Did you know that there are ants that actually jump from the trunks or branches of tall trees? It's easy enough to imagine ants being knocked out of their arboreal homes by heavy winds or pelting raindrops, but apparently certain species of canopy ant, such as Cephalotes atratus,voluntarily drop from great heights if approached by an imposing object. Stephen Yanoviak, Robert Dudley and Michael Kaspari recently reported good news for those of us who might be walking under such trees: these ants are unlikely to land on us, or any other part of the understorey for that matter. Instead, ants of this species exhibit directed descent during free fall and somehow generally manage to land on the trunk of the tree from which they fell rather than hitting the ground.
To investigate the nature and extent of the ants' control during their descent, Yanoviak and colleagues dropped 120 individual worker ants from branches of their resident trees and videotaped their fall trajectories. Falls typically consisted of three phases. First was a period of uncontrolled,vertical free fall, second was a shift in body positioning, and third was a relatively steep glide back toward the trunk, regardless of the animal's original orientation when dropped. The scientists used markings on individuals to determine that they glide abdomen-first. A remarkable 85% actually landed on the trunk and, after a few tumbles, managed to establish their footing. Moreover, within minutes, many of these individuals had climbed back up to the branch from which they had been dropped.
As is the case with most things in biology, size plays an important role here. Smaller ants (inter- and intraspecifically) do better than larger ants;they fall a shorter distance before landing back on the trunk of the tree from which they were dropped. Further experiments in which the scientists blinded the ants revealed a large drop-off in the number making it back to the tree during a fall (10% vs 85%), suggesting the importance of visual stimuli in locating the trunk and controlling the direction of the glide.
The scientists also quantified several gliding performance parameters by analyzing videos of ants dropped within a flight arena near a fabric-covered vertical column. In these trials, they found that the ants' average glide speeds were 4.3 m s–1 and glide angles averaged 75°(relative to the horizontal). Although this angle is steep, it is far from vertical and reflects a much better glide than what Yanoviak and his colleagues calculated for an ant-shaped cylinder falling under similar airflow conditions (i.e. at a comparable Reynolds number).
It turns out that not all tree-dwelling ant species have (or use) this capacity for directed aerial descent. Instead, the behavior may be associated with groups that share certain traits, including arboreal nests, branch-tip foraging and evolutionary origins in flooded habitats. A broader comparative assessment of the distribution, control and mechanics of this gliding behavior is sure to reveal intriguing results. For now, simply pause to reflect on nature's brilliance in the form of wingless ants being led by their rears to safety in the canopies of tropical forests around the world.