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Social substructure can influence pathogen transmission. Modularity measures the degree of social contact within versus between "communities" in a network, with increasing modularity expected to reduce transmission opportunities. We investigated how social substructure scales with network size and disease transmission. Using small-scale primate social networks, we applied seven community detection algorithms to calculate modularity and subgroup cohesion, defined as individuals' interactions within subgroups proportional to the network. We found larger networks were more modular with higher subgroup cohesion, but the association's strength varied by community detection algorithm and substructure measure. These findings highlight the importance of choosing an appropriate community detection algorithm for the question of interest, and if not possible, using multiple algorithms. Disease transmission simulations revealed higher modularity and subgroup cohesion resulted in fewer infections, confirming that social substructure has epidemiological consequences. Increased subdivision in larger networks could reflect constrained time budgets or evolved defences against disease risk. [ABSTRACT FROM AUTHOR]

Many parasites and pathogens are transmitted via water, including through faecal contamination of water sources. Yet water is essential for survival, and some species gain nutritional and other benefits from coprophagy. We investigated how primates balance the risks of faecal pathogen transmission with potential benefits of faeces ingestion in their selection of water sources by conducting behavioural experiments with five species of lemurs (Family Lemuridae) in captivity. Subjects were given a choice between clean water and water 'contaminated' with disinfected faecal material, which contained cues associated with faecally transmitted parasites, but minimal risk. We found that lemurs exhibited strong preferences for the clean water. This pattern was supported even at low levels of faecal contamination and in species adapted to water-limited habitats, for which choosiness about water quality could present a dehydration risk. Our results strongly support the hypothesis that avoiding faecal contamination is important in water selection. [ABSTRACT FROM AUTHOR]