Do animals living in larger groups experience greater parasitism? A meta-analysis.
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.
Department of Human Evolutionary Biology, Harvard University, Peabody Museum, Cambridge, MA 02138, USA.
Joanna L
JL
Rifkin
Charles L
CL
Nunn
László Z
LZ
Garamszegi
D000818:Animals; D001717:Birds; D005399:Fishes; D006790:Host-Parasite Interactions; D007313:Insecta; D008116:Lizards; D008322:Mammals; D010273:Parasitic Diseases, Animal / Q000453:epidemiology / Q000635:transmission*; D010802:Phylogeny; D011156:Population Density; D012306:Risk; D013112:Spiders