title: By-product mutualism with evolving common enemies.
abstract: The common-enemy hypothesis of by-product mutualism states that organisms cooperate when it is in their individual interests to do so, with benefits for other organisms arising as a by-product; in particular, such cooperation is hypothesized to arise when organisms face the common enemy of a sufficiently adverse environment. In an evolutionary game where two defenders can cooperate to defend a common resource, this paper analyzes the common-enemy hypothesis when adversity is endogenous, in that an attacker sets the number of attacks. As a benchmark, we first consider exogenous adversity, where adversity is not subject to evolution. In this case, the common-enemy hypothesis is predicted when the degree of complementarity between defenders' defensive efforts is sufficiently low. When the degree of complementarity is high, the hypothesis is predicted only when cooperation costs are high; when cooperation costs are instead low, a competing hypothesis is predicted, where adversity discourages cooperation. Second, we consider the case of endogenous adversity. In this case, we continue to predict the competing hypothesis for a high degree of complementarity and low cooperation costs. The common-enemy hypothesis, however, only continues to be predicted for the lowest degrees of complementarity.
authors.affiliation: Utrecht University School of Economics, Utrecht University, Utrecht, The Netherlands. Electronic address: k.dejaegher@uu.nl.
authors.forename: Kris
authors.initials: K
authors.lastname: De Jaegher
mesh_terms: D000818:Animals; D005075:Biological Evolution; D003163:Competitive Behavior; D003299:Cooperative Behavior; D000071251:Extreme Environments; D005716:Game Theory; D006801:Humans; D012919:Social Behavior; D013559:Symbiosis
keywords: By-product mutualism; Common enemies; Evolutionary game theory; Harsh environments