through the air in a steep glide ending at the trunk.
To see videos of the behavior, click here. To see videos of non-gliders for comparison click here.
2) Why is this interesting?
The discovery of gliding behavior in ants is interesting and relevant for several reasons:
• It is the first documented example of gliding flight in a living wingless insect, and the first record of intentional backwards gliding in any macroorganism.
• It was completely unexpected; it is not something that anyone, biologist or not, would have predicted.
• It is a good example of a novel adaptation to a strong selective pressure and of parallel evolution; the behavior occurs in unrelated groups of ants (mainly the Pseudomyrmecinae and Cephalotini).
• In a time when extreme reductionism and sub-organismal research are in vogue in biology, it is nice to know that exciting discoveries can still be made through simple observation of organisms in nature.
• It justifies the need for continued research in tropical forest canopies, especially direct observational studies. The discovery of gliding ants shows that an arboreal existence poses unique challenges for many species.
3) How did you discover this behavior?
Two circumstances acted in concert to facilitate this discovery: a) ants are conspicuous and abundant in tropical tree crowns, and b) a biologist who spends a lot of time working 30 m above the ground tends to develop a curiosity (if not an obsession) about how things fall.
The first observation of gliding in ants occurred when I was climbing trees on Barro Colorado Island in Panama in 1998. I accidentally put my hand down on a Cephalotes atratus worker causing its thoracic spines to become lodged in my skin. When I brushed the ant away, I watched its path through the air and noticed that it appeared to glide back to the tree trunk. I dropped a few more ants and saw that the behavior was consistent. Unfortunately, I was at the end of my field season and did not have time to follow up these observations with experiments, but I discussed it briefly with my colleague, Dr. Mike Kaspari, and wrote a quick summary of the observation for future reference. Five years later I was confronted by hundreds of C. atratus workers while climbing a tree in Peru to collect mosquitoes. (For more info on the mosquito project, click here.) I brushed the ants off of the branch and watched in amazement as they glided back to the trunk in a J-shaped cascade. Since then, understanding gliding behavior in ants has occupied most of my free time.
4) Is this an important piece of the puzzle of the evolution of flight in insects?
Ants are a relatively advanced lineage among insects, and ant workers are secondarily wingless, thus gliding behavior in ants clearly is not linked to the evolution of flight. However, this discovery and our ongoing research with jumping bristletails and other basal taxa (e.g., silverfish) likely will shed some light on the evolution of flight in insects.
5) Why do ants glide?
To avoid becoming lost. Ant workers are responsible for (among other tasks) gathering resources from the surrounding environment and bringing them to the nest. This is a high-risk job. Day-active ants foraging in a lowland rain forest canopy are frequently exposed to visual predators (lizards, birds, anteaters) and disturbance from passing mammals (e.g., monkeys) in tree crowns. If a worker ant drops off the tree to avoid predation or is brushed off by a passing primate, it will probably land on unfamiliar vegetation or on leaf litter in the understory. Leaf litter is particularly complex foreign terrain and harbors a variety of predator species that are not found in the canopy. Thus, the understory is very hazardous for a small animal that is adapted for following chemical trails along sunlit branches. The Amazon basin presents an even more dangerous situation because a large fraction of the forest is flooded (and full of surface-feeding carnivorous fish) for several months of the year. An ant falling from a tree in this setting faces certain death. Why does this matter? A worker that is lost or eaten while foraging represents a direct cost to the colony; selection should favor any traits that prevent worker attrition. One example is "sticky" feet -- tarsal modifications that enable an insect to grip smooth surfaces. Now gliding can be added to the list.
6) Do all arboreal ants glide?
No. So far, the behavior occurs primarily in arboreal species in the myrmicine tribe Cephalotini and the subfamily Pseudomyrmecinae. Current field work is focusing on species in the subfamily Formicinae, especially species of Camponotus. I recently found that this genus contains several gliding and non-gliding species, and the gliders appear to descend head-first to the tree (unlike the other gliding ants mentioned above, which do it backwards). Based on data gathered to date, all arboreal dolichoderines and non-cephalotine myrmicines except Daceton armigerum also do not glide. The fact that the behavior crosses taxonomic lines suggests that strong parallel evolution in response to similar ecological pressures has occurred. This is something I am currently working on.
7) How do they do it?
My colleagues, Drs. Robert Dudley and Mike Kaspari, and I are still working on this question. We are certain that the ants use visual cues to locate tree trunks during a fall and we recently showed that they preferentially orient to white or bright-colored columns, which closely resemble their natural targets: lichen-covered tree trunks. We suspect that movements of the hind legs, abdomen, and head are used to align the ant with the tree and adjust the direction of the glide. We are currently using high-speed video and appendage ablation experiments to address this question.
8) What other animals glide?
The most popular and best-studied gliders are "flying" squirrels, sugar gliders and other mammals. Several species of frogs and lizards also glide, and there are some amazing gliding snakes in Asian tropical forests. We recently discovered that ants are not the only canopy arthropods that glide. Stay tuned for more on this ongoing research.