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By-product mutualism with evolving common enemies.

Kris De Jaegher1

  • 1Utrecht University School of Economics, Utrecht University, Utrecht, The Netherlands.

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Organisms cooperate more when facing environmental threats, especially with low complementarity in defense. However, endogenous threats can reduce cooperation, particularly when complementarity is high.

Keywords:
By-product mutualismCommon enemiesEvolutionary game theoryHarsh environments

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Area of Science:

  • Evolutionary Game Theory
  • Behavioral Ecology
  • Mutualism

Background:

  • The common-enemy hypothesis suggests cooperation arises from shared environmental threats.
  • By-product mutualism posits that cooperation benefits others incidentally.
  • Adversity is hypothesized to drive cooperation when facing common enemies.

Purpose of the Study:

  • To analyze the common-enemy hypothesis in an evolutionary game with endogenous adversity.
  • To compare predictions under exogenous versus endogenous adversity.
  • To investigate the role of complementarity and cooperation costs in mutualism.

Main Methods:

  • Developed an evolutionary game model with two defenders and an attacker.
  • Analyzed scenarios with exogenous adversity (fixed environmental threat).
  • Analyzed scenarios with endogenous adversity (attacker-determined threat level).

Main Results:

  • Exogenous adversity predicts the common-enemy hypothesis with low complementarity or high cooperation costs.
  • With high complementarity and low costs, adversity discourages cooperation (competing hypothesis).
  • Endogenous adversity favors the competing hypothesis, with the common-enemy hypothesis only holding at very low complementarity.

Conclusions:

  • The common-enemy hypothesis is sensitive to the nature of adversity (exogenous vs. endogenous).
  • Endogenous threats can undermine cooperation predicted by the common-enemy hypothesis.
  • Complementarity and cooperation costs are critical factors modulating the evolution of cooperation under threat.