Kin Discrimination in a Macropod Marsupial Daniel T. Blumstein*, , Jodie G. Ardron ,à & Christopher S. Evansà *Department of Organismic Biology, Ecology, and Evolution, University of California, Los Angeles, CA, USA;  The Cooperative Research Centre for Conservation and Management of Marsupials, Macquarie University, Sydney; àAnimal Behaviour Laboratory, Department of Psychology, Macquarie University, Sydney, Australia Abstract Differential treatment of kin is ubiquitous in social animals. Parents often behave preferentially towards their dependent offspring. Species in several taxa also bias behaviour towards non-descendent kin. This latter phenomenon has not been demonstrated in marsupials, which are reportedly less social than eutherian mammals. We report the first evidence of non-parental kin-biased behaviour in a macropodid marsupial. Experimental pairing of individuals based on kinship reliably altered the rate of aggression between individuals in pairs of female tammar wallabies (Macropus eugenii). This effect is probably attributable to relatedness rather than to familiarity. Marsupial sociality may be substantially more complex than is currently recognized. Corresponding author: Daniel T. Blumstein, Department of Organismic Biology, Ecology and Evolution, 621 Charles E. Young Drive South, University of California, Los Angeles, CA 90095-1606, USA. E-mail: marmots@ucla.edu Introduction Animals in a variety of vertebrate and invertebrate taxa differentiate kin from non-kin (Holmes and Sherman 1983; Halpin 1991). Such kin discrimination is an important mechanism allowing individuals to maximize their inclusive fitness (Hamilton 1964). Kinship influences reproductive decisions, thus preventing inbreeding and facilitating optimal outbreeding (Shields 1993; Pusey and Wolf 1996). Kinship may also influence decisions about time or energy allocation. In some species, helping behaviour varies as a function of relatedness (Emlen 1991). Similarly, relatedness has been reported to influence the rate of aggressive and other social behaviour (Holmes 1986; Pfennig 1999). Given the potential functional importance of kin discrimination, it is surprising that researchers Ethology 108, 815—823 (2002) Ó 2002 Blackwell Verlag, Berlin ISSN 0179–1613 U. S. Copyright Clearance Center Code Statement: 0179-1613/2002/1089–0815/$15.00/0 www.blackwell.de/synergy