Abstract
Ritualized fighting between conspecifics is an inherently dangerous behavioral strategy, optimized to secure limited resources at minimal cost and risk. To be adaptive, potential rewards, and costs of aggression must be assessed to decide when it would be more opportune to fight or flee. We summarize insights into the proximate mechanisms underlying this decision-making process in field crickets. As in other animals, cricket aggression is enhanced dramatically by motor activity, winning, and the possession of resources. Pharmacological manipulations provide evidence that these cases of experience dependent enhancement of aggression are each mediated by octopamine, the invertebrate counterpart to adrenaline/noradrenaline. The data suggest that both physical exertion and rewarding aspects of experiences can activate the octopaminergic system, which increases the propensity to fight. Octopamine thus represents the motivational component of aggression in insects. For the decision to flee, animals are thought to assess information from agonistic signals exchanged during fighting. Cricket fights conform to the cumulative assessment model, in that they persist in fighting until the sum of their opponent’s actions accumulates to some threshold at which they withdraw. We discuss evidence that serotonin, nitric oxide, and some neuropeptides may promote an insect’s tendency to flee. We propose that the decision to fight or flee in crickets is controlled simply by relative behavioral thresholds. Rewarding experiences increase the propensity to fight to a level determined by the modulatory action of octopamine. The animal will then flee only when the accumulated sum of the opponent’s actions surpasses this level; serotonin and nitric oxide may be involved in this process. This concept is in line with the roles proposed for noradrenaline, serotonin, and nitric oxide in mammals and suggests that basic mechanisms of aggressive modulation may be conserved in phylogeny.
Highlights
AGGRESSION AND THE DECISION TO FIGHT OR FLEE There are many forms of aggression but no uniform definition
What are the proximate mechanisms controlling aggression? How do experiences such as resource possession determine “aggressive motivation” and how is this encoded in the nervous system? How do animals “assess” agonistic signals and by which means do they influence the expression of aggressive behavior? Just how exactly do animals make the “decision to fight or flee”?
It has been shown that the tendency to fight in insects is promoted by the amine octopamine, the analog of noradrenaline
Summary
CRICKETS AS MODEL ANIMALS FOR THE STUDY OF AGGRESSION In this review we summarize insights into these questions gained from studies on insects, primarily field crickets (Gryllus bimaculatus de Geer). Fighting establishes clear winners and losers, whereby winners sing a rivalry song, and the losers avoid other males for hours This is just one example of many illustrating that aggression in crickets is experience dependent. The data suggest that octopamine acts as a modulator that promotes the tendency of insects to fight (Stevenson et al, 2005) and perform agonist acts such as lunging (Hoyer et al, 2008; Zhou et al, 2008) and mandible spreading (Rillich and Stevenson, 2011) This basically corresponds to the modulatory role of octopamine in promoting cholinergic initiating of motor behaviors such as flying (Buhl et al, 2008). This winner effect is transient and lasts www.frontiersin.org
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