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, 8 (1), 11828

Cooperative Foraging Expands Dietary Niche but Does Not Offset Intra-Group Competition for Resources in Social Spiders

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Cooperative Foraging Expands Dietary Niche but Does Not Offset Intra-Group Competition for Resources in Social Spiders

Marija Majer et al. Sci Rep.

Abstract

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.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Boxplots of available (white) and consumed prey size (grey) in the diets of three solitary and three social Stegodyphus species. Data are plotted for solitary S. africanus S. lineatus and S. pacificus, followed by social S. dumicola, S. mimosarum and S. sarasinorum (social species marked with*). Numbers on the y-axis below the species names show the average body size of adult female spiders. Abbreviations next to the boxplots represent sites (see Table 2), where species data collected at different sites are combined. Each boxplot shows the extremes, the inter-quartile range, and the median.
Figure 2
Figure 2
Percentages of the most frequent prey taxa caught in the webs and traps in each site: (a) Solitary species and (b) social species of Stegodyphus. Black bars represent their frequency in webs, while grey bars represent their frequencies in traps. Plus symbols within brackets (+) above the bars indicate that the respective prey order was of significantly larger size in the webs than in traps; minus symbol (−) indicates the opposite (rates of change in size were estimated from the exponents of coefficient estimates in the models; full analysis presented in Table S1). For details on the statistical analyses see Supplementary analyses A.
Figure 3
Figure 3
Web area per capita (a) prey capture rate per capita (b) prey biomass per capita (c) and per capita variance in biomass (d) in relation to group size for each observed nest of three cooperatively foraging species: S. dumicola (N nests = 25 in Namibia, 10 in South Africa, filled and empty circles, respectively), S. mimosarum (N nests = 4, filled triangles) and S. sarasinorum (N nests = 14, empty squares). Prey biomass is shown in mg; web area in cm2. We used nest volume as a proxy for group size, as they are positively correlated (see Methods section).

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