Adventures Among Ants - Mark W. Moffett [45]
SHARING THE LOAD
On Mt. Apo, José and I settled down on the trail for a moment to watch the marauder ants at work. Although, as Frank Sinatra laments in the song “High Hopes,” “an ant can’t move that rubber tree plant,” all ant species are celebrated for the loads they can bear—even as singletons. A marauder ant minor worker is no exception, carrying up to five times her mass. It’s not that she is particularly muscular. Rather, it’s a question of proportion. Total strength is determined by muscle thickness, which is proportional to the animal’s height squared, while weight is proportional to the cube of its height. This means that unwieldy vertebrates end up with too much body weight for too little muscle. Galileo worked this out in 1636, writing that “a small dog could probably carry on his back two or three dogs of his own size, but I believe that a horse could not carry even one of his own size.”7 This formula explains how ants have the power to carry striking weights.
Through group transport, marauders take this excess-weight capacity to unparalleled levels. Not only do they haul food together, but they also gang-transport brood during a migration or in emergencies, the corpses of enemy ants that they dump near their trails, queens endangered during pandemonium at the nest, hunks of refuse bound for the colony dump, and, on occasion, obstructions on a trail or chunks of soil for making their arcades. In each case, they work in groups with greater effectiveness than any other living thing.
The capacity of a minor worker to carry five times her weight on her own sounds impressive, but in Singapore I had figured out that a 10-centimeter-long earthworm, like the largest ones I saw being heaved whole balanced between a hundred ants on the ant trail on Mt. Apo, would require a thousand ants if it had been cut in pieces and carried off one ant at a time—yet the ants gang-transporting the burdens on Mt. Apo were slowed to only about half the speed at which they hauled items by themselves. Even when the worm was five thousand times the weight of a minor worker and ten thousand times as voluminous, a gap was usually visible between the cargo and the ground: marauder ants lift rather than drag burdens. In human terms, that would be equal to getting friends together to run at breakneck speed while lifting overhead 250 tons, which would likely amount to far more than the contents of all their houses combined—an utter impossibility for a human.
Before coming to the Philippines, I had conducted an experiment. Although my breakfast ritual in Singapore was to have roti prata at an Indian food nook, for days when I needed to get going before 7 A.M. I kept a supply of a cheap Australian cereal called Grainut (which, being virtually inedible, has since gone off the market). One morning I was sitting in the Botanic Gardens next to a marauder ant trail eating said cereal, when I decided to crush some chunks among the ants and document the outcome.
As it turned out, the bigger the chunk, the more efficient they were, and the more food each ant was able to move along the trail in a unit of time. Beyond a certain size chunk, however, efficiency declined. It was apparently a matter of geometry: with increasing size, the weight of the chunks increased faster than their circumference, until there wasn’t enough space to accommodate the number of ants needed to lift the food. For the cereal, this occurred with chunks requiring more than fourteen ants.
Marauder ants carrying an earthworm at the Singapore Botanic Gardens.