Charles Darwin recognised that the struggle for existence was most severe between males of the same species. After all, they compete for the same foods, sexual partners and territory. Although there’s much to be won, fighting bears the risk of injury and losing life or limb. Hence, for aggression to be of any advantage, and even to have evolved in the first place, animals must somehow weigh up the potential costs and benefits to decide when to fight, or when it may be wiser to flee. Although much of how they do this remains a mystery, researchers from Leipzig University have lifted the secret in field crickets by manipulating brain chemicals and sensory information exchanged during fighting. Why crickets? “Well, for one,” says Paul Stevenson, who led the study with Jan Rillich, “cricket fights are spectacular and easily quantified, but more importantly their less complex brains make decisions instinctively, that is without conscious reasoning, and are accordingly easier to investigate.” They first found that positive experiences, such as possession of a valuable resource, recruit the brain chemical octopamine (the insect equivalent to noradrenaline), which increases their tendency to keep on fighting. In effect, octopamine raises their threshold to flee – or basically the amount of punishment a cricket is prepared to endure. During a fight then, crickets add up the sensory impact of their opponent’s actions (threats, bites etc.) and flee when the critical threshold is reached. This occurs due to the release of nitric oxide (NO), which gives the command. NO, in turn, recruits the messenger chemical serotonin, which causes losers to behave submissively for some time after fighting. Intriguingly, the same types of brain chemicals also influence aggression in mammals including humans, but their natural role in controlling the decision to fight or flee is still unclear. Stevenson’s team in Leipzig is currently investigating the effects of repeated, intermittent defeats, which induce depression-like symptoms in humans, mice, and apparently also crickets.