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

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.

Loser-effect duration evolves independently of fighting ability

Winning or losing contests can impact subsequent competitive behaviour and the duration of these effects can be prolonged. While it is clear effects depend on social and developmental environments, the extent to which they are heritable, and hence evolvable, is less clear and remains untested. Furthermore, theory predicts that winner and loser effects should evolve independently of actual fighting ability, but again tests of this prediction are limited. Here we used artificial selection on replicated beetle populations to show that the duration of loser effects can evolve, with a realized heritability of about 17%. We also find that naive fighting ability does not co-evolve with reductions in the duration of the loser effect. We discuss the implications of these findings and how they corroborate theoretical predictions.

Winners have higher pre-copulatory mating success but losers have better post-copulatory outcomes

In many animals, the outcomes of competitive interactions can have lasting effects that influence an individual's reproductive success and have important consequences for the strength and direction of evolution via sexual selection. In the fruit fly, Drosophila melanogaster , males that have won previous contests are more likely to win in subsequent conflicts and losers are more likely to lose (winner–loser effects), but the direct fitness consequences and genetic underpinnings of this plasticity are poorly understood. Here, we tested how male genotype and the outcomes of previous male–male conflicts influence male pre- and post-copulatory success. We quantified pre-copulatory success in a choice and no-choice context, and post-copulatory success by quantifying ejaculate offensive and defensive ability. We found that winners have higher reproductive success compared to losers in both pre-copulatory scenarios. However, losers consistently mated for a longer duration, boosted female fecundity and had an increased paternity share when they were the first males to mate, suggesting increased investment into post-copulatory mechanisms. Finally, by using clonal hybrids from the Drosophila Genetic Reference Panel, we documented that genetic variation explained a sizeable proportion of the observed differences between lines, and of the interaction between line and winner and loser effects. Our results place the behavioural data on winner–loser effects in an evolutionary context by documenting the potential fitness gain to males from altering their reproductive strategy based on fighting experience. Our data may also explain the presence and maintenance of trade-offs between different male reproductive strategies.