Warm and arid regions of the world are hotspots of superorganism complexity

Coevolution of larval signalling and worker response can trigger developmental caste determination in social insects

Eusocial insects belong to distinct queen and worker castes, which, in turn, can be divided into several morphologically specialized castes of workers. Caste determination typically occurs by differential nutrition of developing larvae. We present a model for the coevolution of larval signalling and worker task allocation-both modelled by flexible smooth reaction norms-to investigate the evolution of caste determination mechanisms and worker polymorphism. In our model, larvae evolve to signal their nutritional state to workers. The workers evolve to allocate time to foraging for resources versus feeding the brood, conditional on the larval signals and their body size. Worker polymorphism evolves under accelerating foraging returns of increasing body size, which causes selection to favour large foraging and small nursing workers. Worker castes emerge because larvae evolve to amplify their signals after obtaining some food, which causes them to receive more food, while the other larvae remain unfed. This leads to symmetry-breaking among the larvae, which are either well-nourished or malnourished, thus emerging as small or large workers. Our model demonstrates the evolution of nutrition-dependent caste determination and worker polymorphism by a self-reinforcement mechanism that evolves from the interplay of larval signalling and worker response to the signals.

The diversity of social complexity in termites

Sociality underpins major evolutionary transitions and significantly influences the structure and function of complex ecosystems. Social insects, seen as the pinnacle of sociality, have traits like obligate sterility that are considered 'master traits', used as single phenotypic measures of this complexity. However, evidence is mounting that completely aligning both phenotypic and evolutionary social complexity, and having obligate sterility central to both, is erroneous. We hypothesize that obligate and functional sterility are insufficient in explaining the diversity of phenotypic social complexity in social insects. To test this, we explore the relative importance of these sterility traits in an understudied but diverse taxon: the termites. We compile the largest termite social complexity dataset to date, using specimen and literature data. We find that although functional and obligate sterility explain a significant proportion of variance, neither trait is an adequate singular proxy for the phenotypic social complexity of termites. Further, we show both traits have only a weak association with the other social complexity traits within termites. These findings have ramifications for our general comprehension of the frameworks of phenotypic and evolutionary social complexity, and their relationship with sterility.

Environmental quality mediates the ecological dominance of cooperatively breeding birds

Although social species as diverse as humans and ants are among the most abundant organisms on Earth, animals cooperate and form groups for many reasons. How these different reasons for grouping affect a species' ecological dominance remains unknown. Here we use a theoretical model to demonstrate that the different fitness benefits that animals receive by forming groups depend on the quality of their environment, which in turn impacts their ecological dominance and resilience to global change. We then test the model's key predictions using phylogenetic comparative analysis of >6500 bird species. As predicted, we find that cooperative breeders occurring in harsh and fluctuating environments have larger ranges and greater abundances than non-cooperative breeders, but cooperative breeders occurring in benign and stable environments do not. Using our model, we further show that social species living in harsh and fluctuating environments will be less vulnerable to climate change than non-social species.

Body mass and cuticular hydrocarbon profiles, but not queen number, underlie worker desiccation resistance in a facultatively polygynous harvester ant (Pogonomyrmex californicus)

As small-bodied terrestrial organisms, insects face severe desiccation risks in arid environments, and these risks are increasing under climate change. Here, we investigate the physiological, chemical, and behavioral mechanisms by which harvester ants, one of the most abundant arid-adapted insect groups, cope with desiccating environmental conditions. We aimed to understand how body size, cuticular hydrocarbon profiles, and queen number impact worker desiccation resistance in the facultatively polygynous harvester ant, Pogonomyrmex californicus. We measured survival at 0% humidity of field-collected worker ants sourced from three closely situated populations within a semi-arid region of southern California. These populations vary in queen number, with one population dominated by multi-queen colonies (primary polygyny), one population dominated by single-queen colonies, and one containing an even mix of single- and multi-queen colonies. We found no effect of population on worker survival in desiccation assays, suggesting that queen number does not influence colony desiccation resistance. Across populations, however, body mass and cuticular hydrocarbon profiles significantly predicted desiccation resistance. Larger-bodied workers survived longer in desiccation assays, emphasizing the importance of reduced surface area-to-volume ratios in maintaining water balance. Additionally, we observed a positive relationship between desiccation resistance and the abundance of n-alkanes, supporting previous work that has linked these high-melting point compounds to improved body water conservation. Together, these results contribute to an emerging model explaining the physiological mechanisms of desiccation resistance in insects.

Ecological strategies of (pl)ants: Towards a world-wide worker economic spectrum for ants

Current global challenges call for a rigorously predictive ecology. Our understanding of ecological strategies, imputed through suites of measurable functional traits, comes from decades of work that largely focussed on plants. However, a key question is whether plant ecological strategies resemble those of other organisms.Among animals, ants have long been recognised to possess similarities with plants: as (largely) central place foragers. For example, individual ant workers play similar foraging roles to plant leaves and roots and are similarly expendable. Frameworks that aim to understand plant ecological strategies through key functional traits, such as the 'leaf economics spectrum', offer the potential for significant parallels with ant ecological strategies.Here, we explore these parallels across several proposed ecological strategy dimensions, including an 'economic spectrum', propagule size-number trade-offs, apparency-defence trade-offs, resource acquisition trade-offs and stress-tolerance trade-offs. We also highlight where ecological strategies may differ between plants and ants. Furthermore, we consider how these strategies play out among the different modules of eusocial organisms, where selective forces act on the worker and reproductive castes, as well as the colony.Finally, we suggest future directions for ecological strategy research, including highlighting the availability of data and traits that may be more difficult to measure, but should receive more attention in future to better understand the ecological strategies of ants. The unique biology of eusocial organisms provides an unrivalled opportunity to bridge the gap in our understanding of ecological strategies in plants and animals and we hope that this perspective will ignite further interest. Read the free Plain Language Summary for this article on the Journal blog.

DNA methylation and expression of the egfr gene are associated with worker size in monomorphic ants

The reproductive division of labour is a hallmark of eusocial Hymenoptera. Females are either reproductive queens or non-reproductive workers. In ants, workers often display further task specialisation that is associated with variation in size and/or morphology. Because female polyphenism is typically under environmental control, it is thought epigenetic mechanisms (such as DNA methylation) play a central role since they mediate gene-by-environment interactions. Methylation of the growth-promoting gene epidermal growth factor receptor (egfr) was indeed shown to control worker size in a highly polymorphic ant. However, it remains unknown if egfr methylation could also regulate worker size in monomorphic species. By combining experimental pharmacology and molecular biology, we show that worker size is associated with egfr methylation in two monomorphic ants. Furthermore, we functionally demonstrate that EGFR signalling affects worker size. These results indicate that worker size regulation by egfr methylation has been mechanistically conserved in ants but remains unexploited in monomorphic species.

Warm and arid regions of the world are hotspots of superorganism complexity

Biologists have long been fascinated by the processes that give rise to phenotypic complexity of organisms, yet whether there exist geographical hotspots of phenotypic complexity remains poorly explored. Phenotypic complexity can be readily observed in ant colonies, which are superorganisms with morphologically differentiated queen and worker castes analogous to the germline and soma of multicellular organisms. Several ant species have evolved 'worker polymorphism', where workers in a single colony show quantifiable differences in size and head-to-body scaling. Here, we use 256 754 occurrence points from 8990 ant species to investigate the geography of worker polymorphism. We show that arid regions of the world are the hotspots of superorganism complexity. Tropical savannahs and deserts, which are typically species-poor relative to tropical or even temperate forests, harbour the highest densities of polymorphic ants. We discuss the possible adaptive advantages that worker polymorphism provides in arid environments. Our work may provide a window into the environmental conditions that promote the emergence of highly complex phenotypes.