Fostering model explains variation in levels of sociality in a spider system

Anticipated effects of abiotic environmental change on intraspecific social interactions

Social interactions are ubiquitous across the animal kingdom. A variety of ecological and evolutionary processes are dependent on social interactions, such as movement, disease spread, information transmission, and density-dependent reproduction and survival. Social interactions, like any behaviour, are context dependent, varying with environmental conditions. Currently, environments are changing rapidly across multiple dimensions, becoming warmer and more variable, while habitats are increasingly fragmented and contaminated with pollutants. Social interactions are expected to change in response to these stressors and to continue to change into the future. However, a comprehensive understanding of the form and magnitude of the effects of these environmental changes on social interactions is currently lacking. Focusing on four major forms of rapid environmental change currently occurring, we review how these changing environmental gradients are expected to have immediate effects on social interactions such as communication, agonistic behaviours, and group formation, which will thereby induce changes in social organisation including mating systems, dominance hierarchies, and collective behaviour. Our review covers intraspecific variation in social interactions across environments, including studies in both the wild and in laboratory settings, and across a range of taxa. The expected responses of social behaviour to environmental change are diverse, but we identify several general themes. First, very dry, variable, fragmented, or polluted environments are likely to destabilise existing social systems. This occurs as these conditions limit the energy available for complex social interactions and affect dissimilar phenotypes differently. Second, a given environmental change can lead to opposite responses in social behaviour, and the direction of the response often hinges on the natural history of the organism in question. Third, our review highlights the fact that changes in environmental factors are not occurring in isolation: multiple factors are changing simultaneously, which may have antagonistic or synergistic effects, and more work should be done to understand these combined effects. We close by identifying methodological and analytical techniques that might help to study the response of social interactions to changing environments, highlight consistent patterns among taxa, and predict subsequent evolutionary change. We expect that the changes in social interactions that we document here will have consequences for individuals, groups, and for the ecology and evolution of populations, and therefore warrant a central place in the study of animal populations, particularly in an era of rapid environmental change.

Population differences in aggression are shaped by tropical cyclone-induced selection

Extreme events, such as tropical cyclones, are destructive and influential forces. However, observing and recording the ecological effects of these statistically improbable, yet profound 'black swan' weather events is logistically difficult. By anticipating the trajectory of tropical cyclones, and sampling populations before and after they make landfall, we show that these extreme events select for more aggressive colony phenotypes in the group-living spider Anelosimus studiosus. This selection is great enough to drive regional variation in colony phenotypes, despite the fact that tropical cyclone strikes are irregular, occurring only every few years, even in particularly prone regions. These data provide compelling evidence for tropical cyclone-induced selection driving the evolution of an important functional trait and show that black swan events contribute to within-species diversity and local adaptation.

Female-Biased Sex Ratios Increase Colony Survival and Reproductive Output in the Spider Anelosimus studiosus

Negative frequency-dependent selection acting on the sexes is hypothesized to drive populations toward a balanced sex ratio. However, numerous examples of female-biased sex ratios pepper the arthropods. Theoretical examinations have proposed that female-biased populations or groups can have higher chances of surviving and propagating that may be advantageous. We evaluated this hypothesis in the semisocial spider Anelosimus studiosus by creating artificial colonies of varying sex ratios and sizes and observing colony performance at sites with high versus low group extinction rates. We also tested whether colony extinction rates and sex ratios were correlated across 25 collection sites, spanning 10° latitude. We found that colonies with female-biased sex ratios produced more egg cases and were more likely to survive the duration of a field season, suggesting that female-biased sex ratios confer both survival and reproductive advantages on colonies. The effect of sex ratio on colony survival and reproductive output was strongest for small colonies in high extinction areas. Moreover, we found that female-biased sex ratios correlated with greater extinction rates across 25 sites, indicating that female-biased sex ratios may have evolved at some sites in response to high extinction rates. These findings suggest that selection favoring groups with female-biased sex ratios may operate in A. studiosus, shedding light on some of the factors that may drive the evolution of biased sex ratios.

Cuticular bacteria appear detrimental to social spiders in mixed but not monoculture exposure

Much of an animal’s health status, life history, and behavior are dictated by interactions with its endogenous and exogenous bacterial communities. Unfortunately, interactions between hosts and members of their resident bacterial community are often ignored in animal behavior and behavioral ecology. Here, we aim to identify the nature of host–microbe interactions in a nonmodel organism, the African social spider Stegodyphus dumicola. We collected and identified bacteria from the cuticles of spiders in situ and then exposed spiders to bacterial monocultures cultures via topical application or injection. We also topically inoculated spiders with a concomitant “cocktail” of bacteria and measured the behavior of spiders daily for 24 days after inoculation. Lastly, we collected and identified bacteria from the cuticles of prey items in the capture webs of spiders, and then fed spiders domestic crickets which had been injected with these bacteria. We also injected 1 species of prey-borne bacteria into the hemolymph of spiders. Only Bacillus thuringiensis caused increased mortality when injected into the hemolymph of spiders, whereas no bacterial monocultures caused increased mortality when applied topically, relative to control solutions. However, a bacterial cocktail of cuticular bacteria caused weight loss and mortality when applied topically, yet did not detectibly alter spider behavior. Consuming prey injected with prey-borne bacteria was associated with an elongated lifespan in spiders. Thus, indirect evidence from multiple experiments suggests that the effects of these bacteria on spider survivorship appear contingent on their mode of colonization and whether they are applied in monoculture or within a mixed cocktail. We urge that follow-up studies should test these host–microbe interactions across different social contexts to determine the role that microbes play in colony performance.

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.

Social spiders of the genus Anelosimus occur in wetter, more productive environments than non-social species

Latitude, rainfall, and productivity have been shown to influence social organisation and level of sociality in arthropods on large geographic scales. Social spiders form permanent group-living societies where they cooperate in brood care, web maintenance, and foraging. Sociality has evolved independently in a number of unrelated spider genera and may reflect convergent evolutionary responses to common environmental drivers. The genus Anelosimus contains a third of approximately 25 described permanently social spider species, eight to nine species that all occur in the Americas. To test for environmental correlates of sociality in Anelosimus across the Americas, we used logistic regression to detect effects of annual rainfall, productivity, and precipitation seasonality on the relative likelihood of occurrence of social and non-social Anelosimus spiders. Our analyses show that social species tend to occur at higher annual rainfall and productivity than non-social species, supporting the hypothesised effects of these environmental variables on the geographical distribution of social species. We did not find support for the hypothesis that permanently social species occur in areas with low precipitation seasonality. High annual precipitation and, to less extent, high productivity favour the occurrence of permanently group-living Anelosimus spiders relative to subsocial and solitary species. These results are partially consistent with previous findings for the Old World spider genus Stegodyphus, where a link between high habitat productivity and sociality was also found. Unlike Anelosimus, however, Stegodyphus typically occur in dry habitats negating a general importance of high precipitation for sociality. Sociality in spiders thus seems to be strongly linked to productivity, probably reflecting the need for relatively high availability of large prey to sustain social colonies.

Maternal care and subsocial behaviour in spiders

While most spiders are solitary and opportunistically cannibalistic, a variety of social organisations has evolved in a minority of spider species. One form of social organisation is subsociality, in which siblings remain together with their parent for some period of time but disperse prior to independent reproduction. We review the literature on subsocial and maternal behaviour in spiders to highlight areas in which subsocial spiders have informed our understanding of social evolution and to identify promising areas of future research. We show that subsocial behaviour has evolved independently at least 18 times in spiders, across a wide phylogenetic distribution. Subsocial behaviour is diverse in terms of the form of care provided by the mother, the duration of care and sibling association, the degree of interaction and cooperation among siblings, and the use of vibratory and chemical communication. Subsocial spiders are useful model organisms to study various topics in ecology, such as kin recognition and the evolution of cheating and its impact on societies. Further, why social behaviour evolved in some lineages and not others is currently a topic of debate in behavioural ecology, and we argue that spiders offer an opportunity to untangle the ecological causes of parental care, which forms the basis of many other animal societies.

Habitat productivity constrains the distribution of social spiders across continents - case study of the genus Stegodyphus

INTRODUCTION

Sociality has evolved independently multiple times across the spider phylogeny, and despite wide taxonomic and geographical breadth the social species are characterized by a common geographical constrain to tropical and subtropical areas. Here we investigate the environmental factors that drive macro-ecological patterns in social and solitary species in a genus that shows a Mediterranean-Afro-Oriental distribution (Stegodyphus). Both selected drivers (productivity and seasonality) may affect the abundance of potential prey insects, but seasonality may further directly affect survival due to mortality caused by extreme climatic events. Based on a comprehensive dataset including information about the distribution of three independently derived social species and 13 solitary congeners we tested the hypotheses that the distribution of social Stegodyphus species relative to solitary congeners is: (1) restricted to habitats of high vegetation productivity and (2) constrained to areas with a stable climate (low precipitation seasonality).

RESULTS

Using spatial logistic regression modelling and information-theoretic model selection, we show that social species occur at higher vegetation productivity than solitary, while precipitation seasonality received limited support as a predictor of social spider occurrence. An analysis of insect biomass data across the Stegodyphus distribution range confirmed that vegetation productivity is positively correlated to potential insect prey biomass.

CONCLUSIONS

Habitat productivity constrains the distribution of social spiders across continents compared to their solitary congeners, with group-living in spiders being restricted to areas with relatively high vegetation productivity and insect prey biomass. As known for other taxa, permanent sociality likely evolves in response to high predation pressure and imposes within-group competition for resources. Our results suggest that group living is contingent upon productive environmental conditions where elevated prey abundance meet the increased demand for food of social groups.

A Quantitative Index of Sociality and Its Application to Group-Living Spiders and Other Social Organisms

Species are often classified in discrete categories, such as solitary, subsocial, social and eusocial based on broad qualitative features of their social systems. Often, however, species fall between categories or species within a category may differ from one another in ways that beg for a quantitative measure of their sociality level. Here, we propose such a quantitative measure in the form of an index that is based on three fundamental features of a social system: (1) the fraction of the life cycle that individuals remain in their social group, (2) the proportion of nests in a population that contain multiple vs. solitary individuals and (3) the proportion of adult members of a group that do not reproduce, but contribute to communal activities. These are measures that should be quantifiable in most social systems, with the first two reflecting the tendencies of individuals to live in groups as a result of philopatry, grouping tendencies and intraspecific tolerance, and the third potentially reflecting the tendencies of individuals to exhibit reproductive altruism. We argue that this index can serve not only as a way of ranking species along a sociality scale, but also as a means of determining how level of sociality correlates with other aspects of the biology of a group of organisms. We illustrate the calculation of this index for the cooperative social spiders and the African mole-rats and use it to analyse how sex ratios and interfemale spacing correlate with level of sociality in spider species in the genus Anelosimus.

Iterative evolution of increased behavioral variation characterizes the transition to sociality in spiders and proves advantageous

The evolution of group living is regarded as a major evolutionary transition and is commonly met with correlated shifts in ancillary characters. We tested for associations between social tendency and a myriad of abiotic variables (e.g., temperature and precipitation) and behavioral traits (e.g., boldness, activity level, and aggression) in a clade of spiders that exhibit highly variable social structures (genus Anelosimus). We found that, relative to their subsocial relatives, social species tended to exhibit reduced aggressiveness toward prey, increased fearfulness toward predators, and reduced activity levels, and they tended to occur in warm, wet habitats with low average wind velocities. Within-species variation in aggressiveness and boldness was also positively associated with sociality. We then assessed the functional consequences of within-species trait variation on reconstituted colonies of four test species (Anelosimus eximius, Anelosimus rupununi, Anelosimus guacamayos, and Anelosimus oritoyacu). We used colonies consisting of known ratios of docile versus aggressive individuals and group foraging success as a measure of colony performance. In all four test species, we found that groups composed of a mixture of docile and aggressive individuals outperformed monotypic groups. Mixed groups were more effective at subduing medium and large prey, and mixed groups collectively gained more mass during shared feeding events. Our results suggest that the iterative evolution of depressed aggressiveness and increased within-species behavioral variation in social spiders is advantageous and could be an adaptation to group living that is analogous to the formation of morphological castes within the social insects.

Spatio-temporal differentiation and sociality in spiders

Species that differ in their social system, and thus in traits such as group size and dispersal timing, may differ in their use of resources along spatial, temporal, or dietary dimensions. The role of sociality in creating differences in habitat use is best explored by studying closely related species or socially polymorphic species that differ in their social system, but share a common environment. Here we investigate whether five sympatric Anelosimus spider species that range from nearly solitary to highly social differ in their use of space and in their phenology as a function of their social system. By studying these species in Serra do Japi, Brazil, we find that the more social species, which form larger, longer–lived colonies, tend to live inside the forest, where sturdier, longer lasting vegetation is likely to offer better support for their nests. The less social species, which form single-family groups, in contrast, tend to occur on the forest edge where the vegetation is less robust. Within these two microhabitats, species with longer-lived colonies tend to occupy the potentially more stable positions closer to the core of the plants, while those with smaller and shorter-lived colonies build their nests towards the branch tips. The species further separate in their use of common habitat due to differences in the timing of their reproductive season. These patterns of habitat use suggest that the degree of sociality can enable otherwise similar species to differ from one another in ways that may facilitate their co-occurrence in a shared environment, a possibility that deserves further consideration.

How within-group behavioural variation and task efficiency enhance fitness in a social group

How task specialization, individual task performance and within-group behavioural variation affects fitness is a longstanding and unresolved problem in our understanding of animal societies. In the temperate social spider, Anelosimus studiosus, colony members exhibit a behavioural polymorphism; females either exhibit an aggressive 'asocial' or docile 'social' phenotype. We assessed individual prey-capture success for both phenotypes, and the role of phenotypic composition on group-level prey-capture success for three prey size classes. We then estimated the effect of group phenotypic composition on fitness in a common garden, as inferred from individual egg-case masses. On average, asocial females were more successful than social females at capturing large prey, and colony-level prey-capture success was positively associated with the frequency of the asocial phenotype. Asocial colony members were also more likely to engage in prey-capture behaviour in group-foraging situations. Interestingly, our fitness estimates indicate females of both phenotypes experience increased fitness when occupying colonies containing unlike individuals. These results imply a reciprocal fitness benefit of within-colony behavioural variation, and perhaps division of labour in a spider society.

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.

Habitat use by western black widow spiders (Latrodectus hesperus) in coastal British Columbia: evidence of facultative group living

Animal social systems come in a wide range of forms characterized by different types of group-living relationships. Species that express facultative group-living behaviours, where individuals only associate under certain conditions or at certain times, are especially interesting for studying social evolution. We investigated the social structure of the western black widow spider ( Latrodectus hesperus Chamberlin and Ivie, 1935) over 5 years in a coastal British Columbia habitat, and present the first comprehensive evidence of facultative group living in the genus Latrodectus Walckenaer, 1805. Latrodectus hesperus have a flexible social structure that varies seasonally, wherein individuals (mostly females) either form groups or live solitarily. In the fall and early winter, females spontaneously form groups of 2–8 individuals, but live alone during the oviposition season in the spring and summer. When living in groups spiders share large webs and are tolerant of each other, although they appear to forage individually. We also report on the relationships between different ecological factors and the social structure of L. hesperus, including the interaction of these spiders with two co-occurring species of introduced spiders ( Tegenaria agrestis (Walckenaer, 1802) and Tegenaria duellica Simon, 1875). Our findings suggest that L. hesperus is a good model system with which to explore the mechanisms involved in the evolution of sociality.

Frequency-dependent success of cheaters during foraging bouts might limit their spread within colonies of a socially polymorphic spider

Although several studies have demonstrated that frequency-dependent effects can promote the maintenance of cooperative behavior in microbes, experimental evidence of frequency-dependent effects in cooperative animal societies is rare. We staged mixed phenotype feeding bouts in the spider Anelosimus studiosus, which shows a within-population social polymorphism, to determine how phenotype frequency affects the foraging success of the social (cooperative) and asocial (cheater) phenotypes. Foraging performance was inferred from average change in percent mass for the respective phenotypes after staged group foraging events. We then performed a field census of multifemale colonies of A. studiosus to determine the phenotypic composition of naturally occurring colonies. Our data indicate that asocial (i.e., cheater) individuals experience negative frequency-dependent foraging success in staged foraging contests. Asocial individuals outperform social individuals when their representation is low, but lose this competitive advantage as their relative numbers increase. Naturally occurring colonies, on average, contained 58.33% social and 41.67% asocial individuals.

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.

The role of prey size and abundance in the geographical distribution of spider sociality

1. Social species in the spider genus Anelosimus predominate in lowland tropical rainforests, while congeneric subsocial species occur at higher elevations or higher latitudes. 2. We conducted a comparative study to determine whether differences in total biomass, insect size or both have been responsible for this pattern. 3. We found that larger average insect size, rather than greater overall biomass per se, is a key characteristic of lowland tropical habitats correlating with greater sociality. 4. Social species occupied environments with insects several times larger than the spiders, while subsocial species nearing dispersal occupied environments with smaller insects in either high or low overall biomass. 5. Similarly, in subsocial spider colonies, individuals lived communally at a time when they were younger and therefore smaller than the average insect landing on their webs. 6. We thus suggest that the availability of large insects may be a critical factor restricting social species to their lowland tropical habitats.

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.

Smaller colonies and more solitary living mark higher elevation populations of a social spider

1. There appears to be a pattern of decreasing sociality with increasing elevation across social spider species in the genus Anelosimus at tropical latitudes. Our data suggest that this pattern holds within a single species, Anelosimus eximius, on a smaller altitudinal gradient. 2. In comparing colony size at six different altitudes in north-eastern Ecuador, we find that the lowland A. eximius populations tend to have larger colonies and few solitary females. At higher elevations, many of the colonies are small and the proportion of solitary females is greater. 3. Contrary to expectation, we also found no difference in spider density between the upper elevation and lowland populations. This result may be partly due to the fact that upper elevation populations occur only at the forest edge (as opposed to both edge and interior) where populations at all elevations appear more robust.