Behavioural Processes 79 (2008) 85–92

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Behavioural Processes

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Mini review

On the dynamics of rodent social groups

Luis A. Ebenspergera,∗, Loren D. Hayesb

Author's original copy

a Centro de Estudios Avanzados en Ecologıa and Biodiversidad (CASEB), and Departamento de Ecologıa,antia

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tal phacrosspeci. Nataut lesp dynmech

Facultad de Ciencias Biologicas, Pontificia Universidad Catolica de Chile, Casilla 114-D, Sb Department of Biology, University of Louisiana at Monroe, Monroe, LA 71203, USA

a r t i c l e i n f o

Article history:Received 18 March 2008Received in revised form 15 May 2008Accepted 16 May 2008

Keywords:Group-livingImmigrationNatal philopatrySocialitySocial dynamics

a b s t r a c t

A prevailing view is that aprocesses can influence thexisting groups. Given thachanges, the extent to whevolution of sociality. Wethe available evidence supa major importance of nathe reproductive strategygroup dynamics in 26% oftake place simultaneouslysingular breeders (70%), bmain process driving grouimportance of alternative

1. Introduction

Animal social groups range from temporary associations andaggregations to relatively stable social units (Parrish et al., 1997;Krause and Ruxton, 2002). Among the vertebrates, there is consid-erable intra- and inter-specific variation in group size (Lott, 1991).Variation in group size may be coupled to variation in the relativecosts and benefits of sociality (i.e., the tendency to form groups).Benefits-based hypotheses predict that group-living confers netfitness advantages to individuals living in association with con-specifics (Alexander, 1974; Bertram, 1978; Krebs and Davies, 1993;Ebensperger, 2001; Ebensperger and Blumstein, 2006). Obser-vations of reproductive fitness benefits in some fish (Balshineet al., 2001), birds (Brown and Brown, 2004a) and mammals(Clutton-Brock et al., 2001) support these hypotheses. In contrast,group-living results in a net fitness cost to individuals in numer-ous other social species (Cowan, 1987; Boyce and Boyce, 1988; DaSilva et al., 1994; Hoogland, 1995; Woodroffe and Macdonald, 2000;Lacey, 2004; Silk, 2007). Under these circumstances, group-livingmay result when individuals are ecologically constrained to remainin groups despite the associated costs (Brown, 1987; Waser, 1988).Ecological constraints may originate from limited availability of

∗ Corresponding author. Tel.: +562 6862733; fax: +562 6862621.E-mail address: lebensperger@bio.puc.cl (L.A. Ebensperger).

0376-6357/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.beproc.2008.05.006

go, Chile

l social groups are largely determined by natal philopatry. However, otheramics of social groups, including emigration of individuals that join pre-

ess consequences of living in a group may vary depending on how groupsternative mechanisms drive social dynamics is an important theme to theidered the available literature on social rodents to examine (i) whethers single versus multiple mechanisms, (ii) how strongly evidence supportsilopatry, and (iii) whether mechanisms of group formation are linked tos species. While natal philopatry is considered the major process behindes examined, studies on 74% of species indicate two or more mechanismsl philopatry is considered a primary mechanism in communal (56%) ands so in solitary breeders (18%). Thus, the tenet that natal philopatry is theamics in rodents may be premature, and studies aimed at examining theanisms are justified.

© 2008 Elsevier B.V. All rights reserved.

suitable habitat, high population density conditions and habitatsaturation, a patchy distribution of critical resources, or a combi-nation of these (Emlen, 1982; Johnson et al., 2005; Lucia et al., inpress).

Aside from ultimate causes of group living, an emerging theme

in the study of sociality is the proximate mechanisms governingthe dynamics of social groups. Social dynamics involves the origi-nation and subsequent variation in size and composition of thesegroups. A prevailing view among scholars studying vertebrate socialbehavior is that animal social groups are largely determined bynatal philopatry. In the context of social group dynamics, natalphilopatry involves the recruitment of grown offspring born to theparental unit, implying that such offspring delay or refrain fromdispersal and remain in the natal social unit. A consequence ofnatal philopatry is the formation of extended family groups that areinherently unstable (Emlen, 1995). Natal philopatry characterizesthe social dynamics in some communally breeding birds (Brown,1987; Mumme, 1997), carnivores (Moehlman, 1986; Rood, 1986),cetaceans (Connor, 2000), primates (Pusey and Packer, 1987), pro-boscideans (Archie et al., 2006), rodents (Michener, 1983; Solomon,2003; Armitage, 2007; Lacey and Sherman, 2007; Lucia et al., inpress), or mammals in general (Waser, 1988; Lefebvre et al., 2003).As this evidence implies, natal philopatry seems common in specieswith varying social strategies. However, other processes can influ-ence the dynamics of social groups as well, including emigration ofsingle individuals (i.e., dispersal) to join other pre-existing groups,

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emigration of multiple individuals that form new daughter groupsthat remain functionally independent from parental groups, or joinpre-existing groups (Seppa et al., 2008). The break up of groupsand emigration occur in social insects (Queller et al., 2000; Seppaet al., 2008), cooperatively breeding birds (Walters et al., 2004;Williams and Rabenold, 2005), carnivores (Mech, 1987; Rood, 1986,1987), primates (Pusey and Packer, 1987; Lefebvre et al., 2003),proboscideans (Archie et al., 2006), and ungulates (Berger, 1987).

In rodents, the tenet that natal philopatry is the driving forceof social group formation and dynamics comes from the observa-tion that natal philopatry by one or both sexes characterizes socialspecies (Lacey, 2000; Solomon, 2003; Armitage, 2007; Lacey andSherman, 2007; Nunes, 2007; Randall, 2007). However, evidence isaccumulating that the dynamics of social groups in some speciescan be influenced by several processes, including emigration andthe break up of groups (Faulkes and Bennett, 2001). These findingssuggest that the generalization that natal philopatry is the primarymechanism driving the dynamics of rodent social groups could bepremature. Thus, different mechanisms may pertain or be moreimportant in different species.

2. Examination of evidence

The objective of this study was to conduct a preliminary surveyof the mechanisms of social group formation in the rodents. At anintra-specific level, we considered the available primary literatureon social rodents to examine the extent and quality of evidencesupporting alternative mechanisms thought to drive social groupdynamics. We asked two important questions: (i) How exclusive isnatal philopatry in social group dynamics? and (ii) How frequentlyare other mechanisms the primary factor? Thus, we examinedwhether the available evidence supports single versus multiplemechanisms, and how strongly evidence supports a major impor-tance of natal philopatry versus other mechanisms.

To accomplish this endeavour, we first considered the listof social species described in Solomon (2003) and Lacey andSherman (2007) as a starting point. We used the online ISI Webof Science (by LAE) and the Biological Abstracts in EBSCOhost (byLDH) to add additional species for which the dynamics of socialgroup formation have been studied to our list. We independentlytyped keywords such as natal philopatry, emigration, social groupdynamics, rodent dispersal, and combinations of these terms,as well as the scientific names of species reported to be social.In order for a given species to be tallied, we followed the social

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classification of Burda et al. (2000), where “social” species arethose in which individuals of social groups share nests or dens, orform cohesive groups during foraging or other daily activities. Wedid not include “colonial” species i.e., species in which individualsestablish their nests and ranges in spatial proximity, but liveindependently from others. For example, some voles (Salvioni andLidicker, 1995; Pusenius et al., 1998) and squirrels (Michener, 1979;Vestal and McCarley, 1984) fell under this last category and thus,were not included in our analysis. For each species considered, wenoted whether natal philopatry and other processes affecting thecomposition of social groups (e.g., emigration, immigration) havebeen reported and quantified. For each species we categorisedthe importance of natal philopatry, the break up of groups, andthe emigration of single or multiple individuals on the dynamicsof social groups. As a result, we used five categories: (i) “main”,indicates the mechanism is considered by the authors as the mostimportant, (ii) “secondary” indicates that the mechanism doesoccur, but that is considered less important by the authors, (iii)“not quantified” indicates that the mechanism has been reportedbut whose frequency and importance can not be determined,(iv) “unimportant” indicates that the mechanism occurs but is

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marginal, (v) “data not available” indicates that the mechanism hasnot been examined. In an effort to account for potential evolution-ary relationships, we determined the percentage of species withinsuborders that were shown to have one or more mechanisms ofgroup formation. We did not conduct a more rigorous comparativeanalysis by family because of small sample sizes.

In a recent review, Silk (2007) argued that social groups shouldbe categorized based on the degree of reproductive skew (i.e.,amount of shared direct reproduction) among group membersand whether individuals provide communal care. Silk (2007) sug-gested that social species should be categorized as follows: (i)solitary breeders: adults are social during non breeding, but nestsolitarily during breeding time, (ii) singular breeders: a minorityof adult group members breed, non breeders may or may nothelp raising offspring of breeders, and (iii) plural breeders withand without communal care: most adult members breed and reartheir offspring either communally or independently. Among therodents, there is considerable inter-specific variation in the typesof social groups that form (Hayes, 2000). This variation is impor-tant because the fitness outcomes for these strategies may varydepending on the relatedness of group members. For example,when singularly breeding groups consist of closely related kin, aswould be expected if groups form as a result of natal philopa-try, breeders gain immediate direct benefits while non-breedersgain immediate indirect fitness benefits (Solomon and Hayes,in press). In contrast, plural breeders with communal care maygain immediate direct and indirect benefits if communal care isdirected to non-descendent kin (Solomon and Hayes, in press).Given these potential fitness consequences, we also examinedwhether the importance of natal philopatry (tested or presumed)varies across species with varying reproductive strategies. Thus,we addressed the following question: Is the mechanism of groupformation related to reproductive strategy? To do so, every socialspecies examined was categorized as a solitary breeder, singularbreeder, or plural breeder. All of the plural breeders in this anal-ysis provide some care to non-descendent offspring and thus, areconsidered plural breeders with communal care by Silk’s (2007)classification. No species categorized as plural breeders in our studyconsisted of species with multiple breeders without communalcare. Hereafter, we refer to these groups as communal breeders toemphasize the phenomenon of co-breeding with communal care.We used a Pearson’s chi-square to examine the null hypothesisthat studies consider natal philopatry as the main driving mech-

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strategies.

3. Emerging patterns

Support for alternative mechanisms governing group dynam-ics to natal philopatry requires evidence of intra-specific variationin group size and composition, and observations of the spatialecology and movements of juveniles and adults. We found appro-priate information on the social group dynamics of 42 socialrodents (Table 1), a figure that includes 15 new additions to pre-vious analyses (Solomon, 2003; Lacey and Sherman, 2007). Ouranalysis included species from four sub-orders, including Casto-rimorpha (1 species), Hystricomorpha (10 species), Myomorpha(18 species) and Sciuromorpha (13 species) (Table 1). Three specieswere removed from the list generated by Solomon (2003) and Laceyand Sherman (2007) because no appropriate information aboutnatal philopatry or group transfer was found in the primary litera-ture (i.e., Marmota vancouveriensis, Spalacopus cyanus), or becausethe species did not fit to the definition of social living used here(Spermophilus columbianus).

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87Table 1Evidence that natal philopatry, emigration by single or by multiple individuals influence the dynamics of social groups in rodents with varying reproductive strategies

Species Common name Suborder: family Reproductive strategy Natal philopatry Emigration: singleindividuals

Emigration:multipleindividuals

Authorities

Ctenodactylus gundi Gundi Hystricomorpha:Ctenodactylidae

Communal breeder Not quantified Not quantified Nutt (2005, 2007)

Ctenomys sociabilis Social tuco-tuco Hystricomorpha:Ctenomyidae

Communal breeder Main Secondary Lacey et al. (1997), Lacey andWieczorek (2004)

Cynomys gunnisoni Gunnison’s prairiedog

Sciuromorpha:Sciuridae

Communal breeder Main Secondary Rayor (1988), Hoogland (1999)

Cynomys ludovicianus Black-tailed prairiedog

Sciuromorpha:Sciuridae

Communal breeder Main Secondary Notquantified

Tang Halpin (1987), Garrett andFranklin (1988), Hoogland(1995), Manno et al. (2007)

Glis glis Dormouse Sciuromorpha:Gliridae

Communal breeder Not quantified Data not available Marin and Pilastro (1994)

Hydrochaeris hydrochaeris Capybara Hystricomorpha:Hydrochaeridae

Communal breeder Unimportant Not quantified Notquantified

Herrera and Macdonald (1987)

Lagostomus maximus Plains viscacha Hystricomorpha:Chinchillidae

Communal breeder Not quantified Not quantified Branch (1993), Branch et al.(1993)

Lasiopodomys mandarinus Mandarine vole Myomorpha:Cricetidae

Communal breeder Not quantified Not quantified Notquantified

Smorkatcheva (1999)

Marmota caligata Hoary marmot Sciuromorpha: Sciuridae

Communal breeder Not quantified Not quantified Barash (1974), Holmes (1984),Kyle et al. (2007)

Marmota flaviventris Yellow-belliedmarmot

Sciuromorpha:Sciuridae

Communal breeder Main Secondary Armitage (1984), Armitage andGurri-Glass (1994), Blumsteinet al. (2004)

Microtus ochrogaster Prairie vole Myomorpha:Cricetidae

Communal breeder Main (70%) Secondary (30%) Getz et al. (1993), Getz and McGuire (1997)

Mus musculus House mouse Myomorpha:Muridae

Communal breeder Not quantified Not quantified Wilkinson and Baker (1988), Manning et al. (1995)

Neotoma cinerea Bushy-tailedwoodrat

Myomorpha:Cricetidae

Communal breeder Main Secondary Moses and Millar (1992, 1994)

Peromyscus leucopus White-footedmouse

Myomorpha:Cricetidae

Communal breeder Not quantified Secondary Wolff and Durr (1986), Wolff(1994)

Peromyscus maniculatus Deer mouse Myomorpha:Cricetidae

Communal breeder Not quantified Secondary Millar and Derrickson (1992),Wolff and Durr (1986), Wolff(1994)

Rhabdomys pumilio Striped mouse Myomorpha:Muridae

Communal breeder Not quantified Not quantified Schradin and Pillay (2004),Schradin et al. (2006)

Rhombomys opimus Great gerbil Myomorpha:Muridae

Communal breeder Not quantified Secondary Randall et al. (2005)

Xerus inauris South Africanground squirrel

Sciuromorpha:Sciuridae

Communal breeder Main (females) Main (males) Waterman (1995, 2002)

Myocastor coypus Coypu Hystricomorpha:Myocastoridae

Communal breeder? Data not available Not quantified Guichon et al. (2003a, 2003b)

Castor canadensis Canadian beaver Castorimorpha:Castoridae

Singular breeder Not quantified Not quantified Busher et al. (1983), Sun et al.(2000)

Cryptomys damarensis Damara mole-rat Hystricomorpha:Bathyergidae

Singular breeder Main Secondary Jarvis and Bennett (1993),Faulkes and Bennett (2001),Burland et al. (2002, 2004)

Cryptomys darlingi Darling’s mole-rat Hystricomorpha:Bathyergidae

Singular breeder Not quantified Data not available Bennett et al. (1994)

Cryptomys hottentotus Southern Africanmole-rat

Hystricomorpha:Bathyergidae

Singular breeder Main Secondary(3–26%)

Notquantified

Spinks et al. (2000), Bishop etal. (2004)

Cryptomys mechowi Giant mole-rat Hystricomorpha:Bathyergidae

Singular breeder Not quantified Data not available Wallace and Bennett (1998),Scharff et al. (2001)

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85–92Table 1 (Continued )

Species Common name Suborder: family Reproductive strategy Natal philopatry Emigration: singleindividuals

Emigration:multipleindividuals

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Heterocephalus glaber Naked mole-rat Hystricomorpha:Bathyergidae

Singular breeder Not quantified Not quantified Notquantified

Brett (1991), Braude (2000)

Marmota caudata Long-tailedmarmot

Sciuromorpha:Sciuridae

Singular breeder Main Secondary Blumstein and Arnold (1998)

Marmota marmota Alpine marmot Sciuromorpha:Sciuridae

Singular breeder Not quantified Not quantified Arnold (1990), Perrin et al.(1993), Lenti Borero (1999)

Meriones unguiculatus Mongolian jird Muomorpha:Muridae

Singular breeder Not quantified Secondary? Agren et al. (1989)

Microtus pinetorum Woodland vole Myomorpha:Cricetidae

Singular breeder Main Secondary FitzGerald and Madison (1983),Solomon et al. (1998)

Microtus pennsylvanicus Meadow vole Myomorpha:Cricetidae

Solitary and Communal breeder Not quantified Not quantified Madison et al. (1984), McSheaand Madison (1984), McShea(1990)

Myodes (Clethrionomys) glareolus Bank vole Myomorpha:Cricetidae

Solitary breeder Not quantified Not quantified Karlsson (1988), Ylonen et al.(1995)

Myodes (Clethrionomys) rufocanus Gray red-backedvole

Myomorpha:Cricetidae

Solitary breeder Not quantified Not quantified Saitoh (1989), Ishibashi et al.(1998)

Glaucomys volans Southern flyingsquirrel

Sciuromorpha:Sciuridae

Solitary breeder Not quantified Not quantified Layne and Raymond (1994),Winterrowd et al. (2005)

Microtus montanus Montane vole Myomorpha:Cricetidae

Solitary breeder Main Data not available Jannett (1978, 1982)

Microtus townsendii Townsend’s vole Myomorpha:Cricetidae

Solitary breeder Unimportant? Not quantified Lambin and Krebs (1991)

Microtus xanthognathus Taiga vole Myomorpha:Cricetidae

Solitary breeder Not quantified Not quantified Wolff and Lidicker (1980, 1981)

Peromyscus californicus California mouse Myomorpha:Cricetidae

Solitary breeder Unimportant? Not quantified Ribble and Salvioni (1990),Ribble (1992)

Sciurus alberti Albert’s squirrel Sciuromorpha:Sciuridae

Solitary breeder Unimportant? Data not available Edelman and Koprowski (2007)

Sciurus carolinensis Eastern greysquirrel

Sciuromorpha:Sciuridae

Solitary breeder Not quantified Secondary Koprowski (1996)

Sciurus niger Eastern fox squirrel Sciuromorpha:Sciuridae

Solitary breeder Unimportant Main Koprowski (1996)

Marmota olympus Olympic marmot Sciuromorpha:Sciuridae

Solitary breeder* Not quantified Not quantified Barash (1973)

Spalax microphthalmus Greater blindmole-rat

Myomorpha:Spalacidae

Solitary or singular breeder? Unimportant? Not quantified Puzachenko (1993, 1995,personal communication)

Common names and taxonomic affiliations of species follow Wilson and Reeder (2005).

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3.1. How exclusive is natal philopatry in social group dynamics?

Our analysis of the literature yielded evidence to support thatnatal philopatry is an important, but not exclusive, mechanismin the dynamics across social rodents. Studies on 31 out the 42species (i.e., 74%) report evidence for the existence of two ormore mechanisms influencing group dynamics (i.e., species withboth ‘main’ and “secondary” or with “main” and “not quanti-fied” categories; species with “not quantified” and “secondary”categories, and species with two or more “not quantified” cate-gories in Table 1). The percentage of studies showing evidence forthe existence of two or more mechanisms was similar for Hys-tricomorpha (70%, 7/10), Myomorpha (72%, 13/18), Sciuromorpha(77%; 10/13) (Table 1); evidence was not quantified for Casto-rimorpha. Of the 11 species in which only one mechanism isimplicated, 5 (two Hystricomorpha, one Myomorpha, and two Sci-uromorpha) consisted of species in which only one mechanism wasexamined (i.e., species that received a “main”, or a “not quanti-fied”, or an “unimportant”, and a “data not available” category inTable 1).

As data in Table 1 suggest, quantitative rather than qualitativestudies that consider the contribution of natal philopatry relativeto other processes explaining the dynamics of rodent social groupssuch as emigration and group transfer are rare. Exceptions to this“rule” come from studies on the southern African mole rat (Cryp-tomys hottentotus) and the prairie vole (Microtus ochrogaster) allof which indicate that natal philopatry is the main mechanisminvolved. Natal philopatry is suggested to be the major process(“main” in Table 1 behind group dynamics in 11 out of 42 (i.e., 26%)social rodents, a figure not including Xerus inauris, in which natalphilopatry plays a major role for females but not males (Table 1).The percentage of studies indicating that natal philopatry wasthe primary (“main”) mechanism was similar for Hystricomorpha(30%, 3/10), Myomorpha (22%, 4/18), Sciuromorpha (33%; 4/12)(Table 1).

In 23 (55%) other species natal philopatry is reported to playsome role (“not quantified” in Table 1). However, studies on 20 outof these 23 species (i.e., 87%) indicate that emigration has beenrecorded but not quantified (“not quantified” in Table 1) or thatoccurs but is secondary (“secondary” in Table 1). In addition, emi-gration of multiple individuals has been noted in 4 of these species(Table 1). The percentage of species in which natal philopatryplayed some role was similar Hystricomorpha (50%, 5/10), Myomor-

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pha (61%, 11/18), Sciuromorpha (46%; 6/13) (Table 1).

3.2. How frequently are other mechanisms the primary factor?

Studies on 4 species (10%, excluding Xerus inauris) suggest thatemigration is or might be (given that natal philoptary is consideredas secondary or unimportant) the major driving force behind groupdynamics (Table 1). Evidence of group emigration suggests thepotential for the break up of originally larger into small functionallyindependent groups in one of these species (Table 1). Fifteen out ofthe 42 species considered herein (i.e., 36%) report that both natalphilopatry and emigration processes affect, to an unknown extent(“not quantified” in Table 1; 1 Castorimorpha, 3 Hystricomorpha, 7Myomorpha, and 4 Sciuromorpha), the dynamics of social groups.The break up of larger into small groups has been reported to occurin two of these species.

Regarding reports of group transfer through emigration, studieson 10 out of 12 species that have examined this process indicatethat the source of immigrants include other social groups (Table 1).Studies on two other rodents indicate wandering individuals as thesource of immigrants.

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3.3. Is the mechanism of group formation related to reproductivestrategy?

Among the species included in this analysis, 19 of 42 (45%)were communal breeders, 10 others (24%) were singular breed-ers, whereas 11 more (26%) were solitary breeders (Table 1).We found evidence for both communal and solitary breedingin one species, and not enough information to decide betweensolitary and singularly breeding in one other species (Table 1).Across species, the claim that natal philopatry is a main driv-ing mechanism behind social group dynamics (as opposed to thatnatal philopatry occurs but has not been quantified, or is a sec-ondary/unimportant process) tended to vary with reproductivestrategy of species, though this difference was only marginallysignificant (Pearson’s chi-square, �2 = 10.59, 4 df, P = 0.057). In par-ticular, natal philopatry has been considered a primary mechanismin communal breeders (39%; 7/18; including female Xerus inauris)and singular breeders (40% 4/10) and less so in solitary breedingspecies (9%; 1/11).

4. Conclusions and future prospects

The view that natal philopatry is the major process influencingvertebrate social groups has prevailed, possibly as a consequenceof the key role given to ecological constraints as the cause ofgroup formation in cooperative breeding species (Doerr et al.,2007). Our analysis revealed that indeed, the recruitment of pre-vious offspring plays an important role in driving the dynamicsof social groups in some species. However, evidence also sug-gest that emigration and the break up of large sized groups mayaffect the way by which individuals join groups and the over-all dynamics of social groups in rodents. These patterns appearto be consistent when we considered them by the three mainsuborders represented in our sample. Given that the dynamics ofsocial groups are known for a yet limited number of group-livingrodents, it seems premature (and in some cases inappropriate)to conclude that natal philopatry is the main process drivinggroup dynamics in these animals. A representative case that sup-ports this assertion comes from the social dynamics of black-tailedprairie dogs (C. ludovicianus). While natal philopatry of femalesis widespread in these rodents, the splitting of social groups(i.e., coteries) is not uncommon (Manno et al., 2007). Moreover,usurpation of territories between social groups takes place andinvaders may allow some original residents to remain and become

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social members. Our observation that multiple mechanisms seemto influence the dynamics of rodent social groups may mirrorpatterns of other mammals. In particular, social groups of mostprimates include male and female member component that isthe result of emigration and group transfer (Pusey and Packer,1987).

While originally not a major objective of this essay, the anal-ysis also highlighted a need to understand how the underlyingprocesses driving the dynamics of social groups are influencedby the reproductive and social structure across species. Our anal-ysis indicated that the importance of natal philopatry might beinfluenced by reproductive strategies of species. However, futurestudies are called not only to examine this finding in a phyloge-netically controlled framework, but also to use more quantitativemeasures of reproductive strategies. For instance, studies mightuse the extent to which social groups are reproductively skewed,or the extent to which females rear their offspring communally.These approaches would benefit from incorporating the prevalentintra-specific variation linked to these aspects of social behavior.In addition, studies are needed to examine whether the sex com-position within social groups sets the stage for the prevalence of

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alternative mechanisms governing group dynamics. Given the typ-ical male-biased dispersal mode in most mammals (Greenwood,1980), one might predict group transfer through emigration isprobably more important among species whose social units arebased mostly on male–female pairs or in multimale groups. In con-trast, natal philopatry may be a more important (albeit not theonly) process in species with multifemale groups. Variation in thedynamics of social groups and amount of communal care withingroups may also depend on numerous extrinsic factors includingvariation in ecological conditions (Emlen, 1982) and life historycharacteristics (Hatchwell and Komdeur, 2000). Future analysesincluding quantitative measures of habitat conditions in whichstudies were conducted, life history variables (e.g. age, reproduc-tive status) of individuals, and the average relatedness of groupswill likely yield more detailed answers to the questions we presentin this study.

Studies rarely have addressed the relative importance of alterna-tive mechanisms simultaneously, and future long-term field studieson the composition and social group dynamics that examine mul-tiple mechanisms are strongly needed and justified. In particular,different mechanisms may convey different consequences individ-uals living socially. A greater importance of natal philopatry sets thestage for potential indirect fitness benefits and indirect selectionas individuals tend to interact more with close relatives (Clutton-Brock, 2002; West et al., 2002; Nunes, 2007). In contrast, groupswhere unrelated immigrants are common provide a scenario toother forms of cooperation such as reciprocity. Groups in whichunrelated immigrants are common also are more likely to favorthe expression of inter-individual conflicts of interest (although seeWest et al., 2002).

Interestingly, varying consequences linked to different mech-anisms of group dynamics may in turn determine selection toincrease or decrease the occurrence of these mechanisms. Forinstance, mechanisms driving the dynamics of social groups arerelevant to some costs of social living, namely the transmissionof pathogens and disease (Freeland, 1976; Loehle, 1995). Whiledifferences in the social behavior of hosts and life history ofparasites may cause parasite and pathogen infection (Fenton etal., 2002; Wilson et al., 2003; Johnson et al., 2004; Whitemanand Parker, 2004), transmission of contagious pathogens that arepassed via physical contact with infected individuals or feces gen-erally increases with the size of social groups (Cote and Poulin,1995; Tella, 2002). This effect seems to be caused by the higherrate of inter-individual contact (i.e., lateral transmission) in larger

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groups (Brown and Brown, 2004b). Between groups, transmissionis facilitated by territorial interactions and by the permanent andexchange of members between groups (Loehle, 1995; Brown andBrown, 2004a, 2005). Therefore, barriers to dispersal and immigra-tion of foreign members may arise (Freeland, 1976; Loehle, 1995;Wilson et al., 2003), which in turn may result in a greater roleof natal philopatry. Alternatively, inter-individual contact may befavored so that individuals benefit from acquiring an endosym-biont flora that allows them efficient nutrition or defense againstpathogens (Lombardo, 2008). Under such circumstances, disper-sal and immigration of foreign members to groups might befavored, particularly whenever endosymbionts are more easilyobtained from conspecifics rather than from the environment(Lombardo, 2008).

Given the qualitative nature of our analysis, conclusions willremain as preliminary until further comparative or meta-analysesare conducted. Nonetheless, our analysis draws attention to theimportant fact that explanations for the variation and dynamics ofrodent social groups, other than natal philopatry, warrant furtherinvestigation before scholars of social behavior can establish natalphilopatry as a paradigm.

ural Processes 79 (2008) 85–92

Acknowledgements

We are grateful to Robbie Burger, Eileen Lacey, Nancy Solomonand two anonymous reviewers for providing constructive criti-cisms and suggestions on the manuscript. Thanks to Dr. AndreyPuzachenko for providing unpublished insights into the dispers-ing behavior of Spalax microphthalmus. Funding was provided byFONDECYT grant 1060499 to LAE, and by National Science Foun-dation EPSCoR grant #0553910 and Louisiana Board of RegentsResearch and Development grant (LEQSF 2007-09-RD-A-39) toLDH. Other funding sources were the Program 1 of Centro de Estu-dios Avanzados en Ecologıa and Biodiversidad (FONDAP 1501-001),the University of Louisiana at Monroe (ULM) Howard Hughes Med-ical Institute Program, and the Office of Academic Affairs at ULM.

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