Giant Australian cuttlefish use mutual assessment to resolve male-male contests

Game theory models provide a useful framework for investigating strategies of conflict resolution in animal contests. Model predictions are based on estimates of resource-holding potential (RHP) and vary in their assumptions about how opponents gather information about RHP. Models can be divided into self-assessment strategies (energetic war-of-attrition, E-WOA; cumulative assessment model, CAM) and mutual assessment strategies (sequential assessment model, SAM). We used laboratory-staged contests between male giant Australian cuttlefish, Sepia apama, to evaluate RHP traits and to test game theory models. Mantle length was a key indicator of RHP because it predicted contest outcome, whereby larger individuals were more likely to win a contest. Winners and losers did not match behaviours, ruling out the E-WOA. There was no relationship between contest outcome, duration and escalation rates, arguing against the CAM. Persistence to continue a contest was based on RHP asymmetry, rather than loser and/or winner RHP, providing support for the SAM. Motivation to fight was determined from a male's latency to resume a contest following the introduction of a female during a contest. The latency to resume a contest was negatively related to the size of the focal male and positively related to the size of their opponent. These results show that competing males are able to gather information concerning RHP asymmetries, providing support for mutual assessment. Furthermore, males showed significant behavioural differences in their responses to relatively larger than to relatively smaller opponents. Using an integrative approach, our study provides a well-substantiated example of mutual assessment.