Adventures Among Ants - Mark W. Moffett [121]
In his classic 1874 book The Naturalist in Nicaragua, the British geologist and natural historian Thomas Belt described another instance of leafcutters using gravity to save time, in this case when transporting bits of fungus garden during a migration:
I found them busily employed bringing up the ant-food from the old burrows, and carrying it to a new one a few yards distant; and here I first noticed a wonderful instance of their reasoning powers. Between the old burrows and the new one was a steep slope. Instead of descending this with their burdens, they cast them down on the top of the slope, whence they rolled down to the bottom, where another relay of labourers picked them up and carried them to the new burrow. It was amusing to watch the ants hurrying out with bundles of food, dropping them over the slope, and rushing back immediately for more.52
HUNTER AND PREY
The fact that leafcutters live on foliage and fungus doesn’t mean they aren’t as picky about their meals as meat-eating ants. Foragers may largely keep the interests of the fungus in mind—in a sense they are shopping for someone else—but because the adult workers are sustained largely by sap, some of the plants they harvest could reflect personal taste rather than the needs of the gardens. Still, the ants don’t drink the sap while they cut and carry leaves; that typically happens in the nest, where the small ants lick the fragments and regurgitate the liquid to their larger sisters.
While the colony as a whole consumes varied foliage, individual ants become specialists on certain plants growing at sites they get to know intimately.53 Workers are prompt at recruiting assistance to a plant species they know well; conversely, they recruit to an unfamiliar plant only after they assess its quality.54 In this approach they resemble a bumblebee, which, after sampling a variety of flowers, comes to specialize, or major, in a single plant species.55 It’s unclear whether majoring makes a leafcutter in any way better at her job. In any case, tender leaves come and go, and, like bumblebees (and many college students), a leafcutter worker has to change her major now and then.
There are several aspects to leaf desirability—for the ant, the fungus, and the colony. Leafcutters prefer vegetation that is easy to slice. Also, they gravitate toward foliage in direct sunlight, which is the most nutrient rich. Red leaf flushes indicate chemicals toxic to fungi,56 and leafcutters avoid them in favor of older leaves or, ideally, soft, defenseless young leaves with less of the cellulose their fungi can’t assimilate.57 Such foliage is particularly abundant in pioneer trees, which are species that spring up in early-successional habitats—relatively open places where the mature trees of heavily shaded, old growth forest have been felled by storms or old age. Where there are many pioneers, leafcutters have the luxury of selecting the few most desirable plants, whereas in older forests, colonies are forced to constantly sample from dozens of less-choice trees.58
Human land-clearing practices keep vegetation in an early-successional stage, which is why cultivated land is the leafcutters’ favorite grocery store. Many human cultivars are of Old World origin and have no native defenses against leafcutters, or they have had the toxins bred out of them for human consumption, turning them into perfect fungus garden fodder and allowing the ants to strip them bare.59 For these reasons, leafcutter populations have thrived along with human populations to a degree that can be as crippling as a biblical plague of locusts, resulting in hundreds of billions of dollars in damage annually.60
It’s curious that plants don’t do a better job of fighting leafcutter incursions. The munching of caterpillars or beetles can induce plants to produce chemical deterrents that make their