Socioecological models aim to predict the effect of environmental variables on species' ecology and social behaviour. Larger groups should face more within-group food competition than smaller groups but benefit from a reduction in predation pressure and/or between-group food competition. The balance between benefits and costs of living in larger groups may vary between populations and species. Moreover, sample size is usually relatively small in field studies and this increases variation in the results. We used meta-analytical techniques to analyse the effect of group size on behaviour and fitness in an attempt to measure the benefits and costs related to group size differences in primates. Meta-analysis allows the presence of an overall effect to be determined across different studies and taxa while controlling for sample size. We selected published papers from the PrimateLit database (http://primatelit.library.wisc.edu). Larger groups travelled further per day and spent more time feeding than smaller groups. Time spent on social behaviour and resting was not affected by group size differences. We found partial support for a decrease in female fecundity in larger groups whereas no significant difference was found for juvenile survival. These results were relatively unaffected by habitat characteristics, by species' ecology and social structure, and by indirect measures of predation risk. Our findings indicate that folivores and frugivores face similar ecological pressures and suggest that the costs of living in larger groups balance or outweigh the benefits. (C) 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
