Dung fly struggles: Evidence for assessment strategy

1. Dung fly males guard their females from other males while the female oviposits. Occasionally an attacker grasps the female and a struggle for the possession of female begins. These contests obey ‘war of attrition’ rules. Outcomes and durations of struggles were analysed with respect to the various asymmetries present in the contests, using a technique that tests how the two opponents shift their fighting strategies in accordance with variations in a particular asymmetric feature. 2. In struggles, the attacking male is almost invariably larger than the guarding male (the ‘owner’). Males do not appear to play strategies that are fixed in relation to their own size, but play strategies that depend on their size relative to that of their opponent. No significant difference was found between struggle durations for two small opponents and those for two large opponents, though both were significantly different from large versus small opponents. As the relative size of the owner increases, the persistence of the attacker decreases significantly. That of the owner probably increases (though this is not proven). 3. The female's size relative to the owner's size also exerts a significant effect on the outcome. The larger the female, the greater the probability of takeover. This arises from a greater persistence of the attacker. The ‘resource holding power’ (RHP) of the owner is therefore considered to be influenced both by his size relative to that of the attacker and by his size relative to that of this female. In addition, the owner may have an inherent RHP advantage due to his initial grasp on the female. This may contribute to the fact that only 20–25% of struggles end in takeover despite the fact that the attacker is larger than the owner. 4. Another effect that must contribute to the low frequency of take-overs concerns the fact that the mean value of winning (the resource value) is higher for the owner than for the attacker. Resource value relates mainly to the number of eggs that remain to be laid at the time of the struggle. Though the owner may be unable to monitor this directly, there is a strongly significant negative correlation between owner's persistence and the time he had spent guarding. Thus he escalates as resource value increases. In contrast, attackers appeared to be unable to estimate resource value; their persistence was independent of the female's egg content. 5. The data strongly support the notion that dung flies assess RHP and resource value in contests and alter their fighting strategies accordingly. We discuss the data in relation to recent theoretical developments in the study of assessment strategy.