Males of the butterfly Pararge aegeria defend sun spot territories on the forest floor as a strategy to locate females: the defence involves a spiral flight between two competing males. The success of any particular fight has been interpreted as following the simple rule 'resident wins, intruder loses' as predicted by game theory models. Here we use field experiments to show that temperature has a marked effect on the territorial defence of these ectothermic insects. The spiral flights were true contests, not conventional signals, so the flight endurance of the butterflies was an important factor in determining the result of the contest. Butterflies that were warmer could fly for longer and were more likely to win a contest; therefore temperature was an important component of contest asymmetry, and resident butterflies that increased their body temperatures by basking in sun spots were more likely to win than the intruding males which might have cooled down during flight around the site. The rule that the resident always wins is not disputed; however, the causal factors underpinning the rule are questioned.
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