Reproductive skew, fitness costs, and winner-loser effects in social-dominance evolution

Abstract

Social hierarchies can increase reproductive skew in group-living animals. Using game theory we investigate how the opportunity for differently ranked individuals to acquire resources influences reproductive skew, costs of hierarchy formation, and winner and loser effects. Individuals adjust their aggressive and submissive behaviour through reinforcement learning. The learning is based on perceived rewards and penalties, which depend on relative fighting ability. From individual-based simulations we determine evolutionary equilibria of traits that control an individual's learning. We examine situations that differ in the extent of monopolisation of contested resources by dominants and in the amounts of uncontested resources that are distributed independently of rank. With costly fighting, we find that stable dominance hierarchies form, such that reproductive skew mirrors the distribution of resources over ranks. Individuals pay substantial costs of interacting, in particular in high-skew situations, with the highest costs paid by intermediately ranked individuals. For cases where dominants monopolise contested resources there are notable winner and loser effects, with winner effects for high ranks and very pronounced loser effects for lower ranks. The effects are instead weak when acquired resources increase linearly with rank. We compare our results on contest costs and winner-loser effects with field and experimental observations.

Social restructuring during harsh environmental conditions promotes cooperative behaviour in a songbird

“…In response to harsh environmental conditions, cooperation can emerge in the form of reduced aggression rates towards potential competitors that can increase interdependence among individuals (through group size benefits). Such interdependence would further drive the benefits of maintaining relatively stable associations with extra-group members, as repeated low-cost interactions could allow more expensive forms of cooperative behaviours to emerge [ 46 , 47 ]. Such a positive feedback loop between cooperation and social structure [ 2 ] could then drive the emergence of upper social units within multilevel societies.…”

Game theory in biology: 50 years and onwards

“…A recent approach that incorporates these various effects into game-theory models is to assume that individuals use behavioural mechanisms similar to reinforcement learning when forming hierarchies [79,80]. This has, for instance, the potential to explain when winner-loser effects are expected [81].…”

“…The evolution of social dominance in group-living animals shows that mechanism-based large-worlds models can be helpful when exploring previously difficult-to-study phenomena. By implementing behavioural mechanisms similar to reinforcement learning, one can study things like costs of hierarchy formation, winner-loser effects, and bystander effects [79][80][81], all of which have been studied in fieldwork and experiments. Reinforcement learning has also been used to model aspects of cleaning mutualisms [146].…”