Differential treatment of kin is ubiquitous in social animals. Parents often behave preferentially toward their dependent offspring, but species in several taxa also bias behaviour toward 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 manipulations of kinship reliably altered the rate of aggression between individuals in pairs of female tammar wallabies (Macropus eugenii). This effect is attributable to relatedness, rather than to familiarity. Marsupial sociality may be substantially more complex than is currently recognised.

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