Relatedness facilitates cooperation in the subsocial spider, Stegodyphus tentoriicola

Incorporating neurological and behavioral mechanisms of sociality into predator-prey models

Consumer-resource population models drive progress in predicting and understanding predation. However, they are often built by averaging the foraging outcomes of individuals to estimate per capita functional responses (functions that describe predation rate). Reliance on per-capita functional responses rests on the assumption that that individuals forage independently without affecting each other. Undermining this assumption, extensive behavioral neuroscience research has made clear that facilitative and antagonistic interactions among conspecifics frequently alter foraging through interference competition and persistent neurophysiological changes. For example, repeated social defeats dysregulates rodent hypothalamic signaling, modulating appetite. In behavioral ecology, similar mechanisms are studied under the concept of dominance hierarchies. Neurological and behavioral changes in response to conspecifics undoubtedly play some sort of role in the foraging of populations, but modern predator-prey theory does not explicitly include them. Here we describe how some modern approaches to population modeling might account for this. Further, we propose that spatial predator-prey models can be modified to describe plastic changes in foraging behavior driven by intraspecific interaction, namely individuals switching between patches or plastic strategies to avoid competition. Extensive neurological and behavioral ecology research suggests that interactions among conspecifics help shape populations' functional responses. Modeling interdependent functional responses woven together by behavioral and neurological mechanisms may thus be indispensable in predicting the outcome of consumer-resource interactions across systems.

The role of inter-individual intolerance in group cohesion and the transition to sociality in spiders

Conspecific tolerance is key for maintaining group cohesion in animals. Understanding shifts from conspecific tolerance to intolerance is therefore important for understanding transitions to sociality. Subsocial species disperse to a solitary lifestyle after a gregarious juvenile phase and display conspecific intolerance as adults as a mechanism to maintain a solitary living. The development of intolerance towards group members is hypothesized to play a role in dispersal decisions in subsocial species. One hypothesis posits that dispersal is triggered by factors such as food competition with the subsequent development of conspecific intolerance, rather than conspecific intolerance developing prior to and potentially driving dispersal. Consistent with this hypothesis, we show that intolerance (inferred by inter-individual distance) developed post-dispersal in the subsocial spider Stegodyphus lineatus. The development of conspecific intolerance was delayed when maintaining spiders in groups showing plasticity in this trait, which is advantageous when trade-offs are not fixed over time. However, major evolutionary transitions, such as the transition to sociality, can permanently modify trade-offs and cause derived adaptations by the evolution of new or modified traits or evolutionary loss of traits that become redundant. Sociality in spiders has evolved repeatedly from subsocial ancestors, and social life in family groups combined with a lack of interaction with competing groups suggests relaxed selection for the development of conspecific intolerance. In the social Stegodyphus sarasinorum we found no evidence for the development of conspecific intolerance, consistent with the loss of this trait. Instead, we found evidence for conspecific attraction, which is likely to govern group cohesion.

Birds of a Feather Flock Together: Extent of Long-Term Consistency of Colony-Size Choice in Cliff Swallows

Explaining why animal groups vary in size is a fundamental problem in behavioral ecology. One hypothesis is that life-history differences among individuals lead to sorting of phenotypes into groups of different sizes where each individual does best. This hypothesis predicts that individuals should be relatively consistent in their use of particular group sizes across time. Little is known about whether animals’ choice of group size is repeatable across their lives, especially in long-lived species. We studied consistency in choice of breeding-colony size in colonially nesting cliff swallows (Petrochelidon pyrrhonota) in western Nebraska, United States, over a 32-year period, following 6,296 birds for at least four breeding seasons. Formal repeatability of size choice for the population was about 0.41. About 45% of individuals were relatively consistent in choice of colony size, while about 40% varied widely in the colony size they occupied. Birds using the smaller and larger colonies appeared more consistent in size use than birds occupying more intermediate sized colonies. Consistency in colony size was also influenced by whether a bird used the same physical colony site each year and whether the site had been fumigated to remove ectoparasites. The difference between the final and initial colony sizes for an individual, a measure of the net change in its colony size over its life, did not significantly depart from 0 for the dataset as a whole. However, different year-cohorts did show significant net change in colony size, both positive and negative, that may have reflected fluctuating selection on colony size among years based on climatic conditions. The results support phenotypic sorting as an explanation for group size variation, although cliff swallows also likely use past experience at a given site and the extent of ectoparasitism to select breeding colonies.

Metabolite Profiling of the Social Spider Stegodyphus dumicola Along a Climate Gradient

Animals experience climatic variation in their natural habitats, which may lead to variation in phenotypic responses among populations through local adaptation or phenotypic plasticity. In ectotherm arthropods, the expression of thermoprotective metabolites such as free amino acids, sugars, and polyols, in response to temperature stress, may facilitate temperature tolerance by regulating cellular homeostasis. If populations experience differences in temperatures, individuals may exhibit population-specific metabolite profiles through differential accumulation of metabolites that facilitate thermal tolerance. Such thermoprotective metabolites may originate from the animals themselves or from their associated microbiome, and hence microbial symbionts may contribute to shape the thermal niche of their host. The social spider Stegodyphus dumicola has extremely low genetic diversity, yet it occupies a relatively broad temperature range occurring across multiple climate zones in Southern Africa. We investigated whether the metabolome, including thermoprotective metabolites, differs between populations, and whether population genetic structure or the spider microbiome may explain potential differences. To address these questions, we assessed metabolite profiles, phylogenetic relationships, and microbiomes in three natural populations along a temperature gradient. The spider microbiomes in three genetically distinct populations of S. dumicola showed no significant population-specific pattern, and none of its dominating genera (Borrelia, Diplorickettsia, and Mycoplasma) are known to facilitate thermal tolerance in hosts. These results do not support a role of the microbiome in shaping the thermal niche of S. dumicola. Metabolite profiles of the three spider populations were significantly different. The variation was driven by multiple metabolites that can be linked to temperature stress (e.g., lactate, succinate, or xanthine) and thermal tolerance (e.g., polyols, trehalose, or glycerol): these metabolites had higher relative abundance in spiders from the hottest geographic region. These distinct metabolite profiles are consistent with a potential role of the metabolome in temperature response.

Dispersal Risks and Decisions Shape How Non-kin Groups Form in a Tropical Silk-Sharing Webspinner (Insecta: Embioptera)

Relying on silk can promote sharing, especially when its presence means life and its absence, quick death. In the case of Embioptera, they construct silken tubes and coverings exposed on tree bark in humid and warm environments or in leaf litter and underground in dry habitats. These coverings protect occupants from rain and natural enemies. Of note, adult females are neotenous, wingless and must walk to disperse. Evidence is pulled together from two sources to explore mechanisms that promote the establishment of non-kin groups that typify the neotropical Antipaluria urichi (Clothodidae): (1) a review of relevant information from 40 years of research to identify potential drivers of the facultative colonial system and (2) experimental and observational data exploring how dispersal contributes to group formation. To determine risks of dispersal and decisions of where to settle, adult females were released into the field and their ability to survive in the face of likely predation was monitored. Additional captured dispersers were released onto bark containing silk galleries; their decision to join the silk or to settle was noted. An experiment tested which attributes of trees attract a disperser: vertical or horizontal boles in one test and small, medium, or large boles in another. While walking, experimentally released adult female dispersers experienced a risk of being killed of approximately 25%. Dispersers orient to large diameter trees and join silk of others if encountered. These results align with observations of natural colonies in that adults and late-stage nymphs join existing colonies of non-kin. Experiments further demonstrated that dispersing females orient to vertical and larger diameter tree-like objects, a behavior that matched the distribution of field colonies. The ultimate reason for the observed dispersion pattern is probably because large trees support more expansive epiphytic algae and lichens (the food for this species), although the impact of food resources on dispersion has not been tested. Finally, further research questions and other webspinner species (including parthenogenetic ones) that warrant a closer look are described. Given that this group of primitively social insects, with approximately 1,000 species known, has remained virtually unstudied, one hope is that this report can encourage more exploration.

Benefits and Costs of Mixed-Species Aggregations in Harvestmen (Arachnida: Opiliones)

Many animals form aggregations with individuals of the same species (single-species aggregations, SSA). Less frequently, individuals may also aggregate with individuals of other species (mixed-species aggregations, MSA). Although the benefits and costs of SSA have been intensively studied, the same is not true for MSA. Here, we first review the cases of MSA in harvestmen, an arachnid order in which the records of MSA are more frequent than other arthropod orders. We then propose several benefits and costs of MSA in harvestmen, and contrast them with those of SSA. Second, using field-gathered data we describe gregariousness in seven species of Prionostemma harvestmen from Costa Rica. These species form MSA, but individuals are also found solitarily or in SSA. We tested one possible benefit and one possible cost of gregariousness in Prionostemma harvestmen. Regarding the benefit, we hypothesized that individuals missing legs would be more exposed to predation than eight-legged individuals and thus they should be found preferentially in aggregations, where they would be more protected from predators. Our data, however, do not support this hypothesis. Regarding the cost, we hypothesized that gregariousness increases the chances of parasitism. We found no support for this hypothesis either because both mite prevalence and infestation intensity did not differ between solitary or aggregated individuals. Additionally, the type of aggregation (SSA or MSA) was not associated with the benefit or the cost we explored. This lack of effect may be explained by the fluid membership of the aggregations, as we found high turnover over time in the number of individuals and species composition of the aggregations. In conclusion, we hope our review and empirical data stimulate further studies on MSA, which remains one of the most elusive forms of group living in animals.

Family dynamics reveal that female house mice preferentially breed in their maternal community

Whether females breed in their natal group is an important factor in the evolution of extended families in animal sociality. Breeding in natal groups comes with costs and benefits, depending on group size and presence of older relatives, including mothers. Studying the consequences of breeding in the natal versus another group provides insight into the decisions and trade-offs governing the formation and structure of family groups. We investigated the family dynamics of a population of free-ranging commensal house mice. Using dynamic community detection on long-term datasets, we determined which females first bred in their natal group. We then looked at how this influenced breeding success. We found most females (77%) exhibited strong philopatry, breeding in their natal groups. Breeding elsewhere was only somewhat predictable at very large and very small group sizes. Despite their philopatric preference, breeding elsewhere made no difference in how quickly and successfully a female bred. However, presence of their mother did lead females to breed sooner when born during high breeding activity, when competition over reproduction is high. Based on these results, potential loss of reproductive success from leaving the natal group does not seem to be the main driver of philopatry in female house mice. The effect of the presence of mothers suggests that benefiting from established social connections promotes breeding in the natal group. Mothers providing benefits also implies a lack of conflict between generations, which will be important for the development of stable social groups.

Long-term overlap of social and genetic structure in free-ranging house mice reveals dynamic seasonal and group size effects

Associating with relatives in social groups can bring benefits such as reduced risk of aggression and increased likelihood of cooperation. Competition among relatives over limited resources, on the other hand, can induce individuals to alter their patterns of association. Population density might further affect the costs and benefits of associating with relatives by altering resource competition or by changing the structure of social groups; preventing easy association with relatives. Consequently, the overlap between genetic and social structure is expected to decrease with increasing population size, as well as during times of increased breeding activity. Here, we use multi-layer network techniques to quantify the similarity between long-term, high resolution genetic, and behavioral data from a large population of free-ranging house mice (Mus musculus domesticus), studied over 10 years. We infer how the benefit of associating with genetically similar individuals might fluctuate in relation to breeding behavior and environmental conditions. We found a clear seasonal effect, with decreased overlap between social and genetic structure during summer months, characterized by high temperatures and high breeding activity. Though the effect of overall population size was relatively weak, we found a clear decrease in the overlap between genetic similarity and social associations within larger groups. As well as longer-term within-group changes, these results reveal population-wide short-term shifts in how individuals associate with relatives. Our study suggests that resource competition modifies the trade-off between the costs and benefits of interacting with relatives.

Dual pathways in social evolution: Population genetic structure of group-living and solitary species of kleptoparasitic spiders (Argyrodinae: Theridiidae)

Group-living behavior is taxonomically widespread but rare in spiders. The conventional view is that the main pathways to group-living in spiders are either sub-social, where extended maternal care leads to prolonged sibling association; or communal living, where individuals aggregate to exploit a common resource. Female egg-sac guarding behavior occurs throughout kleptoparasitic spiders in the subfamily Argyrodinae (Theridiidae), while individuals in group-living species cohabit in the resource rich webs of their host spiders. These attributes fit both sub-social and communal routes to group-living, which offers new insights to study the early stages of social evolution. We investigated whether members of kleptoparasitic groups in natural populations comprise related individuals by comparing the population structure of two group-living species, Argyrodes miniaceus and A. cf. fissifrons, and two solitary species, A. fasciatus and Neospintharus trigonum. We found that: (1) genetic-spatial autocorrelation in group-living species was highest among spiders sharing the same host web and declined steeply with increasing distance, but no significant autocorrelation at any scale for solitary species; (2) there was high relatedness among group members in two cases of group-living species, which indicated relatedness was not an adhesive agent in most of the groups, but no high relatedness in solitary species; and (3) the host web boundary was not the sole predictor of genetic structures in group-living species. These results suggest that population genetic structure in the group-living species is caused by limited dispersal of group members that is favored by ecological conditions, including the nature and size of resources. In contrast, the absence of genetic structuring in populations of solitary species indicates a high level of dispersal with individual interactions unlikely to have fitness benefits.

Participation in cooperative prey capture and the benefits gained from it are associated with individual personality

In animal societies, behavioral idiosyncrasies of the individuals often guide which tasks they should perform. Such personality-specific task participation can increase individual task efficiency, thereby improving group performance. While several recent studies have documented group-level benefits of within-group behavioral (i.e., personality) diversity, how these benefits are realized at the individual level is unclear. Here we probe the individual-level benefits of personality-driven task participation in the social spider Stegodyphus dumicola. In S. dumicola, the presence of at least one highly bold individual catalyzes foraging behavior in shy colony members, and all group constituents heavily compete for prey. We assessed boldness by examining how quickly spiders resumed normal movement after a simulated predator attack. We test here whether (1) participants in collective foraging gain more mass from prey items and (2) whether bold individuals are less resistant to starvation than shy spiders, which would motivate the bold individuals to forage more. Next, we assembled colonies of shy spiders with and without a bold individual, added one prey item, and then tracked the mass gain of each individual spider after this single feeding event. We found that spiders that participated in prey capture (whether bold or shy) gained more mass than nonparticipators, and colonies containing a single bold spider gained more total mass than purely shy colonies. We also found that bold spiders participated in more collective foraging events and were more susceptible to starvation than shy spiders, suggesting that the aggressive foraging of bold individuals may represent a strategy to offset starvation risk. These findings add to the body of evidence that animal personality can shape social organization, individual performance, and group success.

Producers and scroungers: feeding-type composition changes with group size in a socially foraging spider

In groups of socially foraging animals, feeding behaviour may change with group size in response to varying cost-benefit trade-offs. Numerous studies have described group-size effects on group-average feeding behaviour, particularly emphasizing an increase in scrounging incidence for larger groups, where individuals (scroungers) feed from the food sources others (producers) discovered. However, individual variation in feeding behaviour remains unconsidered in the vast majority of these studies even though theoretical models predict individuals to specialize in feeding tactic and anticipate higher scrounger-type frequencies in larger groups. We combined group-level and individual-level analyses of group-size effects on social foraging in the subsocial spider Australomisidia ergandros Lending novel experimental support to model predictions, we found that individuals specialize in feeding tactic and that higher scrounging and lower producing incidence in larger groups were mediated through shifts in the ratio of feeding types. Further, feeding-type specialization was not explained by innate individual differences in hunting ability as all feeding types were equally efficient in prey capture when foraging alone. Context adaptivity of feeding behaviour might allow this subsocial species to succeed under varying socioecological conditions.

Kinship reinforces cooperative predator inspection in a cichlid fish

Kin selection theory predicts that cooperation is facilitated between genetic relatives, as by cooperating with kin an individual might increase its inclusive fitness. Although numerous theoretical papers support Hamilton's inclusive fitness theory, experimental evidence is still underrepresented, in particular in noncooperative breeders. Cooperative predator inspection is one of the most intriguing antipredator strategies, as it implies high costs on inspectors. During an inspection event, one or more individuals leave the safety of a group and approach a potential predator to gather information about the current predation risk. We investigated the effect of genetic relatedness on cooperative predator inspection in juveniles of the cichlid fish Pelvicachromis taeniatus, a species in which juveniles live in shoals under natural conditions. We show that relatedness significantly influenced predator inspection behaviour with kin dyads being significantly more cooperative. Thus, our results indicate a higher disposition for cooperative antipredator behaviour among kin as predicted by kin selection theory.

Evidence for loss of nepotism in the evolution of permanent sociality

Kin selected benefits of cooperation result in pronounced kin discrimination and nepotism in many social species and favour the evolution of sociality. However, low variability in relatedness among group members, infrequent competitive interactions with non-relatives, and direct benefits of cooperation may relax selection for nepotism. We tested this prediction in a permanently social spider, Stegodyphus dumicola that appears to fulfil these conditions. Sociality is a derived trait, and kin discrimination exists in sub-social closely related congeners and is likely a selective force in the sub-social route to permanent sociality in spiders. We examined whether social spiders show nepotism in cooperative feeding when genetic relatedness among group members was experimentally varied. We found no effect of relatedness on feeding efficiency, growth rate or participation in feeding events. Previous studies on sub-social species showed benefits of communal feeding with kin, indicating nepotistic cooperation. The lack of evidence for nepotism in the social species suggests that kin discrimination has been lost or is irrelevant in communal feeding. Our results are consistent with the hypothesis that the role of nepotism is diminished when cooperation evolves in certain genetic and ecological contexts, e.g. when intra-group genetic relatedness is homogeneous and encounters with competitors are rare.

Re-description of Xysticus bimaculatus L. Koch, 1867 (Araneae, Thomisidae) and characterization of its subsocial lifestyle

Spiders have become an important model to study the evolution of sociality, but a lack of their detailed natural history and taxonomy hinders broader comparative studies. Group-living crab spiders (Thomisidae) provide an excellent contrast to other social spiders since they lack a communal capture web, which was thought to be a critical factor in the evolution of sociality. Only three non-webbuilding crab-spider species are known to be subsocial or social, all of which belong to the genus Diaea Thorell, 1869. The aim of this study is to describe the social lifestyle of Xysticus bimaculatus L. Koch, 1867 for the first time. Furthermore, we present a detailed re-description of this species and discuss its taxonomic implications. Like other subsocial crab spiders, X. bimaculatus builds nests from tree leaves. Nests contain up to 38 spiders and sometimes several adult females, indicating the species may be at a transitory stage between subsociality and permanent sociality.

Evidence of social niche construction: persistent and repeated social interactions generate stronger personalities in a social spider

While there are now a number of theoretical models predicting how consistent individual differences in behaviour may be generated and maintained, so far, there are few empirical tests. The social niche specialization hypothesis predicts that repeated social interactions among individuals may generate among-individual differences and reinforce within-individual consistency through positive feedback mechanisms. Here, we test this hypothesis using groups of the social spider Stegodyphus mimosarum that differ in their level of familiarity. In support of the social niche specialization hypothesis, individuals in groups of spiders that were more familiar with each other showed greater repeatable among-individual variation in behaviour. Additionally, individuals that were more familiar with each other exhibited lower within-individual variation in behaviour, providing one of the first examples of how the social environment can influence behavioural consistency. Our study demonstrates the potential for the social environment to generate and reinforce consistent individual differences in behaviour and provides a potentially general mechanism to explain this type of behavioural variation in animals with stable social groups.

Cuticular antifungals in spiders: density- and condition dependence

Animals living in groups face a high risk of disease contagion. In many arthropod species, cuticular antimicrobials constitute the first protective barrier that prevents infections. Here we report that group-living spiders produce cuticular chemicals which inhibit fungal growth. Given that cuticular antifungals may be costly to produce, we explored whether they can be modulated according to the risk of contagion (i.e. under high densities). For this purpose, we quantified cuticular antifungal activity in the subsocial crab spider Diaea ergandros in both natural nests and experimentally manipulated nests of varying density. We quantified the body-condition of spiders to test whether antifungal activity is condition dependent, as well as the effect of spider density on body-condition. We predicted cuticular antifungal activity to increase and body-condition to decrease with high spider densities, and that antifungal activity would be inversely related to body-condition. Contrary to our predictions, antifungal activity was neither density- nor condition-dependent. However, body-condition decreased with density in natural nests, but increased in experimental nests. We suggest that pathogen pressure is so important in nature that it maintains high levels of cuticular antifungal activity in spiders, impacting negatively on individual energetic condition. Future studies should identify the chemical structure of the isolated antifungal compounds in order to understand the physiological basis of a trade-off between disease prevention and energetic condition caused by group living, and its consequences in the evolution of sociality in spiders.

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.

Could relatedness help explain why individuals lead in bottlenose dolphin groups?

In many species, particular individuals consistently lead group travel. While benefits to followers often are relatively obvious, including access to resources, benefits to leaders are often less obvious. This is especially true for species that feed on patchy mobile resources where all group members may locate prey simultaneously and food intake likely decreases with increasing group size. Leaders in highly complex habitats, however, could provide access to foraging resources for less informed relatives, thereby gaining indirect benefits by helping kin. Recently, leadership has been documented in a population of bottlenose dolphins (Tursiops truncatus) where direct benefits to leaders appear unlikely. To test whether leaders could benefit indirectly we examined relatedness between leader-follower pairs and compared these levels to pairs who associated but did not have leader-follower relationship (neither ever led the other). We found the average relatedness value for leader-follower pairs was greater than expected based on chance. The same was not found when examining non leader-follower pairs. Additionally, relatedness for leader-follower pairs was positively correlated with association index values, but no correlation was found for this measure in non leader-follower pairs. Interestingly, haplotypes were not frequently shared between leader-follower pairs (25%). Together, these results suggest that bottlenose dolphin leaders have the opportunity to gain indirect benefits by leading relatives. These findings provide a potential mechanism for the maintenance of leadership in a highly dynamic fission-fusion population with few obvious direct benefits to leaders.

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.

Effects of within-colony competition on body size asymmetries and reproductive skew in a social spider

Reproductive partitioning is a key component of social organization in groups of cooperative organisms. In colonies of permanently social spiders of the genus Stegodyphus less than half of the females reproduce, while all females, including nonreproducers, perform suicidal allo-maternal care. Some theoretical models suggest that reproductive skew is a result of contest competition within colonies, leading to size hierarchies where only the largest females become reproducers. We investigated the effect of competition on within-group body size variation over six months in S. dumicola, by manipulating food level and colony size. We found no evidence that competition leads to increased size asymmetry within colonies, suggesting that contest competition may not be the proximate explanation for reproductive skew. Within-colony body size variation was high already in the juvenile stage, and did not increase over the course of the experiment, suggesting that body size variation is shaped at an early stage. This might facilitate task specialization within colonies and ensure colony-level reproductive output by early allocation of reproductive roles. We suggest that reproductive skew in social spiders may be an adaptation to sociality selected through inclusive fitness benefits of allo-maternal care as well as colony-level benefits maximizing colony survival and production.

Task specialization in two social spiders, Stegodyphus sarasinorum (Eresidae) and Anelosimus eximius (Theridiidae)

Understanding the social organization of group-living organisms is crucial for the comprehension of the underlying selective mechanisms involved in the evolution of cooperation. Division of labour and caste formation is restricted to eusocial organisms, but behavioural asymmetries and reproductive skew is common in other group-living animals. Permanently, social spiders form highly related groups with reproductive skew and communal brood care. We investigated task differentiation in nonreproductive tasks in two permanently and independently derived social spider species asking the following questions: Do individual spiders vary consistently in their propensity to engage in prey attack? Are individual spiders' propensities to engage in web maintenance behaviour influenced by their previous engagement in prey attack? Interestingly, we found that both species showed some degree of task specialization, but in distinctly different ways: Stegodyphus sarasinorum showed behavioural asymmetries at the individual level, that is, individual spiders that had attacked prey once were more likely to attack prey again, independent of their body size or hunger level. In contrast, Anelosimus eximius showed no individual specialization, but showed differentiation according to instar, where adult and subadult females were more likely to engage in prey attack than were juveniles. We found no evidence for division of labour between prey attack and web maintenance. Different solutions to achieve task differentiation in prey attack for the two species studied here suggest an adaptive value of task specialization in foraging for social spiders.

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.

Strength of social tie predicts cooperative investment in a human social network

Social networks--diagrams which reflect the social structure of animal groups--are increasingly viewed as useful tools in behavioural ecology and evolutionary biology. Network structure may be especially relevant to the study of cooperation, because the action of mechanisms which affect the cost:benefit ratio of cooperating (e.g. reciprocity, punishment, image scoring) is likely to be mediated by the relative position of actor and recipient in the network. Social proximity could thus affect cooperation in a similar manner to biological relatedness. To test this hypothesis, we recruited members of a real-world social group and used a questionnaire to reveal their network. Participants were asked to endure physical discomfort in order to earn money for themselves and other group members, allowing us to explore relationships between willingness to suffer a cost on another's behalf and the relative social position of donor and recipient. Cost endured was positively correlated with the strength of the social tie between donor and recipient. Further, donors suffered greater costs when a relationship was reciprocated. Interestingly, participants regularly suffered greater discomfort for very close peers than for themselves. Our results provide new insight into the effect of social structure on the direct benefits of cooperation.