Journal
AMERICAN NATURALIST
Volume 180, Issue 1, Pages 70-82Publisher
UNIV CHICAGO PRESS
DOI: 10.1086/666081
Keywords
sociality; group size; parasitism; phylogenetic meta-analysis; phylogeny
Categories
Funding
- Harvard University
- National Science Foundation (NSF) [BCS-0923791, EF-0423641]
- Spanish National Research Council (Consejo Superior de Investigaciones Cientificas)
- Spanish government [CGL2009-10652, CGL2009-09439]
- Direct For Social, Behav & Economic Scie
- Division Of Behavioral and Cognitive Sci [0923791] Funding Source: National Science Foundation
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Parasitism is widely viewed as the primary cost of sociality and a constraint on group size, yet studies report varied associations between group size and parasitism. Using the largest database of its kind, we performed a meta-analysis of 69 studies of the relationship between group size and parasite risk, as measured by parasitism and immune defenses. We predicted a positive correlation between group size and parasitism with organisms that show contagious and environmental transmission and a negative correlation for searching parasites, parasitoids, and possibly vector-borne parasites (on the basis of the encounter-dilution effect). Overall, we found a positive effect of group size (r = 0.187) that varied in magnitude across transmission modes and measures of parasite risk, with only weak indications of publication bias. Among different groups of hosts, we found a stronger relationship between group size and parasite risk in birds than in mammals, which may be driven by ecological and social factors. A metaregression showed that effect sizes increased with maximum group size. Phylogenetic meta-analyses revealed no evidence for phylogenetic signal in the strength of the group size-parasitism relationship. We conclude that group size is a weak predictor of parasite risk except in species that live in large aggregations, such as colonial birds, in which effect sizes are larger.
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