Top-heaviness of ant nests is related to soil properties

The structural diversity of ant nests can be explained by various processes, including natural history and ecology. It is important to uncover the drivers of nest structural diversity because of the functional importance of nests for colony behavior and survival. The vertical distribution of chambers in subterranean nests impacts a variety of processes. Protection from predation can be facilitated by workers aggregating in deep chambers, and foraging regulation and brood development often occur in chambers near the soil surface. Here we examine what factors relate to chamber distribution in subterranean ant nests. We developed a quantitative measure of nest ‘top-heaviness’, which is the proportion of nest chambers found in the top half of the nest (near the soil surface) relative to the number of chambers in the bottom of the nest. We then used a database of subterranean nests from 29 ant species to ask whether phylogeny, foraging strategy, and soil density relate to nest top-heaviness. We did not find a phylogenetic signal for nest top-heaviness, nor did we find a relationship between nest top-heaviness and foraging strategy. However, we found a positive relationship between soil density and nest top-heaviness. These results suggest that the energetics of soil excavation may play an important role in shaping the distribution of chambers in subterranean ant nests. Uncovering drivers of variation in nest structure across ant species can reveal potential selective pressures on nest architecture.