Individual recognition, dominance hierarchies and winner and loser effects

Winner and loser effects are defined as an increased probability of winning an aggressive interaction at time T, based on victories at time T-1, T-2, etc., and an increased probability of losing at time T, based on losses at time T-1, T-2, etc., respectively. Prior theoretical work on dominance hierarchy formation has demonstrated that when players are not capable of individual recognition, loser effects always produce a clear top-ranked (alpha) individual, but all other ranks in a group remain unclear; whereas winner effects always produce strict linear hierarchies in which the rank of each individual is clear. Paradoxically, however, when individual recognition--a phenomenon long thought to stabilize hierarchies--is possible, winner and loser effects have no impact on the probability of forming strict linear hierarchies.     

N-person games and the evolution of co-operation: a model based on predator inspection in fish

Two N-person game theoretical models examining the evolution of co-operation during predator inspection in fish are presented. Predator inspection occurs in small shoals of fish, in which one to a few individuals, the "inspectors" (co-operators) break away from the shoal and cautiously approach a predator to obtain information on this potential danger. In the models presented here, remaining with the shoal and not inspecting is considered an act of defection. Both model I and II produce a stable internal polymorphism of inspectors and noninspectors. While the equilibrial frequency of inspectors can be low (i.e. less than 10%) at large shoal size, the proportion of shoals containing any inspectors--and therefore exhibiting the inspection behavior--is much greater. Both models presented here, and N-person games in general are equivalent to intrademic group selection models of evolution in structured populations, in which shoals are trait groups and co-operation evolves by between-shoal selection. While the results are cast in terms of predator inspection, the model itself is general and applies to any multi-group scenario where co-operators benefit entire groups at their own expense. The results presented here add to the mounting theoretical and empirical evidence that co-operation is frequently not a pure evolutionarily stable strategy, and that many metapopulations should be polymorphic for both co-operators and defectors.     

Tendency to inspect predators predicts mortality risk in the guppy (Poecilia reticulata)

Although predator inspection behavior in fishes has become amodel system for examining game theoretical strategies suchas Tit for Tat, the direct costs of inspection behavior havenot been quantified. To begin quantifying such costs, I conductedan experiment that examined mortality due to predation as afunction of predator inspection in the guppy (Poecilia reticulata).Before being subjected to a "survivorship" experiment, guppieswere assayed for their tendency to inspect a predator. Groupswere then composed of six guppies that differed in their tendencyto inspect. These groups were placed into a pool containinga predator, and survivorship of guppies with different inspectiontendencies was noted 36 and 60 h later. Results indicate thatindividuals that display high degrees of inspection behaviorsuffer greater mortality than their noninspecting shoalmates.     

Interpopulational differences in the use of the Tit-For-Tat strategy during predator inspection in the guppy,Poecilia reticulata

Summary Prior work (Dugatkin, 1991a) has argued that guppies, originating from an area of high predation pressure, employ the Tit-For-Tat strategy during predator inspection visits. Guppies from the high predation site show all three characteristics associated with Tit-For-Tat; nice, retaliatory and forgiving behaviour. Since predation is the major selective force favouring Tit-For-Tat during predator inspection, we predicted fish from an area of low predation would not display the Tit-For-Tat strategy. Our results confirm this prediction; males display retaliatory, but not nice and forgiving behaviour, while females display none of the three characteristics. Thus as predation pressure changes, so too does the conditional nature of the cooperative strategy used during predator inspection.     

Play and the evolution of fairness: a game theory model

Bekoff [J. Consci. Stud. 8 (2001) 81] argued that mammalian social play is a useful behavioral phenotype on which to concentrate in order to learn more about the evolution of fairness. Here, we build a game theoretical model designed to formalize some of the ideas laid out by Bekoff, and to examine whether ‘fair’ strategies can in fact be evolutionarily stable. The models we present examine fairness at two different developmental stages during an individual's ontogeny, and hence we create four strategies—fair at time 1/fair at time 2, not fair at time 1/not fair at time 2, fair at time 1/not fair at time 2, not fair at time 1/fair at time 2. Our results suggest that when considering species where fairness can be expressed during two different developmental stages, acting fairly should be more common than never acting fairly. In addition, when no one strategy was evolutionarily stable, we found that all four strategies we model can coexist at evolutionary equilibrium. Even in the absence of an overwhelming database from which to test our model, the general predictions we make have significant implications for the evolution of fairness.     

Antibiotic resistance and the evolution of group-beneficial traits. II: a metapopulation model

Inspired by the evolution of antibiotic resistance in bacteria, we have developed a model that examines the evolution of "producers" (who secrete a substance that breaks down antibiotics) and non-producers. In a previous study, we found that frequency-dependent selection could favor an intermediate frequency of producers in a single, large population. Here we develop a metapopulation model that examines the evolution of producers and non-producers. Our results indicate that in a metapopulation with many groups, each of size N, the equilibrial frequency of producers decreases with group size. Even when N is high (e.g. 150 individuals/group), however, a significant frequency of producers is still predicted. We also found that the equilibrial frequency of producers increases as the minimum numbers of producers necessary to provide protection to non-producers increases. Lastly, increasing the benefit/cost ratio (b/c) for producers increases their equilibrial frequency.     

Altruism,tit for tat and ‘outlaw’ genes

Summary In a prior study we combined game theory and inclusive fitness models to examine whether the guarded altruism that can evolve among non-relatives (tit for tat, TFT) might also evolve among close relatives, supplanting unconditional altruism. In most cases, TFT replaced unconditional altruism in family-structured models. Even when TFT is selected at a single locus, however, by withholding altruism from non-reciprocating relatives it may qualify as an outlaw from the standpoint of modifier genes at other loci. Here we examine this possibility with a series of haploid, two-locus models in which a modifier gene transforms TFT into unconditional altruism. The modifier allele spreads to fixation whenever Hamilton's Rule is satisfied, resulting in an unconditional altruist replacing the TFT strategy. As such, TFT may be regarded as an outlaw vulnerable to suppression by alleles at other loci.     

Extrinsic effects, estimating opponents' RHP, and the structure of dominance hierarchies

We examined the impact of winner and loser effects on dominance hierarchy formation when individuals are capable of estimating their opponent's resource holding power (RHP). The accuracy of such estimates was a variable in our simulations, and we considered cases in which all individuals err within the same bounds, as well as cases in which some individuals consistently overestimate, while others consistently underestimate their opponent's fighting RHP. In all cases, we found a clearly defined linear hierarchy. In most simulations, the vast majority of interactions were 'attack-retreats', and the remainder of interactions were almost all 'fights'. Error rates had no effect on the linearity of the hierarchy or the basic attack-retreat nature of interactions, and consistent over and underestimation did not affect the ultimate position of an individual in a hierarchy.     

Juvenile three-spined sticklebacks avoid parasitized conspecifics

Synopsis Juvenile three-spined sticklebacks,Gasterosteus aculeatus, were given a series of four choice tests to determine whether they avoided schools of conspecifics in which individuals were parasitized with the ectoparasiteArgulus canadensis. Results from these tests indicate that juvenile sticklebacks can avoid schools of parasitized conspecifics. Furthermore, parasites alone did not elicit an avoidance response, suggesting that it is both the presence of the parasite and its effect on stickleback behavior that causes avoidance of parasitized individuals.     

Breaking up fights between others: a model of intervention behaviour

To examine when and why animals break up fights between others in their group, I modelled whether ''winner'' and ''loser'' effects might be one element driving the evolution of intervention behaviour. I considered one particular type of intervention: when the intervener simply breaks up fights between two others, but does not favour either party in so doing. When victories at time T + 1 are more likely given a victory at time T (i.e. winner effects), intervention is often favoured. Intervention is favoured in these circumstances because the intervening party in essence stops others from ''getting on a roll'' and climbing up any hierarchy that exists. However, when loser effects alone are at work (defeats at time T + 1 are more likely given a defeat at time T), breaking up fights between others is never selected. If both winner and loser effects are operating simultaneously, then the likelihood of intervention behaviour evolving is a function of the relative strength of these two effects. The greater the winner effect relative to the loser effect, the more likely intervention behaviour is to evolve.