Evaluation of aggression

Unless stated otherwise, all tested crickets were mature adult males that had no social contact to conspecifics for at least 24 h (“naive”), after which all known effects of previous social interactions on aggressive behavior have abated (Stevenson and Rillich, 2013). The aggressive behavior of test crickets that received various pre-treatments (see below) was evaluated by matching them in dyadic contests against equally sized males (<5% weight difference) that were induced to be hyper-aggressive by flying them in a wind stream for 3 min shortly before the match (cf. Hofmann and Stevenson, 2000). Since in these experiments the hyper-aggressive crickets always won the contest, they served as a standard, against which the aggressiveness of test crickets could be directly compared in dyadic contests (see also Stevenson and Rillich, 2013, 2015).

For each test, a pair of crickets were placed at opposite ends of a clear Perspex-glass rectangular fighting arena (l. w. h.: 16 × 9 × 7 cm) with a sand-covered floor divided halfway along its length by an opaque sliding door. On removing the door, the animals' interactions follow a stereotyped sequence typical for fights in the field (Alexander, 1961) which we score on a scale of 0-6 to denote aggressive escalation (Hofmann and Stevenson, 2000; Stevenson et al., 2000): Level 0: mutual avoidance without aggression. Level 1: one cricket attacks, the other retreats. Level 2: antennal fencing. Level 3: mandible spreading by one cricket. Level 4: mandible spreading by both crickets. Level 5: mandible engagement. Level 6: grappling, an all-out fight. Contests can finish at any level with the retreat of one opponent, and fight duration was measured to the nearest second with a stopwatch, deducting pauses that occasionally occurred when the animals lost contact. Since the hyper-aggressive individual always won, the level of aggression gives the level to which it had to escalate in order to induce the test cricket to retreat.