Jessica Purcell

The expression and impact of antifungal grooming in ants.

Parasites can cause extensive damage to animal societies in which many related individuals frequently interact. In response, social animals have evolved diverse individual and collective defences. Here, we measured the expression and efficiency of self‐grooming and allo‐grooming when workers of the ant Formica selysi were contaminated with spores of the fungal entomopathogen Metarhizium anisopliae. The amount of self‐grooming increased in the presence of fungal spores, which shows that the ants are able to detect the risk of infection. In contrast, the amount of allo‐grooming did not depend on fungal contamination. Workers groomed all nestmate workers that were re‐introduced into their groups. The amount of allo‐grooming towards noncontaminated individuals was higher when the group had been previously exposed to the pathogen. Allo‐grooming decreased the number of fungal spores on the surface of contaminated workers, but did not prevent infection in the conditions tested (high dose of spores and late allo‐grooming). The rate of disease transmission to groomers and other nestmates was extremely low. The systematic allo‐grooming of all individuals returning to the colony, be they contaminated or not, is probably a simple but robust prophylactic defence preventing the spread of fungal diseases in insect societies.

Altitudinal Patterns of Spider Sociality and the Biology of a New Midelevation Social Anelosimus Species in Ecuador

To the extent that geography correlates with particular environmental parameters, the geographical distribution of phylogenetically related social and nonsocial organisms should shed light on the conditions that lead to sociality versus nonsociality. Social spiders are notorious for being concentrated in tropical regions of the world, occupying a set of habitats more restricted than those available to the phylogenetic lineages in which they occur. Here we document a parallel pattern involving elevation in the spider genus Anelosimus in America and describe the biology of a newly discovered social species found at what appears to be the altitudinal edge of sociality in the genus. We show that this is a cooperative permanent‐social species with highly female‐biased sex ratios but colonies that are one to two orders of magnitude smaller than those of a low‐elevation congener of similar body size. We suggest that the absence of subsocial Anelosimus species in the lowland rain forest may be due to an increased probability of maternal death in this habitat due to greater predation and/or precipitation, while absence of a sufficient supply of large insects at high elevations or latitudes may restrict social species to low‐ to midelevation tropical moist forests. We refer to these as the “maternal survival” and “prey size” hypotheses, respectively, and suggest that both in combination may explain the geographical distribution of sociality in the genus.

Geographic patterns in the distribution of social systems in terrestrial arthropods

The role of ecology in the evolution and maintenance of arthropod sociality has received increasing research attention in recent years. In some organisms, such as halictine bees, polistine wasps, and social spiders, researchers are investigating the environmental factors that may contribute to high levels of variation in the degree of sociality exhibited both among and within species. Within lineages that include only eusocial members, such as ants and termites, studies focus more on identifying extrinsic factors that may contribute to the dramatic variation in colony size, number of queens, and division of labour that is evident across these species. In this review, I propose a comparative approach that seeks to identify environmental factors that may have a common influence across such divergent social arthropod groups. I suggest that seeking common biogeographic patterns in the distribution of social systems or key social traits may help us to identify ecological factors that play a common role in shaping the evolution of sociality across different organisms. I first review previous studies of social gradients that form along latitudinal and altitudinal axes. Within families and within species, many organisms show an increasing degree of sociality at lower latitudes and altitudes. In a smaller number of cases, organisms form larger groups or found nests cooperatively at higher latitudes and altitudes. I then describe several environmental factors that vary consistently along such gradients, including climate variables and abundance of predators, and outline their proposed role in the social systems of terrestrial arthropods. Finally, I map distributions of a social trait against several climatic factors in five case studies to demonstrate how future comparative studies could inform empirical research.

Gradients of precipitation and ant abundance may contribute to the altitudinal range limit of subsocial spiders: insights from a transplant experiment

Species range boundaries often form along environmental gradients that dictate the success of the phenotypes present in each habitat. Sociality may allow colonization of environments where related species with a solitary lifestyle cannot persist. Social spiders in the genus Anelosimus appear restricted to low- and mid-elevation moist environments in the tropics, while subsocial spiders, common at higher elevations and latitudes, appear to be absent from the lowland tropical rainforest. Here, we seek factors that may simultaneously prevent subsocial Anelosimus species from colonizing the lowland rainforest while favouring species with large social groups in this habitat. To this end, we transplanted small groups of a subsocial species, which contain the offspring of a single female, from cloud forest habitat in the centre of its natural range to lower montane rainforest on the range margin and to lowland rainforest outside of the species range. Groups transplanted at the range margin and below their range limit were less likely to disperse and experienced increased mortality. This was correlated with greater rainfall intensity and ant abundance. We show that protection from rainfall enhances the performance of small groups of spiders in the lowland rainforest, and suggest that predation or disturbance by ants may influence the geographical range limits of this species.

Transitions in social complexity along elevational gradients reveal a combined impact of season length and development time on social evolution

Eusociality is taxonomically rare, yet associated with great ecological success. Surprisingly, studies of environmental conditions favouring eusociality are often contradictory. Harsh conditions associated with increasing altitude and latitude seem to favour increased sociality in bumblebees and ants, but the reverse pattern is found in halictid bees and polistine wasps. Here, we compare the life histories and distributions of populations of 176 species of Hymenoptera from the Swiss Alps. We show that differences in altitudinal distributions and development times among social forms can explain these contrasting patterns: highly social taxa develop more quickly than intermediate social taxa, and are thus able to complete the reproductive cycle in shorter seasons at higher elevations. This dual impact of altitude and development time on sociality illustrates that ecological constraints can elicit dynamic shifts in behaviour, and helps explain the complex distribution of sociality across ecological gradients.

Differential allocation and deployment of direct and indirect defences by Vicia sepium along elevation gradients

Summary 1. Dissecting drivers of plant defence investment remains central for understanding the assemblage of communities across different habitats. There is increasing evidence that direct defence strategies against herbivores, including secondary metabolites production, differ along ecological gradients in response to variation in biotic and abiotic conditions. In contrast, intraspecific variation in indirect defences remains unexplored. 2. Here, we investigated variation in herbivory rate, resistance to herbivores and indirect defences in ant-attracting Vicia species along the elevation gradient of the Alps. Specifically, we compared volatile organic compounds (VOCs) and ant attraction in high- and low-elevation ecotypes. 3. Consistent with adaptation to the lower herbivory conditions that we detected at higher elevations in the field, high-elevation plants were visited by fewer ants and were more susceptible to herbivore attack. In parallel, constitutive volatile organic compound production and subsequent ant attraction were lower in the high-elevation ecotypes. 4. We observed an elevation-driven trade-off between constitutive and inducible production of VOCs and ant attraction along the environmental cline. At higher elevations, inducible defences increased, while constitutive defence decreased, suggesting that the high-elevation ecotypes compensate for lower indirect constitutive defences only after herbivore attack. 5. Synthesis. Overall, direct and indirect defences of plants vary along elevation gradients. Our findings show that plant allocation to defences are subject to trade-offs depending on local conditions, and point to a feedback mechanism linking local herbivore pressure, predator abundance and the defence investment of plants.

The influence of social structure on brood survival and development in a socially polymorphic ant: insights from a cross-fostering experiment

Animal societies vary in the number of breeders per group, which affects many socially and ecologically relevant traits. In several social insect species, including our study species Formica selysi, the presence of either one or multiple reproducing females per colony is generally associated with differences in a suite of traits such as the body size of individuals. However, the proximate mechanisms and ontogenetic processes generating such differences between social structures are poorly known. Here, we cross‐fostered eggs originating from single‐queen (= monogynous) or multiple‐queen (= polygynous) colonies into experimental groups of workers from each social structure to investigate whether differences in offspring survival, development time and body size are shaped by the genotype and/or prefoster maternal effects present in the eggs, or by the social origin of the rearing workers. Eggs produced by polygynous queens were more likely to survive to adulthood than eggs from monogynous queens, regardless of the social origin of the rearing workers. However, brood from monogynous queens grew faster than brood from polygynous queens. The social origin of the rearing workers influenced the probability of brood survival, with workers from monogynous colonies rearing more brood to adulthood than workers from polygynous colonies. The social origin of eggs or rearing workers had no significant effect on the head size of the resulting workers in our standardized laboratory conditions. Overall, the social backgrounds of the parents and of the rearing workers appear to shape distinct survival and developmental traits of ant brood.

Bidirectional shifts in colony queen number in a socially polymorphic ant population.

The breeding system of social organisms affects many important aspects of social life. Some species vary greatly in the number of breeders per group, but the mechanisms and selective pressures contributing to the maintenance of this polymorphism in social structure remain poorly understood. Here, we take advantage of a genetic dataset that spans 15 years to investigate the dynamics of colony queen number within a socially polymorphic ant species. Our study population of Formica selysi has single‐ and multiple‐queen colonies. We found that the social structure of this species is somewhat flexible: on average, each year 3.2% of the single‐queen colonies became polygynous, and conversely 1.4% of the multiple‐queen colonies became monogynous. The annualized queen replacement rates were 10.3% and 11.9% for single‐ and multiple‐queen colonies, respectively. New queens were often but not always related to previous colony members. At the population level, the social polymorphism appeared stable. There was no genetic differentiation between single‐ and multiple‐queen colonies at eight microsatellite loci, suggesting ongoing gene flow between social forms. Overall, the regular and bidirectional changes in queen number indicate that social structure is a labile trait in F. selysi, with neither form being favored within a time‐frame of 15 years.

Functional diversity decreases with temperature in high elevation ant fauna

Severe environmental conditions filter community species compositions, forming clines of functional diversity along environmental gradients. Here, the changes in functional diversity in ant assemblages with severe environmental conditions in the Swiss Alps were investigated. Eight sites were sampled along an elevation gradient (1800–2550 m). The variation in functional diversity was analysed along an elevation gradient considering four traits: social structure (monogynous vs. polygynous), worker size, pupal development, and nest structure. Ant species richness and functional diversity decreased with decreasing temperature. Species found in colder habitats tended to live in subterranean nests rather than in mounds and exhibit a polymorphism in queen number, either within or across populations. The phylogenetic diversity did not decrease at colder temperature: Formicinae and Myrmicinae occupied the full range of elevations investigated. An insulation experiment indicated that mounds are more thermally insulated against the cold compared with soil. The absence of a mound‐building ant from high elevations probably results from a reduction in the amount of vegetal materials provided by coniferous trees. More severe abiotic conditions at higher elevations act as a filter on ant assemblages, directly through physiological tolerances to the abiotic conditions and indirectly as the vegetation necessary for nest building shifts with elevation.

The Evolution of Inbred Social Systems in Spiders and Other Organisms: From Short-Term Gains to Long-Term Evolutionary Dead Ends?

The question of why some social systems have evolved close inbreeding is particularly intriguing given expected short- and long-term negative effects of this breeding system. Using social spiders as a case study, we quantitatively show that the potential costs of avoiding inbreeding through dispersal and solitary living could have outweighed the costs of inbreeding depression in the origin of inbred spider sociality. We further review the evidence that despite being favored in the short term, inbred spider sociality may constitute in the long run an evolutionary dead end. We also review other cases, such as the naked mole rats and some bark and ambrosia beetles, mites, psocids, thrips, parasitic ants, and termites, in which inbreeding and sociality are associated and the evidence for and against this breeding system being, in general, an evolutionary dead end.