Cooperation and grouping are regularly studied as separate traits. The evolution of sociality however requires both that individuals get together in groups and that they cooperate within them. ...Because the level of cooperation can influence selection for group size, and vice versa, it is worth studying how these traits coevolve. Using a generally applicable two-trait optimization approach, we provide analytical solutions for three specific models. These solutions describe how cooperative associations of non-relatives evolve, and predict how large and how cooperative they will be. The analytical solutions help understand how changes in parameter values, such as the group carrying capacity and the costs of cooperation, affect group size and the level of cooperation in equilibrium. Although the analytical model makes a few simplifying assumptions—populations are assumed to be monomorphic for grouping as well as for cooperative tendencies, and group size is assumed to be deterministic—simulations show that its predictions are matched quite closely by results for settings where these assumptions do not hold.
A first step toward understanding why sociality has evolved in a particular taxonomic group is to establish comparison points by studying the organization of different social systems. We examined the ...social organization and spatial distribution of individuals in colonies of the undescribed colonial spider Leucauge sp. (Araneae: Tetragnathidae). The social organization of this species was typical of a colonial species, with spiders maintaining individual territories (orb webs) within a scaffolding of shared support lines maintained by the group. Furthermore, we observed a size-dependent vertical stratification of spiders within colonies, with large spiders occupying the highest positions, followed by medium, extra-small and small individuals, a spacing pattern that was consistent across colonies of all sizes. Spiders captured and consumed prey individually and displayed territorial behaviors involving web defense. This study provides a new example of a colonial spider species that shows a distinctive within-group spatial distribution. We discuss possible reasons underlying this species' spatial arrangement in the context of social evolution.
We explored the role of group living and cooperation in resource use in a spider community where 4 congeneric species of similar body size, but with behaviors ranging from near-solitary to fully ...social, co-occur. We found that the range of insect sizes captured by each species reflected their nest and colony size so that species with larger colonies and prey capture webs captured larger insects than less social species. Yet, among those species whose webs did not differ significantly in size-the 2 with the largest and the 2 with the smallest webs-one captured significantly larger insects than the other. This pattern was apparently due to differences in the extent to which nest mates cooperated in the capture of prey, as in only one of the species in each pair did the size of the insects captured increase with colony size. The 4 species were thus packed along the spectrum of available insect sizes from least to most social, with limited overlap between contiguous species. The pattern of resource use was thus more overdispersed than expected by chance, as would be expected if the species had been assembled or differentiated to avoid extensive dietary overlap. We consider alternative hypotheses to explain these patterns and suggest that group size and level of cooperation may play an important role in the dietary segregation of sympatric organisms.
1. The evolution of sociality alters the genetic structure of populations, often leading to an increase in the level of inbreeding and a concomitant decrease in the ratio between the benefits and ...costs of dispersal. 2. The association between an absence of dispersal and sociality in spiders has so far been established indirectly from the age distribution of colonies, their genetic structure and their sex ratio. Using a functional and mechanistic approach, we investigated the dispersal tendencies and abilities of individuals of both sexes in species of spiders with different degrees of sociality. We predicted that dispersal tendencies and abilities would decrease when sociality increased. 3. In seven species of Anelosimus (Theridiidae), we tested whether there is a sharp transition between dispersing subsocial species and non-dispersing social ones, or whether dispersal ability decreases more gradually as level of sociality increases and whether both sexes are equally affected. Our assays measured the propensity and ability to bridge, a common aerial locomotion mode in spiders that is especially relevant during pre-mating dispersal. 4. We found that the tendency to disperse by bridging and, at least in males, the ability to disperse, significantly decrease as the level of sociality increases, and that this occurs without a pronounced threshold between subsocial and social species. Additionally, we detected a reduction of leg length relative to body size with increasing sociality and decreasing dispersal abilities in males, which may be mechanistically related to their reduced ability to bridge. 5. We propose that the loss of dispersal ability associated with the evolution of sociality in spiders, especially in males, could contribute to the maintenance of their inbred social systems by substantially increasing dispersal costs.
Animals living in cooperative groups experience fundamentally different environments than their nonsocial relatives, potentially changing the strength of natural selection on some aspects of their ...behavior. Using a comparative approach, we examined a potential example of this phenomenon: an association between reduced levels of maternal care behavior and sociality in cobweb spiders. We compared 6 different measures of maternal care behavior between species from 2 independently derived social clades and subsocial species from sister clades. In natural nests, we measured the mean distance between egg sacs and the nearest female and the proportion of egg sacs being attended. In the lab, we measured a female's willingness to accept an egg sac, abandon her egg sac when disturbed, repair a damaged egg sac, and the speed at which a female reclaimed her egg sac when separated from it. Social species from both social clades scored significantly lower than subsocial species from sister clades on 6 and 4 of 6 of these assays of maternal care, respectively. We discuss alternative explanations of this pattern, including the potential role of relaxed natural selection in a social environment in permitting the evolution of a novel "low-parenting" phenotype.
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We explored the extent to which differences between elevations in arthropod composition – insects and arachnids – are reflected by different sampling methods and in the diet of local social and ...subsocial spiders.
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We surveyed two low‐elevation tropical rainforest and two upper montane cloud forest sites in eastern Ecuador using blacklighting, sweeping, malaise traps, beating, and visual search. We also observed the prey captured by social (lowland rainforest) and subsocial (upper montane cloud forest) spider colonies in each habitat and related their diets to the insect composition yielded by the individual and combined set of techniques.
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The most notable differences between high‐ and low‐elevation sites in eastern Ecuador were an increase in the relative abundance of Hymenoptera, in particular ants, and a concomitant reduction in the representation of homopterans, dipterans and coleopterans at lower elevations. Differences between elevations, however, were only detected by three of the techniques employed (beating, sweeping and blacklighting).
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The proportions of major taxa categories in the spider diets were only significantly different from samples from their respective environments for the upper elevation subsocial spider against blacklighting and the combined set of all techniques, excluding blacklighting. Nonetheless, only sweeping had similarity indices greater than 75% for both species, with beating and malaise being the next most similar. The more advanced level of sociality and larger nests of the social species may facilitate exploitation of a more representative range of insect types from its environment.
The effects of colony size on individual fitness and its components were investigated in artificially established and natural colonies of the social spider Anelosimus eximius (Araneae: Theridiidae). ...In the tropical rain forest understory at a site in eastern Ecuador, females in colonies containing between 23-107 females had india significantly higher lifetime reproductive success than females in smaller colonies. Among larger colonies, this trend apparently reversed. This overall fitness function was a result of the conflicting effects of colony size on different components of fitness. In particular, the probability of offspring survival to maturity increased with colony size while the probability of a female reproducing within the colonies decreased with colony size. Average clutch size increased with colony size when few or no wasp parasitoids were present in the egg sacs. With a high incidence of egg sac parasitoids, this effect disappeared because larger colonies were more likely to be infected. The product of the three fitness components measured-probability of female reproduction, average clutch size, and offspring survival-produced a function that is consistent with direct estimates of the average female lifetime reproductive success obtained by dividing the total number of offspring maturing in a colony by the number of females in the parental generation. Selection, therefore, should favor group living and intermediate colony sizes in this social spider.
We examine the social characteristics and sex ratio of the recently described Anelosimus oritoyacuAgnarsson 2006. We find that this spider, whose nests occur on tree crowns and bushes in open fields ...near Baeza, Ecuador, lives in colonies that may contain from one to several thousand adult females and their progeny. It differs from most other social congeners in that it occurs at relatively high elevations (1800–1900 m) and its primary sex ratio, 2.5 females per male, is the least biased of any known social species in the genus. The low sex ratio bias may reflect a low colony turnover rather than high gene flow among colonies, as the colonies occurred in complexes that were few and far between, but appeared to be long-lived. The relatively small body size of adult females and a web that appears to allow the capture of insects from all directions, combined with individual and group foraging, may allow the formation of large colonies at an elevation where insects, albeit abundant, are for the most part small.
Abstract Aim To evaluate the relative importance of environmental factors relevant to specific hypotheses that have been proposed to explain the distribution of social and subsocial A nelosimus ...spiders in the American continent and E cuador. For social species, we test the seasonality and prey size hypotheses, which posit that species forming long‐lived multifemale colonies require aseasonal conditions and warm and productive environments in which large insects can develop. For subsocial species, we test the rain intensity and predation risk hypotheses, which posit that species whose colonies contain a single female and her offspring cannot occupy areas where strong rains cause frequent damage to their webs and where warm temperatures and high productivity promote greater predation. Location A merica, E cuador. Methods Using generalized linear models, hierarchical variance partitioning, M axent distribution modelling and phylogenetically controlled regressions, we analysed the relative importance of environmental variables that either directly or indirectly relate to these non‐exclusive hypotheses – temperature and precipitation seasonality (seasonality hypothesis), annual temperature and net primary productivity (insect size and predation hypotheses) and rain intensity (rain intensity hypothesis). Results Temperature seasonality, followed by annual temperature and rain intensity, were the most important predictors of the distribution of spider sociality across America, whereas temperature and rain intensity predominated in the largely aseasonal E cuador. In general, social species were associated with lower temperature seasonality, warmer temperatures and higher rain intensity than subsocial species. Main conclusion The association of social A nelosimus with warm and wet areas in the tropics is consistent with both the seasonality and prey size hypotheses, i.e. both aseasonal conditions and warm temperatures, which allow large insects to develop, are needed for large social colonies to form. That subsocial A nelosimus drop‐out from tropical areas with warm temperatures and high rain intensity is consistent with the hypotheses that high predation risk and disturbance by strong rains exclude subsocial A nelosimus from the lowland rain forest.
Permanent cooperative sociality is rare in spiders. Here we describe sociality in a cobweb (Theridiidae) genus with no other known social species. In five areas of eastern Ecuador, we found nests of ...Theridion nigroannulatum containing from a single to several thousand adults living together in a communal web. Spiders cooperated in prey capture and shared their food. Subadult to young adult sex ratios were highly female biased, suggesting a strongly subdivided population structure as in other permanent-social spiders. Unusual aspects of T. nigroannulatum's biology include the existence of adult females of two discrete sizes, an extreme boom and bust pattern of colony growth, and the presence, within larger colonies, of populations of a predatory spider (Faiditus spp., Argyrodinae) that may play the role of a colony-level parasite. The existence of females of two sizes may be the result of alternative female reproductive strategies, intracolony competition for resources, or an incipient caste system in this species. A redescription and phylogenetic analysis corroborate the placement of this species in Theridion, indicating that sociality has evolved independently in at least three theridiid genera.
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