Altitudinal patterns of spider sociality and the biology of a new midelevation social Anelosimus species in Ecuador

The Andes as a semi-permeable geographical barrier: Genetic connectivity between structured populations in a widespread spider

Geographical barriers like mountain ranges impede genetic exchange among populations, promoting diversification. The effectiveness of these barriers in limiting gene flow varies between lineages due to each species' dispersal modes and capacities. Our understanding of how the Andes orogeny contributes to species diversification comes from well-studied vertebrates and a few arthropods and plants, neglecting organisms unable to fly or walk long distances. Some arachnids, such as Gasteracantha cancriformis, have been hypothesized to disperse long distances via ballooning (i.e. using their silk to interact with the wind). Yet, we do not know how the environment and geography shape its genetic diversity. Therefore, we tested whether the Andes contributed to the diversification of G. cancriformis acting as an absolute or semi-permeable barrier to genetic connectivity between populations of this spider at opposite sides of the mountain range. We sampled thousands of loci across the distribution of the species and implemented population genetics, phylogenetic, and landscape genetic analyses. We identified two genetically distinct groups structured by the Central Andes, and a third less structured group in the Northern Andes that shares ancestry with the previous two. This structure is largely explained by the altitude along the Andes, which decreases in some regions, possibly facilitating cross-Andean dispersal and gene flow. Our findings support that altitude in the Andes plays a major role in structuring populations in South America, but the strength of this barrier can be overcome by organisms with long-distance dispersal modes together with altitudinal depressions.

Matriphagy in five species of the genus Anelosimus (Araneae: Theridiidae)

Parental care has evolved numerous times in many taxonomic groups of animals. Matriphagy, as an extreme example of parental care, is present in many social species, subsocial species, and even in solitary spiders. Here, we describe matriphagy in five species of Anelosimus of different levels of sociality: social (A. dubiosus), intermediate social (A. jabaquara), subsocial (A. vierae, A. baeza), and solitary (A. nigrescens). Each group contained a female and its brood, maintained under standardized laboratory conditions. All species showed matriphagy, regardless of their social level. Further studies are necessary to clarify whether matriphagy is a necessary precondition for the evolution of sociality in spiders, or if it is phylogenetically conserved in some families.

Ecological Transitions in Grouping Benefits Explain the Paradox of Environmental Quality and Sociality

Both benign and harsh environments promote the evolution of sociality. This paradox-societies occur in environments of such contrasting quality-may be explained by the different types of benefits that individuals receive from grouping: resource defense benefits that derive from group-defended critical resources versus collective action benefits that result from social cooperation among group members. Here, we investigate cooperative behavior in the burying beetle Nicrophorus nepalensis along an elevational gradient where environmental quality (climate and competition) varies with altitude. We show that climate (temperature) and competition (both intra- and interspecific) independently and synergistically influence sociality via different grouping benefits that vary along the gradient. At low elevations where interspecific competition for resources is intense, groups gain from the collective action benefit of increased interspecific competitive ability. In contrast, pairs have higher fitness at intermediate elevations where intraspecific competition for resources is greatest because resource defense is the key grouping benefit. However, groups and pairs have similar fitness at high elevations, suggesting that there is no grouping benefit in such physiologically challenging environments. Our results demonstrate that sociality is favored for different reasons under a range of environmental conditions, perhaps explaining why animal societies occur in environments of such contrasting quality.

Prey to predator body size ratio in the evolution of cooperative hunting-a social spider test case

One of the benefits of cooperative hunting may be that predators can subdue larger prey. In spiders, cooperative, social species can capture prey many times larger than an individual predator. However, we propose that cooperative prey capture does not have to be associated with larger caught prey per se, but with an increase in the ratio of prey to predator body size. This can be achieved either by catching larger prey while keeping predator body size constant, or by evolving a smaller predator body size while maintaining capture of large prey. We show that within a genus of relatively large spiders, Stegodyphus, subsocial spiders representing the ancestral state of social species are capable of catching the largest prey available in the environment. Hence, within this genus, the evolution of cooperation would not provide access to otherwise inaccessible, large prey. Instead, we show that social Stegodyphus spiders are smaller than their subsocial counterparts, while catching similar sized prey, leading to the predicted increase in prey-predator size ratio with sociality. We further show that in a genus of small spiders, Anelosimus, the level of sociality is associated with an increased size of prey caught while predator size is unaffected by sociality, leading to a similar, predicted increase in prey-predator size ratio. In summary, we find support for our proposed 'prey to predator size ratio hypothesis' and discuss how relaxed selection on large body size in the evolution of social, cooperative living may provide adaptive benefits for ancestrally relatively large predators.

Are personalities genetically determined? Inferences from subsocial spiders

Background

Recent research has revealed that polymorphic behavioral strategies shape intra-and interspecific interactions and contribute to fitness in many animal species. A better understanding of the proximate mechanisms underlying these behavioral syndromes will enhance our grasp this phenomenon. Spiders in the genus Anelosimus exhibit inter-individual behavioral variation on several axes: individuals have consistent responses to stimuli (e.g. bold vs. shy individuals) and they are subsocial (exhibiting extended maternal care and sibling cooperation) across most of their range, but they sometimes form permanent social groups in northern temperate regions. Here, we seek genetic variants associated with boldness and with social structure in a socially polymorphic population of the spider Anelosimus studiosus. We also develop preliminary genomic resources, including a genome assembly and linkage map, that support this and future genomic research on this group.

Results

Remarkably, we identify a small genomic scaffold (~ 1200 bp) that harbors seven single nucleotide polymorphisms (SNPs) associated with boldness. Moreover, heterozygotes are less common than expected based on Hardy-Weinberg equilibrium, suggesting that either assortative mating or selection against heterozygotes may be occurring in this system. We find no loci significantly associated with social organization. Our draft genome assembly allows us to localize SNPs of interest in this study and to carry out genetic comparisons with other published genomes, although it remains highly fragmented.

Conclusions

By identifying a locus associated with a well-studied animal personality trait, this study opens up avenues for future research to link behavioral studies of animal personality with genotype and fitness.

Resolving the Paradox of Environmental Quality and Sociality: The Ecological Causes and Consequences of Cooperative Breeding in Two Lineages of Birds

Cooperatively breeding animals occur in virtually every ecosystem on earth. Comparative and biogeographic studies suggest that both benign and harsh-as well as stable and fluctuating-environments can favor the evolution of cooperative breeding behavior. The fact that cooperative societies occur in environments of such contrasting quality creates a paradox of environmental quality and sociality. The dual benefits framework-which leads to the prediction that the ecological consequences of sociality (e.g., range size) vary depending on the benefits that individuals of each species receive by forming social groups-offers a potential resolution to this paradox. Here we use a case study of two avian lineages, starlings (Sturnidae) and hornbills (Bucerotidae), in which environmental unpredictability appears to have opposite effects on the evolution of cooperation to test the dual benefits framework. Consistent with previous work, harsh and unpredictable environments promote cooperative breeding behavior in starlings, which in turn leads to larger geographic ranges. However, cooperatively breeding hornbills occur in benign and stable environments, but sociality does not influence range size. Our study suggests that the paradox of environmental quality and sociality arises largely because cooperative breeding is an umbrella term encompassing social species that form groups for different reasons. We demonstrate that differentiating among the functional causes of social group formation is critical for developing a predictive framework for understanding the evolution of cooperative breeding behavior.

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.

Cooperative foraging expands dietary niche but does not offset intra-group competition for resources in social spiders

Group living animals invariably risk resource competition. Cooperation in foraging, however, may benefit individuals in groups by facilitating an increase in dietary niche. To test this, we performed a comparative study of social and solitary spider species. Three independently derived social species of Stegodyphus (Eresidae) occupy semi-arid savannas and overlap with three solitary congeners. We estimated potential prey availability in the environment and prey acquisition by spiders in their capture webs. We calculated dietary niche width (prey size) and breadth (taxonomic range) to compare resource use for these six species, and investigated the relationships between group size and average individual capture web production, prey biomass intake rate and variance in biomass intake. Cooperative foraging increased dietary niche width and breadth by foraging opportunistically, including both larger prey and a wider taxonomic range of prey in the diet. Individual capture web production decreased with increasing group size, indicating energetic benefits of cooperation, and variance in individual intake rate was reduced. However, individual biomass intake also decreased with increasing group size. While cooperative foraging did not completely offset resource competition among group members, it may contribute to sustaining larger groups by reducing costs of web production, increasing the dietary niche and reducing the variance in prey capture.

Trait overdispersion and the role of sociality in the assembly of social spider communities across the Americas

Among the factors that may lead to differences in resource use among closely related species, body size and morphology have been traditionally considered to play a role in community assembly. Here we argue that for animals that live and forage in groups, level of sociality, reflecting differences in group size and cooperative tendencies, can be an additional and powerful dimension separating species in niche space. We compare 50+ communities of the social spider genus Anelosimus across the Americas against a null model that accounts for known effects of biotic and abiotic factors on the distribution of social systems in the genus. We show that these communities are more overdispersed than expected by chance in either or both body size and level of sociality, traits we have previously shown to be associated with differences in resource utilization (prey size, microhabitat, and phenology). We further show that the contribution of sociality to differences in the size of the prey captured is two to three times greater than that of body size, suggesting that changes in group size and cooperative tendencies may be more effective than changes in body size at separating species in niche space.

The ecology of cooperative breeding behaviour

Ecology is a fundamental driving force for the evolutionary transition from solitary living to breeding cooperatively in groups. However, the fact that both benign and harsh, as well as stable and fluctuating, environments can favour the evolution of cooperative breeding behaviour constitutes a paradox of environmental quality and sociality. Here, we propose a new model - the dual benefits framework - for resolving this paradox. Our framework distinguishes between two categories of grouping benefits - resource defence benefits that derive from group-defended critical resources and collective action benefits that result from social cooperation among group members - and uses insider-outsider conflict theory to simultaneously consider the interests of current group members (insiders) and potential joiners (outsiders) in determining optimal group size. We argue that the different grouping benefits realised from resource defence and collective action profoundly affect insider-outsider conflict resolution, resulting in predictable differences in the per capita productivity, stable group size, kin structure and stability of the social group. We also suggest that different types of environmental variation (spatial vs. temporal) select for societies that form because of the different grouping benefits, thus helping to resolve the paradox of why cooperative breeding evolves in such different types of environments.

The legacy effects of keystone individuals on collective behaviour scale to how long they remain within a group

The collective behaviour of social groups is often strongly influenced by one or few individuals, termed here 'keystone individuals'. We examined whether the influence of keystone individuals on collective behaviour lingers after their departure and whether these lingering effects scale with their tenure in the group. In the social spider, Stegodyphus dumicola, colonies' boldest individuals wield a disproportionately large influence over colony behaviour. We experimentally manipulated keystones' tenure in laboratory-housed colonies and tracked their legacy effects on collective prey capture following their removal. We found that bolder keystones caused more aggressive collective foraging behaviour and catalysed greater inter-individual variation in boldness within their colonies. The longer keystones remained in a colony, the longer both of these effects lingered after their departure. Our data demonstrate that, long after their disappearance, keystones have large and lasting effects on social dynamics at both the individual and colony levels.

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.

Cooperative breeding favors maternal investment in size over number of eggs in spiders

The transition to cooperative breeding may alter maternal investment strategies depending on density of breeders, extent of reproductive skew, and allo-maternal care. Change in optimal investment from solitary to cooperative breeding can be investigated by comparing social species with nonsocial congeners. We tested two hypotheses in a mainly semelparous system: that social, cooperative breeders, compared to subsocial, solitarily breeding congeners, (1) lay fewer and larger eggs because larger offspring compete better for limited resources and become reproducers; (2) induce egg size variation within clutches as a bet-hedging strategy to ensure that some offspring become reproducers. Within two spider genera, Anelosimus and Stegodyphus, we compared species from similar habitats and augmented the results with a mini-meta-analysis of egg numbers depicted in phylogenies. We found that social species indeed laid fewer, larger eggs than subsocials, while egg size variation was low overall, giving no support for bet-hedging. We propose that the transition to cooperative breeding selects for producing few, large offspring because reproductive skew and high density of breeders and young create competition for resources and reproduction. Convergent evolution has shaped maternal strategies similarly in phylogenetically distant species and directed cooperatively breeding spiders to invest in quality rather than quantity of offspring.

Differences in group size and the extent of individual participation in group hunting may contribute to differential prey-size use among social spiders

We have previously shown that the range of prey sizes captured by co-occurring species of group-hunting social spiders correlates positively with their level of sociality. Here, we show that this pattern is probably caused by differences among species in colony size and the extent to which individuals participate in group hunting. We assess levels of participation for each species from the fraction of individuals responding to the struggling prey that partake as attackers and from the extent to which the number of attackers increases with colony size. Of two species that form equally large colonies, the one that captures on average larger prey engaged as attackers a significantly larger fraction of individuals that responded to struggling prey and also increased its number of attackers in larger colonies when presented with large prey items. Surprisingly, a third co-occurring species previously found to capture smaller insects than the other two exhibited the highest levels of participation. This species, however, typically forms small single-family colonies, thereby being limited in the size of insects it can capture. It is thus a combination of colony size and the extent of individual participation (or cooperation) that probably determines patterns of resource use in this community of co-occurring social predators.

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.

Community-wide body size differences between nocturnal and diurnal insects

Examining community-wide patterns for the most diverse animal group, insects, is fundamental to our understanding of the ecological and evolutionary factors that maintain tropical diversity. Using several sampling techniques (malaise traps, pitfall traps, visual searches, and social spider nest captures), we investigated the day-night community composition of active insects to reveal differences in body size at three elevations in eastern Ecuador. We show that insects active at night are, on average, larger than those active during the day. Even though insect size decreased with increasing elevation, the observed diel pattern was consistent across elevations, and for most insect orders. All sampling techniques consistently detected day--night differences in insect size, except for social spider captures at the two higher elevations, probably due to the reduced range of colony sizes at the higher elevations and possibly lower spider activity at night. We suggest that the observed diel patterns in insect size may be driven by a combination of factors, including increased risk imposed on large insects by diurnal visual predators, mainly insectivorous birds, and physiological responses to diel changes in abiotic conditions.

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.

Loss of genetic variability in social spiders: genetic and phylogenetic consequences of population subdivision and inbreeding

The consequences of population subdivision and inbreeding have been studied in many organisms, particularly in plants. However, most studies focus on the short-term consequences, such as inbreeding depression. To investigate the consequences of both population fragmentation and inbreeding for genetic variability in the longer term, we here make use of a natural inbreeding experiment in spiders, where sociality and accompanying population subdivision and inbreeding have evolved repeatedly. We use mitochondrial and nuclear data to infer phylogenetic relationships among 170 individuals of Anelosimus spiders representing 23 species. We then compare relative mitochondrial and nuclear genetic variability of the inbred social species and their outbred relatives. We focus on four independently derived social species and four subsocial species, including two outbred–inbred sister species pairs. We find that social species have 50% reduced mitochondrial sequence divergence. As inbreeding is not expected to reduce genetic variability in the maternally inherited mitochondrial genome, this suggests the loss of variation due to strong population subdivision, founder effects, small effective population sizes (colonies as individuals) and lineage turnover. Social species have < 10% of the nuclear genetic variability of the outbred species, also suggesting the loss of genetic variability through founder effects and/or inbreeding. Inbred sociality hence may result in reduction in variability through various processes. Sociality in most Anelosimus species probably arose relatively recently (0.1–2 mya), with even the oldest social lineages having failed to diversify. This is consistent with the hypothesis that inbred spider sociality represents an evolutionary dead end. Heterosis underlies a species potential to respond to environmental change and/or disease. Inbreeding and loss of genetic variability may thus limit diversification in social Anelosimus lineages and similarly pose a threat to many wild populations subject to habitat fragmentation or reduced population sizes.

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.

Direct fitness correlates and thermal consequences of facultative aggregation in a desert lizard

Social aggregation is a common behavioral phenomenon thought to evolve through adaptive benefits to group living. Comparing fitness differences between aggregated and solitary individuals in nature--necessary to infer an evolutionary benefit to living in groups--has proven difficult because communally-living species tend to be obligately social and behaviorally complex. However, these differences and the mechanisms driving them are critical to understanding how solitary individuals transition to group living, as well as how and why nascent social systems change over time. Here we demonstrate that facultative aggregation in a reptile (the Desert Night Lizard, Xantusia vigilis) confers direct reproductive success and survival advantages and that thermal benefits of winter huddling disproportionately benefit small juveniles, which can favor delayed dispersal of offspring and the formation of kin groups. Using climate projection models, however, we estimate that future aggregation in night lizards could decline more than 50% due to warmer temperatures. Our results support the theory that transitions to group living arise from direct benefits to social individuals and offer a clear mechanism for the origin of kin groups through juvenile philopatry. The temperature dependence of aggregation in this and other taxa suggests that environmental variation may be a powerful but underappreciated force in the rapid transition between social and solitary behavior.

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.

Systematics of new subsocial and solitary Australasian Anelosimus species (Araneae:Theridiidae)

Species of the cobweb spider genus Anelosimus range from solitary to subsocial to social, and sociality has evolved repeatedly within the genus. Thus, this genus allows studies of the traits that play a role in social evolution. However, taxonomic knowledge of Anelosimus is geographically narrow and nearly all sociobiological studies have been done in the Americas. Only one behaviourally unknown species has been described from all of Australasia. Here, I describe seven new Anelosimus from Papua New Guinea (Anelosimus potmosbi, sp. nov., Anelosimus pomio, sp. nov., Anelosimus eidur, sp. nov. and Anelosimus luckyi, sp. nov.), Bali (Anelosimus bali, sp. nov.), Australia (Anelosimus pratchetti, sp. nov.) and an unknown locality (Anelosimus terraincognita, sp. nov.), ranging from solitary to subsocial. A phylogenetic analysis supports the inclusion of these species in Anelosimus, and suggests that solitary Papuan species represent a second reversal from subsocial behaviour. Both solitary species inhabit the beachfront, a habitat that appears not to be conducive to social behaviour in spiders. Subsocial species, as in other parts of the world, are found in montane tropical forests of Papua New Guinea, and at relatively high latitudes in Australia. Thus, a global ecological pattern of sociality in Anelosimus is emerging as taxonomic, phylogenetic and ethological knowledge extends beyond the Americas.

Within-group behavioral variation promotes biased task performance and the emergence of a defensive caste in a social spider

The social spider Anelosimus studiosus exhibits a behavioral polymorphism where colony members express either a passive, tolerant behavioral tendency (social) or an aggressive, intolerant behavioral tendency (asocial). Here we test whether asocial individuals act as colony defenders by deflecting the suite of foreign (i.e., heterospecific) spider species that commonly exploit multi-female colonies. We (1) determined whether the phenotypic composition of colonies is associated with foreign spider abundance, (2) tested whether heterospecific spider abundance and diversity affect colony survival in the field, and (3) performed staged encounters between groups of A. studiosus and their colony-level predator Agelenopsis emertoni (A. emertoni)to determine whether asocial females exhibit more defensive behavior. We found that larger colonies harbor more foreign spiders, and the number of asocial colony members was negatively associated with foreign spider abundance. Additionally, colony persistence was negatively associated with the abundance and diversity of foreign spiders within colonies. In encounters with a colony-level predator, asocial females were more likely to exhibit escalatory behavior, and this might explain the negative association between the frequency of asocial females and the presence of foreign spider associates. Together, our results indicate that foreign spiders are detrimental to colony survival, and that asocial females play a defensive role in multi-female colonies.

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.

Cooperative capture of large prey solves scaling challenge faced by spider societies

A decrease in the surface area per unit volume is a well known constraint setting limits to the size of organisms at both the cellular and whole-organismal levels. Similar constraints may apply to social groups as they grow in size. The communal three-dimensional webs that social spiders build function ecologically as single units that intercept prey through their surface and should thus be subject to this constraint. Accordingly, we show that web prey capture area per spider, and thus number of insects captured per capita, decreases with colony size in a neotropical social spider. Prey biomass intake per capita, however, peaks at intermediate colony sizes because the spiders forage cooperatively and larger colonies capture increasingly large insects. A peaked prey biomass intake function would explain not only why these spiders live in groups and cooperate but also why they disperse only at large colony sizes, thus addressing both sociality and colony size range in this social spider. These findings may also explain the conspicuous absence of social spiders from higher latitudes and higher elevations, areas that we have previously shown to harbor considerably fewer insects of the largest size classes than the lowland tropical rainforests where social spiders thrive. Our findings thus illustrate the relevance of scaling laws to the size and functioning of levels of organization above the individual.