Adventures Among Ants - Mark W. Moffett [91]
11 negotiating the physical world
Arboreal ants scramble around the labyrinth of the canopy, spreading out along interconnected branches and vines, coursing up and down the trunks of trees. Vertigo is a human problem: most canopy-dwelling animals, ants included, are indifferent to height. What matters for them is finding the conditions and resources suited for their survival. Potential habitats occur at many levels—both on different parts of a plant and within each layer of growth in the plant community, from herbs to shrubs to shaded understory trees to tall trees with crowns in the sun, to the occasional emergent tree that sticks up above all the others.1
The ants’ choices among these parts and layers contribute to the diverse mosaic of their species in a forest. At one site in Borneo, a quarter of the ant species restricted themselves to the ground, another quarter to the understory shrubs, and another quarter to the lower canopy, leaving only a quarter to move freely through all three layers.2 That’s a remarkably low percentage of wide-ranging ants given the mobility of most workers. Low plants offer the ants living there conditions similar to those on the ground—shade, coolness, and moisture. The uppermost foliage is more fickle: in the blazing sun it is hot and dry, during periods of rain or fog it is cool and damp, and at night the temperature drops sharply. Between the extremes at top and bottom, an elevation change of a few meters up or down a tree can be equivalent to the environmental transformation we experience while traveling kilometers over the ground, say from inland mountain slopes to the seaside.
A cross-sectional view of rainforest along the Amazon River near Iquitos, Peru, showing the canopy layers, including a tall emergent tree at left, as well as vines and epiphytes. Different ants confine themselves to particular strata.
Even ecosystems with less height than a forest can offer similar variety. The overarching grasses and wildflowers in a meadow, for example, form an upper canopy. A suburban lawn can have layers of vegetation as well defined as those of a forest 90 meters high—from groundhuggers like the procumbent pearlwort and fairy flax, to midlevel scramblers such as sweet vernal, to the upright stalks of the lawn canopy giants: white clover and any of a variety of grasses.3 For a human on the ground, forest interiors provide a mild climate relative to the sun-roasted air above the trees, and a lawn’s interior offers similar conditions—for an ant, anyway.
Ants that stay within one canopy stratum, perhaps nesting and foraging their entire lives on the same tree branch, need not climb any more than ants on terra firma. However, nearly all ant species are natural climbers. Camponotus gigas carpenter ants, the biggest of all ant workers at a length of 2.8 centimeters, walk all over trees in the same Malaysian forests that weaver ants occupy. I remember huffing and puffing up one immense tree, assisted by ropes and gear, and looking over at the trunk to see a chunky Camponotus gigas major worker race ahead of me to the crown from a barrack nest at the base of the tree. Why was her ascent so effortless? When walking on flat ground, an ant burns a lot of energy relative to her mass because she has to move her little legs quickly to get anywhere; for her the added cost of a climb, compared to moving horizontally, is almost nil.4
For many people, going up a small tree can be a pleasure—at least for those of us who’ve kept a little bit of our kid selves inside. But in the tropics, even small trees harbor risks. At the Tiputini Research Station in Ecuador, I free-climbed a slim tree near the residence cabins during a much-needed break from rainy hours photographing the falling behavior of turtle ants—an effort that required scaring one worker after another off a little ledge and entailed over 6,300 clicks of my camera. I had gone up the tree because I surmised there was a turtle ant nest in it, which would provide me with a fresh supply of workers. What I found instead were workers of the giant