Aggregation is a costly strategy for any species, because individuals must compete for resources such as food, shelter and mating opportunities. Hamilton (1971) provides the best-established explanation of aggregation, which is security. For reasons ranging from increased vigilance to cooperative mobbing of aggressors, living in a group can provide benefits as well as costs. But security may not be the only explanation for aggregation. Aggregation can also occur by chance rather than choice, as individuals may come to the same location simply be seeking the same resources, and it may not be worth eort of driving others away from these. One strong indication that there may be more than one selective pressure for aggregation is when a group can be described at multiple levels of hierarchy. A fission- fusion (FF) society is characterised this way. FF societies consist of both parties — relatively small groups travel together, and a community or troop from which parties are drawn. Party size may well be determined by security, but in some species it is not clear that the entire community ever aggregates at the same time. Lehmann et al. (2007) suggest that knowledge exchange may be one selective pres- sure for community-level aggregation in fission-fusion species where party composition is flexible. This is an attractive theory in species like great apes where some commu- nities are known to posses behaviour not seen in other communities. Such 'culture' is particularly obvious when the communities are completely isolated from each other by geographic features such as rivers, but otherwise live in identical ecosystems (Whiten et al., 1999). Here we present new preliminary results concerning aggregation in Asiatic wild asses Equus hemionus. In examining available data simulating multiple hypotheses on their social behaviour, we have identified potential new costs as well as benefits. Our collaboration began because Kaczensky and colleagues were looking for a mecha- nism that could explain the exceptional ability of Asiatic asses to exploit unexpected resource bonanzas in the form of transient oases caused by rainfall in the Gobi desert. Mongolian or Asiatic wild asses are primarily adapted to arid desert steppes and semi-deserts of the Gobi. They can venture well away from water sources and are able to exploit resources that vary in space and time. Asiatic wild asses also live in