Molecular Ecology (2007) 16, 199–208 doi: 10.1111/j.1365-294X.2006.03131.x © 2006 The Authors Journal compilation © 2006 Blackwell Publishing Ltd Blackwell Publishing Ltd Does soil type drive social organization in southern hairy-nosed wombats? FAITH M. WALKER, ANDREA C. TAYLOR and PAUL SUNNUCKS Australian Centre for Biodiversity Analysis, Conservation and Management, School of Biological Sciences, Monash University, Clayton, Victoria 3800, Australia Abstract Spatiotemporal distributions of key resources are hypothesized to underpin sociobiological patterns. Burrow availability and quality is of paramount importance to fossorial animals. The southern hairy-nosed wombat (Lasiorhinus latifrons) burrows in both hard and friable soils. Theoretical and empirical studies suggest that the harder substrate should promote closer geographical clustering of burrows than in softer soils. Clustered burrows are expected to be associated with larger group sizes. If sociality is driven by constraint rather than advantage, patterns of spatial and temporal distribution of animals within and among groups may show indications of avoidance or even antagonism, and ‘making the best of a bad job’ via positive kin associations to offset the disadvantages of high-density living. To test these ideas, we compared warren relatedness and social structure of L. latifrons on friable soils (Nullarbor Plain) and hard calcrete (Brookfield Conservation Park, BCP). Individuals were sampled by noninvasive collection of hairs for genotyping to identify individuals and to estimate their space-use and associative behaviour with respect to relatedness. Burrows in calcrete were indeed more clumped, and warren and group size larger. Differences in spatiotemporal organization and relatedness structure between sites were in the expected direction: (i) Nullarbor males associated and shared warrens less than at BCP; and (ii) Nullarbor spatial relatedness patterning data were not consistent with proposed female breeding dispersal, in contrast to those at BCP. Under Nullarbor (low density) conditions, cooperation or tolerance between males may be less advantageous, and accessing or digging burrows should be less of a constraint for juvenile females. Keywords: fossorial, marsupial, microsatellite, resource structure, sociobiology Received 5 May 2006; revision received 26 July 2006; accepted 21 August 2006 Introduction Studies attributing social evolution to environmental constraints are rare (Lott 1991; Foster & Endler 1999). Species exhibiting social plasticity are most informative regarding the evolution of social systems. Within such species, ecological variables can alter strategies employed by interacting conspecifics (Maher 2004; Schradin & Pillay 2005). Genetic relatedness is an important driver of levels of social interactions, and advances in molecular tools in the last decade or so allow kin theory to be integrated with assessment of resource and habitat structures (Travis et al. 1995; Faulkes et al. 1997; Cornuet et al. 1999; Stow & Sunnucks 2004; Randall et al. 2005; Waits & Paetkau 2005). Soil type is a particularly influential ecological variable for fossorial (burrowing) animals because substrate can constrain burrow distribution and promote clustering by influencing the relative energetic costs of digging new burrows and expanding existing ones (Cowan 1987; Lovegrove 1989; Lacey & Sherman 1997; Ebensperger & Cofré 2001). For instance, in a classic example of sociality by constraint, soil type was found to influence rabbit (Orycto- lagus cuniculus) social structure and population dynamics (Cowan & Garson 1985). In stable soils, burrows were clus- tered into complex warrens, were a limited resource for females, and were associated with substantial variation in male reproductive success. In contrast, rabbits on sandy soils (in which burrows are easy to dig) tended to live as Correspondence: Faith M. Walker, 2700 Woodlands Village Blvd., Ste. 300-407, USA. Fax: (928) 773-9201; E-mail: faith.walker@sci.monash.edu.au