To determine whether animals assess each other's fighting ability in contests, researchers usually regress contest duration over the sizes of the contestants. The predominant trend in recent studies is for the contest duration to correlate positively with the size of the smaller opponent but to have no obvious relationship with the size of the larger opponent. This indicates that animals make contest decisions based on their own abilities ('self assessment') and displaces the once-popular belief that they assess their opponents ('mutual assessment'). These tests, however, are based on the implicit but never stated assumption that animals adopt only one assessment approach throughout an entire contest. By examining the contest behaviours of a killifish, we show that (1) the fish adopt mutual assessment at earlier stages when deciding whether to escalate the contest from the mutual display to the attack phase and (2) once a contest is escalated, the fish switch to self assessment to decide how long to escalate. Our results show that individuals may adopt multiple assessment approaches in one contest; contest behaviours in different stages (where applicable) of a contest should be analysed separately to better elucidate animal contest assessment strategies.
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