Rodent sociality and parasite diversityIntroductionMaterial and methodsParasitic dataSociality dataComparative analysisResultsDiscussionWe thank Tina Wey and two anonymous reviewers for a variety of constructive comments that helped us improve our presentation.head10Biol. Lett. (2007) 3, 692–694doi:10.1098/rsbl.2007.0393Published online 9 October 2007Evolutionary biologyRodent sociality andparasite diversityFre´de´ric Bordes1, Daniel T. Blumstein2and Serge Morand1,3,*,†1Institut des Sciences de l’e´volution, CNRS-UM2, CC065,Universite´de Montpellier 2, 34095 Montpellier, France2Department of Ecology and Evolutionary Biology,University of California, Los Angeles, CA 90095-1606, USA3UR22 Faune Sauvage CIRAD, Campus International de Baillarguet,34398 Montpellier Cedex 05, France*Author for correspondence ([email protected]).†Present address: Institut des Sciences de l’e´volution, CNRS-UM2,CC065, Universite´de Montpellier 2, 34095 Montpellier, FranceThe risk of parasitism is considered to be ageneral cost of sociality and individuals living inlarger groups are typically considered to bemore likely to be infected with parasites.However, contradictor y results have beenreported for the relationship between g roup sizeand infection by directly transmitted parasites.We used independent contrasts to examine therelationship between an index of so ciality inrodents and the diversity of their macroparasites(helminths and arthropods such as fleas, ticks,suckling lice and mesostigmatid mites). Wefound that the species richness of directly trans-mitted ectoparasites, but not endoparasites,decreased significantly with the level of rodentsociality. A greater homogeneity in the bioticenvironment (i.e. a reduced number of cohabit-ing host species) of the more social species mayhave reduced ectoparasites’ diversity by impair-ing ectoparasites transmission and exchange.Our finding may also result from beneficialoutcomes of social living that include be ha-vioural defences, like allogrooming, and theincreased avoidance of parasites throughdilution effects.Keywords: rodents; sociality; parasite speciesrichness; ectoparasites1. INTRODUCTIONInfectious and parasitic diseases play a major role inmammalian demography ( Tompkins et al. 2001) andin the emergence of immunological and behaviouralcounterstrategies to avoid or eliminate infection(Moore 2002). Social (or group living) species areexpected to have higher parasitic loads or diseaseprevalence because parasite transmission is usuallydensity dependent (Arneberg et al. 1998; Coˆte´&Poulin 1995; Altizer et al. 2003). Thus, parasites areexpected to create a ‘cost’ of sociality (Alexander1974). This theoretical prediction has been confirmedin several empirical studies that found positive corre-lations between group size (or densities) and parasiticloads (i.e. abundance, prevalence or parasite speciesrichness (PSR)) for a variety of directly transmittedectoparasites or helminths (Arneberg et al. 1998;Stanko et al. 2002), as well as for vector-bornearthropods ( Nunn & Heymann 2005). Other studiesthat focused on investment in immune function as anindirect measure of parasitic pressures have alsofound positive relationships between the group sizeand the level of immunological investment (Mølleret al. 2001; Brown & Brown 2002).Notably, however, some studies have failed to findsuch positive correlations (Arnold & Lichtenstein1993; Van Vuren 1996; Ezenwa et al. 2006), andrecent theoretical studies suggest that spatial segre-gation of individuals into social groups can actuallyreduce the spread of disease (Wilson et al. 2003).Group size alone is a somewhat imprecise measure ofsociality because it does not recognize more complexsocial relationships ( Wey et al. in press).The aim of this study was to identify, in rodents,the relationship between host sociality and PSR usingan index of sociality. We expected to find a positiverelationship between the level of host sociality and thespecies richness of directly transmitted ectoparasites,because a main determinant of ectoparasite speciesrichness is host density (Stanko et al. 2002). Bycontrast, the relationship between host sociality andthe species richness of indirectly transmitted endopar-asites is difficult to predict, as these parasites aretransmitted by animals ingesting them. The maindeterminants of their species richness are host diet,prey abundance and diversity, and host density(Sˇimkova´et al. 2003).2. MATERIAL AND METHODS(a) Parasitic dataData for both ecto- and endoparasite species richness were obtainedfrom published studies. We used only surveys where sample sizewas given and rejected descriptive parasitic reports for a given hostspecies. We used PSR—the number of parasite species found in ahost species—as a measure of parasitic load. For endoparasites wefocused on helminths (nematodes, cestodes, trematodes andacanthocephalans) and for ectoparasites we focused on fleas,suckling lices, ticks and mesostigmatid mites. We obtained endopar-asite species richness for 46 rodent species and data for both theendo- and the ectoparasites for 33 species (electronic supple-mentary material 1).(b) Sociality dataIt is remarkably difficult to obtain consistently collected data thatallow broad-scale comparative study of sociality. Thus, manyresearchers use group size as a metric of sociality. Social behaviour,however, is more complex than simply the number of potentiallyinteracting individuals. Therefore, we used a three-level index ofsociality that tried to focus on the nature and complexity of socialinteractions (sensu Blumstein & Armitage 1998), but was indepen-dent of group size or density. For this study, rodent species wereclassified into one of the three groups: group 1, species that areessentially solitary; group 2, species that aggregate only seasonally,that nest communally during the breeding season or that aggregateto form colonies but in which all members of the colony liveindividually; and group 3, species living in well-established groupsthroughout their active season continually sharing space. Groupsizes were highly variable in this third, most social, category. Intotal, we obtained data for 46 rodent species from eight familiesthat represented the full range of sociality (electronic supple-mentary material 1).(c) Comparative analysisWe used independent contrasts to study the coevolution of socialityand parasite abundance. The phylogeny and its sources arepresented in