• L.A. Dugatkin. Can kinship ever inhibit the evolution of cooperation? EcoScience 4, 460-464 (1997)
  ABSTRACT: A 2-person game-theoretical model of the evolution of cooperation is presented. The model compares the fate of cooperation when pairs are formed by randomly selecting individuals and when groups are composed of close kin. Results indicate that for some cost/benefit scenarios, kinship can inhibit, rather than facilitate, the evolution of cooperation. Although kinship increases between-group variance (and thus facilitates cooperation), it also causes cooperators to continue to cooperate when they wouldn't otherwise, thereby providing a large payoff to "free-riding" cheaters. The model presented here, in conjunction with other recent theoretical developments, should serve as a caveat that although kinship often facilitates sociality, under some reasonable conditions it may, in fact, do just the opposite. Rather than simply assuming that relatedness per se selects for sociality, we need to step back and consider the population structure, as well as the rules that are available to animals, when examining the evolution of cooperation, and more generally, social behavior.
• L.A. Dugatkin. The evolution of cooperation. Bioscience 47, 355-362 (1997)
  ABSTRACT: Four conceptual frameworks have been developed for studying the evolution of cooperation in animals—kin-selected cooperation, reciprocity, byproduct mutualism and group selection—and there is now ample empirical evidence available to put these models to the test. Here, I outline these four paths and provide two studies highlighting each. I then close with some thoughts of where the study of animal cooperation might head in the future.
• L.A. Dugatkin. Winner and loser effects and the structure of dominance hierarchies. Behavioral Ecology 8, 583-587 (1997)
  ABSTRACT: In the literature on dominance hierarchies, ``winner'' and ``loser'' effects usually are defined as an increased probability of winning at time T, based on victories at time T-1, T-2, etc., and an increased probability of losing at time T, based on losing at T-1, T-2, etc., respectively. Despite some early theoretical work on winner and loser effects, these factors and how they affect the structure of dominance hierarchies have not been examined in detail. A computer simulation was developed to examine winner and loser effects when such effects are independent of one another (as well as when they interact) and when combatants assess each other's resource holding power. When winner effects alone were important, a hierarchy in which all individuals held an unambiguous rank was found. When only loser effects were important, a clear alpha individual always emerged, but the rank of others in the group was unclear because of the scarcity of aggressive interactions. Increasing winner effects for a given value of the loser effect increases the number of individuals with unambiguous positions in a hierarchy and the converse is true for increasing the value of the loser effect for a given winner effect. Although winner and loser effects have been documented in a number of species, no study has documented both winner and loser effects (using some controlled, pairwise testing system) and the detailed nature of behavioral interactions when individuals are in groups. I hope the results of this model will spur on such studies in the future.
• L.A. Dugatkin. Breaking up fights between others: a model of intervention behaviour. Proceedings of the Royal Society of London B 265, 433-437 (1998)
  ABSTRACT: In order to examine when and why animals break up fights between others in their group, I modeled whether ``winner'' and ``loser'' effects might be one element driving the evolution of intervention behaviour. I considered one particular type of intervention - namely, when the intervener simply breaks up fights between two others, but does not favor 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 favored. Intervention is favored 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.
• L.A. Dugatkin. A model of coalition formation in animals. Proceedings of the Royal Society of London B 265, 2121-2125
  ABSTRACT: A simple three player model is presented for the evolution of coalitions. The model demonstrates that under certain conditions ``winner'' and ``loser'' effects both favour coalition formation. Increasing the strength of loser effects or winner effects, or the strength of an individual's position in the hierarchy, makes coalition formation in general more likely, while increasing the costs of giving aid does the opposite.
  ----The model does not assume any form of reciprocity, but rather examines whether reciprocity emerges from the model itself. When either winner or loser effects exist, reciprocal coalition formation (e.g., i helps j against k and j helps i against K) between b and a or a and g were possible, reciprocal aid giving between g and b was never favoured. Thus, we have the counterintuitive result that while a coalition between the two lowest members of a hierarchy against the dominant individual is possible (as selection may favour g aiding b against a), such a coalition is not predicted to be reciprocal in kind.
  ----Interpopulational comparisons examining winner/loser effects and coalition formation would allow a test of many of the models most basic predictions. Unfortunately, most work on coalitions has been undertaken in primates, while work on winner and loser effects has focused on rodents and more recently in fish. Hopefully, the model presented here will spur future work looking at all these factors simultaneously in primates, fish, and many other taxa.