Leafcutter ants are normally thought of as tropical animals. These ubiquitous insects are commonly featured in Amazonian nature documentaries, for example, and they constitute one of the most damaging pests of South American agriculture.
But the ants, in spite of our latitudinal stereotyping, do extend into the temperate zone. Atta texana occupies the humid climes of Texas and Louisiana, forming the northern edge of the distribution of all leafcutters. But why do leafcutter ants stop there? Why wouldn’t they live in New York, or Manitoba?
Last week Ulrich Mueller’s lab at the University of Texas published an ambitious study in PNAS examining one factor that might limit the northern range of the ants: the ability of their symbiotic Attamyces fungus to withstand cold temperatures. (Remember that leafcutters are farmers, feeding only from a fungus cultivated in their underground nests.) Mueller et al gathered fungal samples from dozens of colonies at varying latitudes and tested their survival when challenged with cold. The results were clear: fungus from northern nests survived longer under cold temperatures than fungus from southern nests.
That Attamyces from different latitudes have different cold tolerances suggests that winter temperature exerts significant selection pressure on the system. Thus, Mueller et al’s study is indirect evidence that the low temperatures of the North American winter might be the key barrier preventing the pesty leafcutters from chewing their way across the midwestern soybean belt.
The chronic contrarian that I am, though, I can’t just let Mueller et al’s findings sit at that.
I’d like to offer a further relevant observation having to do with the seasonality of the fungus’s food. In temperate North America, most green vegetation dies back in October and doesn’t reappear until March or April. That’s 5 months without the fresh vegetation that Attamyces requires. So it might also be that the system simply starves out if stretched too far north.
In way of support, consider Trachymyrmex septentrionalis, a related fungus-growing ant:
Trachymyrmex does not cultivate its fungus with live vegetation like Atta but with bits of dead vegetative detritus. These food sources are available for more months of the year. And here’s the kicker. Trachymyrmex is found as far north as Illinois and Long Island. If they required fresh-cut leaves I’m not sure they’d make it.
Anyway, I am just a guy with a hunch typing away on a blog. Mueller et al have a peer-reviewed paper and piles of data. Make of it what you will.
Source: Mueller, U. G. et al 2011. Evolution of cold-tolerant fungal symbionts permits winter fungiculture by leafcutter ants at the northern frontier of a tropical ant–fungus symbiosis. PNAS published ahead of print, doi:10.1073/pnas.1015806108