Social insects are among the most abundant arthropods in terrestrial ecosystems, where they provide ecosystem services. The effect of subterranean activity of ants on soil is well-studied, yet little is known about nest architecture due to the difficulty of observing belowground patterns. Furthermore, many species’ ranges span environmental gradients, and their nest architecture is likely shaped by the climatic and landscape features of their specific habitats. We investigated the effects of two temperature treatments on the shape and size of nests built by Formica podzolica ants collected from high and low elevations in the Colorado Rocky Mountains in a full factorial experiment. Ants nested in experimental chambers with soil surface temperatures matching the local temperatures of sample sites. We observed a plastic response of nest architecture to conditions experienced during excavation; workers experiencing a high temperature excavated deeper nests than those experiencing a cooler temperature. Further, we found evidence of local adaptation to temperature, with a significant interaction effect of natal elevation and temperature treatment on nest size and complexity. Specifically, workers from high elevation sites built larger nests with more tunnels when placed in the cool surface temperature treatment, and workers from low elevation sites exhibited the opposite pattern. Our results suggest that subterranean ant nest architecture is shaped by a combination of plastic and locally adapted building behaviors; we suggest that the flexibility of this ‘extended phenotype’ likely contributes to the widespread success of ants.

Author Contributions: MB and MS conceived and designed the study. MS performed the field work and analyzed the data. MB and MS wrote the manuscript.

Ants alter soil moisture and nutrient distributions during foraging and nest construction. Here, we investigated how the effects of ants on soil vary with elevation. We compared moisture, carbon, and nitrogen levels in soil samples taken both within nests and nearby the nests (control) of two subterranean ant species. Using a paired design, we sampled 17 sites along elevation gradients in two California mountain ranges (Formica francoeuri in the San Jacinto mountains and Formica sibylla in the Sierra Nevada). We observed an interaction between soil carbon and nitrogen composition and elevation in each mountain range. At lower elevations, nest soil had lower amounts of carbon and nitrogen than control soil, but at higher elevations, nest soil had higher amounts of carbon and nitrogen than control soil. However, our sampling method may only breach the interior of ant nests in some environments. The nest soil moisture did not show any elevational patterns in either mountain range. Ants likely modulate soil properties differently across environmental gradients, but testing this effect must account for variable nest architecture and other climate and landscape differences across diverse habitats.

Invasive social insects rank among the most damaging of terrestrial species. They are responsible for extensive damage and severely threaten the biodiversity of environments where they are introduced. Variation in colony social form commonly occurs in introduced populations of yellowjacket wasps (genus Vespula). In particular, invasive colonies may contain multiple queens (i.e., polygyne) and persist several years, while in the native range, the colonies are usually annual and harbor a single queen (i.e., monogyne). In this study, we used genome-wide loci obtained by double digest restriction site-associated DNA sequencing (RADseq) to investigate the genetic structure and queen turnover in colonies of the western yellowjacket, Vespula pensylvanica, in their introduced range in Hawaii. Of the 27 colonies monitored over four months (October-January), 19 were polygyne and already contained multiple queens on the first day of sampling. Contrary to previous speculation, this finding suggests that polygyny often arises early in the annual colony cycle, before the production of new queens in the fall. Furthermore, polygyne colonies exhibited a prolonged average lifespan relative to those headed by a single queen. As a result, there is no clear window during which colony eradication efforts would be more effective than upon first discovery. The relatedness among nestmate queens was slightly above zero, indicating that these colonies are generally composed of nonrelatives. The queen turnover within each colony was low, and we detected some full-sibling workers sampled up to four months apart.Finally, we did not detect any population structure among colonies, suggesting that queens disperse up to several kilometers. Taken together, our results provide the first insights into the requeening dynamics in this invasive and incipiently polygyne population and illuminate the early establishment of multiple long-lasting queens in these damaging colonies.

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