An~m.Behav., 1981, 29, 221-240 ROLE ASSESSMENT, RESERVE STRATEGY, AND ACQUISITION OF INFORMATION IN ASYMMETRIC ANIMAL CONFLICTS BY G. A. PARKER* & D. I. RUBENSTEIN King's College Research Centre, King's College, Cambridge CB2 1 ST Abstract. It was formerly argued that alternative evolutionarily stable strategies (ESSs) are possible for animal contests characterized by some asymmetry that can be perceived with perfect accuracy. Where roles A and B refer to the asymmetry between opponents, ESSs are: 'fight when A, retreat when B', and vice versa. Either can be an ESS, but only if the 'reserve strategy' ( ---- what an animal does when it fights) is sufficiently damaging. We examine the 'war of attrition' (winner ~ opponent that persists longer). In a population at either ESS, reserve strategy is never normally shown; it is therefore subject to drift unless the selective action of rare individuals which break the convention is considered. These could arise either by mutation or by mistakes in role assessment. When mutations and mistakes simply specify that occasionally an animal fights when it 'should' retreat, selection adjusts reserve strategy to a level where only one ESS (the 'commonsense' ESS) is possible, if the asymmetry is relevant to payoff. Thus for asymmetries in fighting ability or resource value, the individual with the lower score will retreat. However, we are particularly concerned with cases where both payoff-relevant aspects (fighting ability and resource value) are asymmetric. If opponents sustain contest costs at rates KA and KB, and their resource values are VA and VB, an 'optimal assessor' strategy defined by the interaction between the two asymmetries, is a unique ESS. It obeys the rule 'fight on estimating role A, where VA/KA VB/KB; retreat in B'. If mistakes can occur in both roles, but are very rare, the ESS is not funda- mentally altered though there will be infinitesimal tendencies for persisting in role B. Selection to im- prove assessment abilities intensifies as abilities improve, but is weak if roles A and B are rather similar. Over a range of similarity between roles, an 'owner wins' convention may be adopted if ownership correlates positively with role A and an individual cannot tell when it would otherwise pay him to break the convention. We also examine a contest in which information about roles can be acquired only during a contest itself, and at a cost. Much depends on the rate at which information is acquired relative to the rate at which costs are expended, and on whether contests normally escalate in intensity, remain at the same level, or de-escalate. Selection favours short contests when costs are high relative to resource value, where the outcome of a round contains much information about fighting ability, and where the actual disparity in fighting ability is large. Introduction J. Maynard Smith's pioneering approach to the evolutionary theory of animal fighting created a landmark in sociobiology (Maynard Smith 1972; Maynard Smith & Price 1973). It also instigated the development of the evolutionarily stable strategy (ESS) concept, an idea that has major implications for the whole of evolutionary biology. A strategy is an ESS if, when fixed in a population, it cannot be invaded genetically by any alternative strategy. In early models of animal contests, two identi- cal opponents were assumed to be in dispute over a resource of equal value to each individual (Maynard Smith 1972; Maynard Smith & Price 1973; Maynard Smith 1974). The game is thus assumed to be symmetric. But in most competi- *Present address: Department of Zoology, University of Liverpool, Liverpool L69 3BX. rive situations asymmetries play a major role in determining the outcomes of contests. Two sorts of asymmetries are of importance in animal fights (Parker 1974; Maynard Smith & Parker 1976). (i) Asymmetry in fighting ability or 'resource holding potential' (RHP). Individuals may differ in some intrinsic features (e.g. size, strength, weaponry, etc.) or in some extrinsic feature (e.g. 'postural' differences to do with the relative placing of the two opponents). RHP asym- metries affect the probable allocation of contest costs. (ii) Asymmetries in resource (reward) value. The contested resource may be worth (in terms of fitness units) more to one opponent than the other. Maynard Smith (1974) noted that an asym- metry that is entirely uneorrelated with either of 221