Assessment strategies and fighting patterns in animal contests: a role for serotonin?

Abstract

Accurate assessment of the probability of success in an aggressive confrontation with a conspecific is critical to the survival and fitness of the individuals. Various game theory models have examined these assessment strategies under the assumption that contests should favor the animal with the greater resource-holding potential (RHP), body size typically being the proxy. Mutual assessment asserts that an individual can assess their own RHP relative to their opponent, allowing the inferior animal the chance to flee before incurring unnecessary costs. The model of self-determined persistence, however, assumes that an individual will fight to a set personal threshold, independent of their opponent’s RHP. Both models have been repeatedly tested using size as a proxy for RHP, with neither receiving unambiguous support. Here we present both morphological and neurophysiological data from size-matched and mismatched stalk-eyed fly fights. We discovered differing fighting strategies between winners and losers. Winners readily escalated encounters to higher intensity and physical contact and engaged in less low-intensity, posturing behaviors compared with losers. Although these fighting strategies were largely independent of size, they were associated with elevated levels of 5-HT. Understanding the neurophysiological factors responsible for mediating the motivational state of opponents could help resolve the inconsistencies seen in current game theory models. Therefore, we contend that current studies using only size as a proxy for RHP may be inadequate in determining the intricacies of fighting ability and that future studies investigating assessment strategies and contest outcome should include neurophysiological data.

Winner–loser effects overrule aggressiveness during the early stages of contests between pigs

“…Confinement in the contest arena may also have influenced contest strategies,weaker individuals had limited opportunity to retreat and therefore might have used attack as a form of defence. The absolute and relative RHP of opponents influences later stages of a contest, such as persistence in mutual fighting 29 (for assessment strategies used in the full contests of the current study, see 34 ), although contest outcome is weakly predicted by body weight in pigs 65 and fighting style can also be influenced by neurophysiological differences 66 . Contest decisions were highly impacted by the sex of the C2 contestants, but not by a contrast between the C1 and C2 opponent sex or size, implying that winner–loser effects are generalised to future contests 19 rather than associated with specific characteristics of the opponent.…”

Editorial The role of monoamines in modulating behavior

“…2014b ), they aim to distinguish between two commonly used models of assessment strategies, self-determined persistence and mutual rival assessment, as described by Taylor and Elwood (2003) . Bubak et al. (2016) clearly demonstrate that assessment strategies, decisions regarding whether to escalate aggression or retreat, and contest outcome are better predicted when behavioral and neurophysiological measurements are incorporated alongside morphological indices of resource-holding potential.…”

“…The issue starts with a synthetic contribution by Bubak et al. (2016) , who use a well-established model system for aggression, stalk-eyed flies ( Bubak et al.…”

Sex differences in aggression: Differential roles of 5-HT2, neuropeptide F and tachykinin

“…Serotonin (5-HT) appears to promote aggression in invertebrates [1,2], in contrast to the largely inhibitory effect seen in vertebrates ([3], but see [4]). Much of the empirical support for this dichotomy comes from studies using arthropod invertebrates, with increased expression of overt aggressive behavior and greater willingness to engage in conflict seen in decapod crustaceans [5–8], crickets [9], ants [10,11], and dipteran flies [12–15] following pharmacological or genetic elevations of 5-HT at the systemic level.…”

“…Interestingly, contest duration remains the same whether the smaller male is treated with 5-HT or not [23]. In contrast, when contestants are of equal size, both aggression intensity and contest duration appear to be modulated as a function of the difference in brain 5-HT between opponents [2]. In other words, size-matched opponents with closer brain 5-HT concentrations will engage in prolonged high intensity contests regardless of whether 5-HT has been increased in one male, with exogenous manipulation only making contests shorter and less intense if it causes brain 5-HT to be substantially elevated above that of the opponent [2].…”