Adventures Among Ants - Mark W. Moffett [93]
Whereas ants who don’t live in the trees tend to tumble willy-nilly and hit the ground blindly, canopy species frequently seem to be able to control their falls. Jack Longino of Evergreen State College and I have spent some time dangling from ropes, where we’ve often contributed to ant rain by knocking various species of ants off branches, for the most part unintentionally. More often than not, it seemed to us, the ants would land back on the tree, as if they could control their flight path and hit a target. We couldn’t understand how they did it.
Stephen Yanoviak, then at the University of Oklahoma, noticed the same thing and set out to prove that certain ants from Peru and Panama indeed can glide.8 The species he concentrated on was Cephalotes atratus, a slate-black “turtle ant” with a flattened body. High-speed videos proved that when dislodged from a tree, a turtle ant stretches out her body and limbs and aligns herself with respect to the ground so that she doesn’t turn head over heels. Detecting a tree trunk by its relative brightness against the dark greenery, she twists in the air to point her abdomen in that direction, glides backward at a steep angle—a behavior that I was eventually able to capture for this species with my camera in Tiputini, Ecuador—and grabs the trunk on impact.9
Other ant species make a tight spiral as they fall, directing their bodies with apparent intention, much like a parachutist who aims well enough to strike the earth at a good spot and on his feet. In a rainforest, numerous leaves lie between a plummeting ant and the ground. I believe that if she can slow her descent while keeping her clingy feet oriented downward, a worker can greatly improve her chances of landing securely on one of these leaves rather than bouncing off, as she’d surely do if she were tumbling head over tarsi.
The method employed by a falling worker depends on where she lives and the dangers she faces. Weaver ants neither glide nor spiral down, but rather plummet head over heels, a reflection of how little a fall matters to them, whereas Daceton are virtuoso gliders. Both the turtle ant and Daceton nest in tree trunks or thick branches. When one of these ants falls, a trunk is likely to be in range, and gliding to it is the obvious choice. For species that nest farther out among the twigs it makes little sense to aim for a trunk that may be too far away to see, let alone reach by gliding. Foliage is a sensible target, and parachuting in a tight spiral is the way to make a firm landing.
TRAVELING IN THE CANOPY
What convoluted territories arboreal ants inhabit! The navigational problem faced by a small ant is that a tree for her is a highly warped surface, one that is much more complicated than the surface of the earth is for us. She can usually monitor her movements up and down by gauging the influence of gravity on her body segments, but these gravitational effects can be masked by the movement of a plant in a breeze.10 Because she may often have no idea which way is up, a worker in the canopy does not experience the geometry of the world the way we do. She can walk in one direction and find herself back at her starting point (she circled a trunk or branch). If she makes a ninety-degree turn, she may either reach the end of the world (a branch tip) or be lost forever (having walked down the trunk and onto the ground).
It helps that individual trees have some common features, such as a limited number of branching patterns—compare the alternate branching rhythms of an oak to the terminal leaf clumps of palm, for instance.11 Unlike a rat forced to navigate a psychologist’s maze that has been constructed with no thought to the geometry of nature, arboreal animals can use a tree’s predictable structure as a navigation aid.
Ants exploit many aspects of plant architecture. On trunks, columns of workers often follow grooves