Organisms with lower dispersal abilities tend to have more genetically dissimilar populations. The same is true for parasites, whose transmission frequency may depend on the population structure of ...the host. This should be especially true when hosts and parasites face similar barriers to dispersal. Here, we considered the similarities between host and parasite population structure in a social spider system. In this system, host colonies are typified by rapid growth via internal recruitment followed by budding or fission events when colonies grow too large, with each colony representing a distinct population. Host colonies provide habitat for kleptoparasitic spiders, which steal prey from and may also feed directly on host individuals. We asked whether kleptoparasites exhibit a similar degree of population subdivision as their host. Under the free‐mixing hypothesis (i.e., horizontal transmission), kleptoparasites would have well‐mixed populations across broader regions than a single host nest, whereas host populations would be strongly genetically structured. Under the host‐tracking hypothesis (i.e., vertical transmission), kleptoparasites would have a population structure that parallels that of the host. We conducted a genotype‐by‐sequencing study to assess the population structure of both hosts and kleptoparasites within three nearby regions in eastern Ecuador. We found strong signatures of population differentiation and bottlenecks in the host species, which is congruent with past studies. However, we found that kleptoparasite populations were well mixed across host nests, with no evidence of recent bottlenecks. These results support our free‐mixing hypothesis, suggesting that kleptoparasites follow patterns of horizontal transmission in this social spider system.
Abstract
A defining feature of parasitism is the harm parasites cause to their host via a reduction in lifetime reproductive success. Harm, also referred to as “virulence,” may involve host mortality ...or sublethal effects, such as a decreased body condition or protracted development of immature individuals. We considered a system where colonies of the social spider
Anelosimus eximius
serve as hosts to confamilial kleptoparasitic spiders that steal food resources. In a laboratory experiment with parasitized and non‐parasitized colonies, we tested whether this host–parasite interaction meets the criterium of harm to host individuals and colonies. We assessed survival and measured scaled mass indices (SMI) before and after the experimental period. Linear mixed effects models demonstrated that colonies exposed to kleptoparasites had lower SMI values at the end of the experiment compared to controls, but found no effect of kleptoparasitism on mortality in treatment and control groups. We conclude that kleptoparasites meet the criterium of harm to their host to be considered parasitic and provide the first empirical measure of virulence for this study system.
Predation, which is a fundamental force in ecosystems, has been found to decrease in intensity with elevation and latitude. The mechanisms behind this pattern, however, remain unaddressed. Using ...visual sampling of potential predators and live flies as baits, we assessed predation patterns along 4,000-m elevation transects on either side of the equatorial Andes. At the lower elevations, we found that around 80% of predation events on our insect baits were due to ants. The decline in predation with elevation was driven mainly by a decline in the abundance of ants, whose importance relative to other predators also declined. We show that both predator density and activity (predation rate per individual predator) decreased with elevation, thus ascribing specific mechanisms to known predation patterns. We suggest that changes in these two mechanisms may reflect changes in primary productivity and metabolic rate with temperature, factors of potential relevance across latitudinal and other macroecological gradients, particularly for ectotherm predators and prey.
In some ways, social spiders are a biological novelty item. They are not extraordinarily successful either evolutionarily or ecologically, and their societies suffer a variety of disadvantages that ...render them more brittle than other social systems. Yet, from an empiricist's perspective, these attributes make them uniquely poised for addressing a variety of research questions. Here we provide a brief overview of the biology of social spiders for the general reader. We then highlight a variety of ecological and evolutionary challenges suffered by these animals that renders them at risk of extinction in the short and long term. We finally discuss how these hardships have given rise to a variety of individual and group level adaptations that are rare or entirely absent in other spiders, as well as in most other social animals. Throughout this article, we highlight gaps in our current understanding of these creatures and draw attention to some of the more promising frontiers for future research. To this end, we have two goals. First, we would like to draw the attention of general behavioural ecologists interested in social evolution to the biology of social spiders, and emphasize a variety of reasons why one might consider these animals for their next research question. Second, for those already inculcated in the social spider literature, we hope that this article will raise the reader's consciousness to various underexplored but promising avenues for future research. With the right research question, social spiders promise to be a high-profile and high-throughput model system.
•Sociality is rare in spiders and is not associated with great ecological success.•Sociality imposes a series of novel selection pressures on spiders.•Spiders show novel adaptations to these pressures rarely observed in other taxa.•Much of social spider biology is unknown and the field is ripe for new researchers.
Ineffective solitary hunters likely fail to pass on their genes, but social hunters may benefit from others’ spoils and be shielded from natural selection. We examine hunting performance in solitary ...and group-living cobweb spider species, finding that individuals from group-living species, especially those from large groups, were slower to react to prey and captured fewer prey. Group-living may thus negatively affect individual performance, potentially impeding social species’ ability to evolve back to less social states.
Abstract
Ineffective hunters in cooperative foraging groups may be shielded from natural selection by their more effective group mates, whereas those living solitarily would starve and thus be removed from the population. The problem may be exacerbated in large groups where it may be easier for individuals to withhold participation. Group foragers may thus be ineffective individual hunters or exhibit greater inter-individual variation in hunting abilities, in particular, when living in large groups. We test these hypotheses in spider species of the genus Anelosimus that differ in their level of sociality and, among social species, in colonies of different sizes. We found that individuals from the more social species, and those from larger groups, reacted more slowly to prey than those from solitary species or small groups. Individuals from these categories also had greater inter-individual variation in reaction times. Individuals from large social groups also had lower prey-capture success than those from small ones. These differences may have been driven by the size of the group from which the social individuals were taken, as those from small colonies behaved similarly to individuals of the 2 less social species. This finding suggests that hunting ability may develop as a phenotypically plastic trait.
The transition from solitary to social life is a major phenotypic innovation, but its genetic underpinnings are largely unknown. To identify genomic changes associated with this transition, we ...compare the genomes of 22 spider species representing eight recent and independent origins of sociality. Hundreds of genes tend to experience shifts in selection during the repeated transition to social life. These genes are associated with several key functions, such as neurogenesis, behavior, and metabolism, and include genes that previously have been implicated in animal social behavior and human behavioral disorders. In addition, social species have elevated genome-wide rates of molecular evolution associated with relaxed selection caused by reduced effective population size. Altogether, our study provides unprecedented insights into the genomic signatures of social evolution and the specific genetic changes that repeatedly underpin the evolution of sociality. Our study also highlights the heretofore unappreciated potential of transcriptomics using ethanol-preserved specimens for comparative genomics and phylotranscriptomics.
Animals may develop mutualistic associations with other species, whereby prey offer resources or services in exchange for protection from predators. Alternatively, prey may offer resources or ...services directly to their would-be predators in exchange for their lives. The latter may be the case of hemipterans that engage in mutualistic interactions with ants by offering a honeydew reward. We test the extent to which a honeydew offering versus partner recognition may play a role as proximate mechanisms deterring ants from predating upon their hemipteran partners. We showed that, when presented with a choice between a hemipteran partner and an alternative prey type, mutualist ants were less likely to attack and more likely to remain probing their hemipteran partners. This occurred even in the absence of an immediate sugary reward, suggesting either an evolved or learned partner recognition response. To a similar extent, however, ants were also less likely to attack the alternative prey type when laced with honey as a proxy for a honeydew reward. This was the case even after the honey had been depleted, suggesting an ability of ants to recognize new potential sources of sugars. Either possibility suggests a degree of innate or learned partner recognition.
1. Web building spider communities represent a vehicle for addressing how abiotic and biotic factors interact to structure natural communities.
2. The present study investigates how intense rainfall ...and potential predation by ants affect the proportion of three‐dimensional (3D) versus two‐dimensional (2D) spider webs along precipitation gradients.
3. Besides capturing prey, 3D webs may provide protection against predators, but they require a much greater material investment to be built than 2D webs. If costs take precedence over predator protection benefits, the proportion of 3D webs would decrease as precipitation increases (the ‘rain intensity’ hypothesis). Alternatively, if predator protection benefits take precedence, and the abundance of ants and other predators increases with precipitation, the proportion of 3D webs would increase with precipitation (the ‘predation risk’ hypothesis).
4. Seven sites were selected along a rain gradient of relatively uniform elevation and latitude in western Ecuador. Rain intensity, ant abundance and vegetation lushness (leaf area, canopy cover, tree diameter) were all observed to increase along the gradient, as did vegetation cover immediately above each web.
5. Relative to 2D webs, 3D webs increased in frequency with annual rainfall, consistent with the predation risk hypothesis but counter to the rain intensity hypothesis.
6. In areas of greater precipitation, however, lusher vegetation provided greater immediate vegetation cover to webs. Microhabitat factors may thus mitigate the destructive power of intense rainfall, thus allowing the predator protection benefits of 3D webs to be realised despite the occurrence of strong rains.
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 Anelosimus spiders in the ...American continent and Ecuador. 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: America, Ecuador. METHODS: Using generalized linear models, hierarchical variance partitioning, Maxent 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 Ecuador. 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 Anelosimus 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 Anelosimus 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 Anelosimus from the lowland rain forest.
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