The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

Do parasites constrain group size? A phylogenetic comparative study and meta-analysis


1Department of Human Evolutionary Biology, Harvard University, 2Program in Genetics and Genomics, Duke University, 3Department of Evolutionary Ecology, Estación Biológica de Doñana-CSIC

Thursday 11:30-11:45, Galleria South Add to calendar

Living in a group has benefits, but also costs. Among these costs, parasitism has been viewed as a key constraint on group size. However, the association between group size and parasitism is remarkably variable. While some field studies find a positive link between parasites and sociality, others find a negative association; comparative studies have also been inconclusive.

To help resolve these conflicting results, we conducted phylogenetic analyses using data from within and across species. First, we used new phylogenetic comparative methods to assess the association between group size and parasite richness across species of primates. Using a Bayesian approach that controlled for phylogenetic uncertainty and incorporated phylogenetic signal, we failed to find a significant association between group size and parasitism.

Next, we systematically examined the association between group size and parasitism with meta-analysis techniques. Drawing on intraspecific and interspecific studies of 42 animal species, we found evidence for a positive relationship between group size and parasitism. However, the effect was weak and varied across studies and taxonomic groups. Vector-borne parasites showed the strongest association with group size, consistent with larger groups attracting more vectors. The pattern was considerably stronger in birds than in mammals, with mammals having an effect size that was statistically indistinguishable from zero. Phylogeny failed to explain significant variation in effect sizes.

Our analyses call into question whether group size is the main dimension of sociality that influences parasitism. Recent work instead suggests that intra-group contact patterns vary with group size and predict disease transmission.

This research was funded by the NSF (BCS-0923791), Harvard University, the Spanish National Research Council (Consejo Superior de Investigaciones Científicas), and the Plan Nacional program of the Spanish government (ref. no. CGL2009-10652 and CGL2009-09439).

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