By-product mutualism with evolving common enemies.

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.