Many associations between organisms, both intra-and interspecific, can be usefully regarded as 'producer/scrounger' (P/S) relationships. One or more individuals or species (scroungers) within the association in some way use the behavioural or physiological investment of others (pro- ducers) to reduce their costs of obtaining a limited resource. Examples are legion: many parasites take up more or less permanent residence in or on a host individual...