Modularity and connectivity of nest structure scale with colony size

Abstract

Large body sizes have evolved structures to facilitate resource transport. Like unitary organisms, social insect colonies must transport information and resources. Colonies with more individuals may experience transport challenges similar to large-bodied organisms. In ant colonies, transport occurs in the nest, which may consist of structures that facilitate movement. We examine three attributes of nests that might have evolved to mitigate transport challenges related to colony size: (1) subdivision-nests of species with large colonies are more subdivided to reduce crowd viscosity; (2) branching-nest tunnels increase branching in species with large colonies to reduce travel distances; and (3) shortcuts-nests of species with large colonies have cross-linking tunnels to connect distant parts of the nest and create alternative routes. We test these hypotheses by comparing nest structures of species with different colony sizes in phylogenetically controlled meta-analyses. Our findings support the hypothesis that nest subdivision and branching evolved to mitigate transport challenges related to colony size. Nests of species with large colonies contain more chambers and branching tunnels. The similarity in how ant nests and bodies of unitary organisms have evolved in response to increasing size suggests common solutions across taxa and levels of biological organization.

Foraging behaviour affects nest architecture in a cross-species comparison of ant nests

“…Larger nests can house larger colonies (i.e. with more ants) and species with larger colonies have more chambers in their nests [11]. Thus, if species with mass or stable trail foraging tend to have larger colonies than species that use solitary or group foraging [37] then the relationship we found between foraging strategy and nest size could be explained by larger colonies having deeper nests with more chambers.…”

“…We began with the dataset used in Miller et al . 2022 [11], which contains chamber counts of 296 nests from 43 species in 24 genera, chamber widths from 188 nets from 37 species in 21 genera, and networks of 170 nests from 38 species in 21 genera. We added newly published nest images to the dataset used in Miller et al .…”

“…We added newly published nest images to the dataset used in Miller et al . 2022 [11] by searching the terms ‘ant nest’, ‘ant nest architecture’ and ‘ant nest excavation’ on Google Scholar. We included in our meta-analysis manuscripts from which we could extract the nest features that are detailed below from three or more nests of the same species.…”

“…For example, the structure of nest chambers can differ based on their use—with chambers used for growing fungus in Atta leaf-cutter ants being oval-shaped, while waste chambers are conical [22]. Tunnels are used for moving within the nest and their length and connectivity may impact the speed of transport [11]. Chambers differ in the degree to which they are connected to one another; many subterranean nests are ‘top heavy’, with many highly connected chambers near the soil surface and ‘chains’ of less connected chambers deeper inside [8,9].…”

“…Examples of this diversity include using silk from larvae to weave leaves into nests in tree canopies, living in hollowed twigs, branches, or specialized plant cavities, excavating elaborate subterranean nests in soils, and even residing within bivouacs made from the bodies of interlinked workers, as seen in army ants [10]. Nests from species living in the same strata may differ widely in characteristics because of the function the nests provide [11], colony size and demographics [12][13][14][15], and the morphology of the workers [16]. For example, species with very similar ecologies can differ in the widths of the tunnels they dig because of variation in worker body size [17], and species with larger colonies excavate nests with more chambers that are better connected than nests of species with smaller colonies [11].…”