The non-additive effects of body size on nest architecture in a polymorphic ant

Abstract

Like traditional organisms, eusocial insect societies express traits that are the target of natural selection. Variation at the colony level emerges from the combined attributes of thousands of workers and may yield characteristics not predicted from individual phenotypes. By manipulating the ratios of worker types, the basis of complex, colony-level traits can be reduced to the additive and non-additive interactions of their component parts. In this study, we investigated the independent and synergistic effects of body size on nest architecture in a seasonally polymorphic harvester ant, Using network analysis, we compared wax casts of nests, and found that mixed-size groups built longer nests, excavated more sand and produced greater architectural complexity than single-sized worker groups. The nests built by polymorphic groups were not only larger in absolute terms, but larger than expected based on the combined contributions of both size classes in isolation. In effect, the interactions of different worker types yielded a colony-level trait that was not predicted from the sum of its parts. In nature, colonies with fewer fathers produce smaller workers each summer, and produce more workers annually. Because body size is linked to multiple colony-level traits, our findings demonstrate how selection acting on one characteristic, like mating frequency, could also shape unrelated characteristics, like nest architecture.This article is part of the theme issue 'Interdisciplinary approaches for uncovering the impacts of architecture on collective behaviour'.

Interdisciplinary approaches for uncovering the impacts of architecture on collective behaviour

“…In social insects, the building process is an emergent collective behaviour that has been studied extensively both empirically and using modelling [64 -73]. In this theme issue, Kwapich et al [19] show that the composition of the colony that is excavating a structure can substantially impact nest topology. In a polymorphic species of ant, Veromessor pergandei, smaller individuals build shorter and less complex nests than larger individuals.…”

Response thresholds alone cannot explain empirical patterns of division of labor in social insects

“…Empirically, worker behavior in social insect colonies often correlates with individual traits [16]. For example, within a colony, workers of different age [32][33][34][35], experience [36], genotype [37] (e.g., patrilines [38,39] or matrilines [40]), or morphology (e.g., size [38,[41][42][43][44]) can vary in their propensity to engage in tasks such as foraging, nursing, or nest construction. Such behavioral variation is often attributed to the developmental or genetic modulation of response thresholds.…”

Modularity and connectivity of nest structure scale with colony size

“…To examine nest connectivity, we depicted each nest as a network, as in (Buhl et al 2004b; Perna et al 2008; Viana et al 2013; Pinter-Wollman 2015a; Kwapich et al 2018), by manually assigning a node ID to each chamber, nest entrance, tunnel junction, and ends of tunnels that did not reach a chamber (‘end nodes’) and connecting these nodes with edges depicting tunnels. When quantifying nest casts as networks we excluded the “mushroom top” where the casting material pools around the nest entrance when it is poured (Tschinkel 2010).…”