Emotions and courtship help bonded pairs cooperate, but emotional agents are vulnerable to deceit

Significance Well-coordinated pair bond relationships occur in many species, humans included. Decisions in these relationships may be influenced by persistent emotions toward a partner (emotional bookkeeping). Here, we use a simulation model to show that animals guided by emotional bookkeeping can evolve cooperative pair bonds that are resilient to mistakes and miscommunication. Deceit, however, can eradicate cooperation, and emotional bookkeeping can make this even worse. We show that one possible defense against deceit is the evolution of a courtship process where animals can develop relationships without paying their full costs.

) and mortality (m = 0.005, 0.05).Evolution of cooperation is more successful in larger populations with lower mortality, and does not occur in small populations with very low mortality.At high mortality, courtship is more effective and emotions less.This is unsurprising since the benefits of cooperation and emotional bookkeeping accrue over long relationships.(b) Mutation rate σ = 0.01, 0.1; extremely high mutation rates increase cooperation in the absence of courtship (c) All initial trait values = -1.0,initial variance = 0.0001, 0.25.Cooperation takes longer to emerge when the initial population is strongly biased against cooperation and relationships, but still benefits from emotions and courtship.
Fig. S1.Average number of courtship interactions in new relationships, across all noise levels.The panels show results with and without emotional bookkeeping, costly (4.0) divorce, and costly (1.6) deceit.
Fig. S2.Variations on the model for the baseline scenario (costly divorce, no deceit m = 0.01, cdiv = 4.0, ccou = 0.0).(a) Using a smooth sigmoid curve for decisions instead of a piecewise linear one does not influence results.(b) Results with Snowdrift payoffs (R = 1, T = 3, S = 1, P = −1) where T and S maximise social welfare.In this case we do not see high levels of R, but instead T and S are often very high.Across conditions, T and S are highest when both emotions and courtship are allowed.Emotions without courtship are slightly beneficial to T /S, but courtship without emotions reduces it.(c) Under Staghunt payoffs (R = 3, T = 1, S = −1, P = 1), courtship alone has little benefit to cooperation, but emotional bookkeeping does, and their combination yields even higher cooperation.

Fig. S4 .
Fig. S4.Action noise scenario: mutual cooperation rate with and without emotions and courtship graphed against noise level.Panels indicate increasing deceit cost (left to right) and increasing divorce cost (top to bottom).N/A indicates that divorce or deceit was disallowed (bottom row / rightmost column respectively).

Fig. S5 .
Fig. S5.Mutual cooperation rate with different courtship costs graphed against noise level without emotions for action noise.Panels indicate increasing deceit cost (left to right) and increasing divorce cost (top to bottom).

Fig. S6 .
Fig. S6.Mutual cooperation rate with different courtship costs graphed against noise level with emotions for action noise.Panels indicate increasing deceit cost (left to right) and increasing divorce cost (top to bottom).

Fig. S7 .
Fig. S7.Perceptual noise scenario: Mutual cooperation rate with and without emotions and courtship graphed against noise level.Panels indicate increasing deceit cost (left to right) and increasing divorce cost (top to bottom).N/A indicates that divorce or deceit was disallowed (bottom row / rightmost column respectively).

Fig. S8 .
Fig. S8.Mutual cooperation rate with different courtship costs graphed against noise level without emotions for perceptual noise.Panels indicate increasing deceit cost (left to right) and increasing divorce cost (top to bottom).N/A indicates that divorce or deceit was disallowed (bottom row / rightmost column respectively).

Fig. S9 .
Fig. S9.Mutual cooperation rate with different courtship costs graphed against noise level with emotions for perceptual noise.Panels indicate increasing deceit cost (left to right) and increasing divorce cost (top to bottom).N/A indicates that divorce or deceit was disallowed (bottom row / rightmost column respectively).