The evolution of social systems can place novel selective forces on investment in expensive neural tissue by changing cognitive demands. Previous hypotheses about the impact of sociality on neural ...investment have received equivocal support when tested across diverse taxonomic groups and social structures. We suggest previous models for social behavior-brain relationships have overlooked important variation in social groups. Social groups vary significantly in structure and function, and the specific attributes of a social group may be more relevant to setting cognitive demands than sociality in general. We have identified intragroup competition, relationship differentiation, information sharing, dominance hierarchies, and task specialization and redundancy as attributes of social behavior which may impact selection for neural investment, and outline how variation in these attributes can result in increased or decreased neural investment with transitions to sociality in different taxa. Finally, we test some of the predictions generated using this framework in a phylogenetic comparison of neural tissue investment in Anelosimus social spiders. Social Anelosimus spiders engage in cooperative prey capture and brood care, which allows for individual redundancy in the completion of these tasks. We hypothesized that in social spider species, the presence of redundancy would reduce selection for individual neural investment relative to subsocial species. We found that social species had significantly decreased investment in the arcuate body, the cognitive center of the spider brain, supporting our predictions. Future comparative tests of brain evolution in social species should account for the special behavioral characteristics that accompany social groups in the subject taxa.
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.
1. There are many benefits of group living, but also substantial costs, one of which is competition for resources. How scarce food resources are distributed among different members of a population or ...social group — whether via scramble or contest competition — can influence not only the variance in individual fitness, but also the stability and therefore survival of the group or population. 2. Attributes of the food resources themselves, such as their size, may influence the type of intraspecific competition that occurs and therefore the intrinsic stability of a group or population. 3. By experimentally manipulating the size of prey fed to artificial colonies of the social spider Anelosimus eximius, we investigated whether prey size could alter the degree of scramble vs. contest competition that takes place and, thus, potentially influence colony population dynamics. 4. We found that large prey were shared more evenly than small prey and that individuals in poor condition were more likely to feed when prey were large than when prey were small. Additionally, we show that individuals participating in prey capture are also more likely to feed on the captured prey. 5. We developed a simple mathematical model to explore the prey sizes that would be energetically worth defending, i.e. prey that are 'economically défendable'. The model shows that neither very small prey, nor prey above a certain size is worth monopolizing, with only intermediate size prey being 'economically defendable'. We therefore suggest the small and large prey in our experiment corresponds to our model's intermediate and large prey categories, respectively. 6. As the size of prey captured by social spider colonies increases with colony size, our findings suggest that scramble competition may predominate in large colonies. Scramble competition, combined with the fact that prey biomass per capita declines as colonies grow beyond a certain size, would then explain why extremely large colonies of this social spider may suddenly go extinct. Our project thus illustrates the potential triple link between characteristics of the resources, individual behaviour and population dynamics, a link rarely considered in an empirical setting.
1. Body size and other morphological traits play key roles in an animal's performance and thus their ability to locate, capture and handle different resources. For species that live and forage in ...groups, group-level characteristics, such as foraging group size and level of cooperation, may also influence performance and further shape resource use. 2. We explored the simultaneous role of individual (body size) and social traits on performance (i.e. prey capture efficiency) in two sympatric social spiders in Ecuador. 3. Given a fivefold difference in body size, the large species captured on average significantly larger insects than the small species. However, because the large species captured both small and large insects, its prey size range included that of the small species, which is thus said to exhibit an included niche. 4. The small species compensated for its small body size by having greater density of individuals within the nests, faster reaction times, and greater participation of individuals of all age classes in prey capture. As a result, the smaller species had a steeper increase in the size of the insects it captured with increasing colony size than the larger species. As predicted by included niche theory, the small species was also more efficient within the shared range as it was less likely to miss or ignore small prey than the large species. 5. Our study contributes to our understanding of how social traits may jointly interact with individual-level traits to shape foraging performance and resource use in communities of social organisms, a problem little explored in the past.
Explaining the evolution of sociality is challenging because social individuals face disadvantages that must be balanced by intrinsic benefits of living in a group. One potential route towards the ...evolution of sociality may emerge from the avoidance of dispersal, which can be risky in some environments. Although early studies found that local competition may cancel the benefits of cooperation in viscous populations, subsequent studies have identified conditions, such as the presence of kin recognition or specific demographic conditions, under which altruism will still spread. Most of these studies assume that the costs of cooperating outweigh the direct benefits (strong altruism). In nature, however, many organisms gain synergistic benefits from group living, which may counterbalance even costly altruistic behaviours. Here, we use an individual based model to investigate how dispersal and social behaviour co-evolve when social behaviours result in synergistic benefits that counterbalance the relative cost of altruism to a greater extent than assumed in previous models. When the cost of cooperation is high, selection for sociality responds strongly to the cost of dispersal. In particular, cooperation can begin to spread in a population when higher cooperation levels become correlated with lower dispersal tendencies within individuals. In contrast, less costly social behaviours are less sensitive to the cost of dispersal. In line with previous studies, we find that mechanisms of global population control also affect this relationship: when whole patches (groups) go extinct each generation, selection favours a relatively high dispersal propensity, and social behaviours evolve only when they are not very costly. If random individuals within groups experience mortality each generation to maintain a global carrying capacity, on the other hand, social behaviours spread and dispersal is reduced, even when the latter is not costly.
► Altruism can invade if dispersal and cooperation are negatively correlated. ► Dispersal levels are less important after cooperation increases in frequency. ► Synergistic benefits of cooperation expand the conditions where sociality evolves. ► Demographic parameters remain influential when altruism is less costly. ► This work generalizes previous model results to a wider range of social organisms.
1. Manipulation of host behaviour by parasitoids has long captured the imagination of ecologists. Parasitoid wasps in the Polysphincta group of genera develop as external parasitoids of spiders.
2. ...In the present study, the previously undescribed interaction between a
Zatypota
sp. wasp (Ichneumonidae) and a social spider
Anelosimus eximius
(Theridiidae) is described. The larva of this
Zatypota
wasp is found to induce its host to disperse from their communal web and build an entirely enclosed web consisting of densely spun silk.
3. The wasp is observed to target primarily immature
A. eximius
individuals, with 37.5–44% of nests in a given area being parasitised. Of those nests, approximately 1.3–2.0% of individuals are hosts to the parasitoid larvae. Larger spider colonies had a significantly higher probability of harbouring parasitoids.
4. This interaction results in unusual behaviours for
A. eximius
induced by the parasitoid: (i) leaving the protection of the social nest and (ii) building a unique, altered web that it would not otherwise build.
It is suggested that the wasp may be tapping into ancestral dispersal behaviours in its host and that a social species provides this wasp an evolutionary advantage by allowing a stable host source.
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.
One of the enduring problems in the study of social evolution has been to understand how cooperation can be maintained in the presence of freeloaders, individuals that take advantage of the more ...cooperative members of groups they are eager to join. The freeloader problem has been particularly troublesome when groups consist of nonrelatives, and no inclusive fitness benefits accrue to individuals that contribute more heavily to communal activities. These theoretical difficulties, however, are not mirrored by the numerous examples of cooperative or even altruistic behaviors exhibited by groups of nonrelatives in nature (e.g., many human groups, communally nesting bees, multiple queen-founding ants, cellular slime molds, and social bacteria). Using a model in which cooperation and grouping tendencies are modeled as coevolving dynamical variables, I show that the freeloader problem can be addressed when group-size effects on fitness are considered explicitly. I show that freeloaders, whose presence is reflected in the development of linkage disequilibrium between grouping and cooperation, increase in frequency when rare, but are selected against when common due to the reduced productivity of the groups they overburden with their presence. Freeloader frequencies thus periodically rise and fall around an equilibrium shown here to be dynamic. These results highlight the importance of group-level effects in the origin and maintenance of sociality, illustrate the dynamic nature of equilibria when multiple levels of selection are involved, and provide a solution to the freeloaders paradox.
Social and subsocial spiders of the genus Anelosimus exhibit an altitudinal pattern in their geographic distribution at tropical latitudes in the Americas. Social species, which capture prey ...cooperatively, occur primarily in the lowland rain forest and are absent from higher elevations, whereas subsocial species are common at higher elevations but absent from the lowland rain forest. Previous studies have suggested that differences in the size of potential insect prey along altitudinal gradients may explain this pattern as insects were found to be, on average, larger in lowland rain forests than at higher elevations. These studies, however, may have under-sampled the insect size composition of each habitat because only one sampling technique was used. Using a number of collection methods we sampled the insect size composition in the environments of social and subsocial spiders in this genus. We found that the average insect size in lowland rain forest habitats was indeed larger than at high-elevation cloud forests in eastern Ecuador. We also found that, even though the various techniques differed in the size of the insects they captured (visual searching and blacklighting yielding larger insects than beating, sweeping, or malaise trapping), they all caught, on average, larger insects in the lowlands. Overall, spider colonies in the lowlands caught larger prey than did spider colonies at higher elevations, paralleling differences in insect size distribution obtained by the various techniques in their respective environments.
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 (genusAnelosimus). 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, andAnelosimus 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.
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