THE EVOLUTION OF TRAITS THAT DETERMINE ABILITY IN COMPETITIVE CONTESTS

We analyse mathematical models of the evolution of a trait that determ ines ability in contest competition. We assume that the value of the c ompetitive trait affects two different components of fitness, one meas uring the benefit of winning contests and the other measuring the cost of developing the competitive trait. Unlike previous analyses, we inc lude the population dynamical consequences of larger competitive trait values. Exaggeration of the competitive trait reduces the mean probab ility of survival during the non-competitive stage of the life cycle. The resulting lower population density reduces competition and, theref ore, reduces the advantages of greater competitive ability. Models wit hout population dynamics often predict dimorphism in the competitive t rait when resource possession is decided by interactions with many oth er individuals. If the competition involves a contest with a single ot her individual, models without population dynamics often predict cycle s of increase and collapse in the trait or a continual increase, possi bly resulting in extinction. When population dynamics are included, bo th of these results become less likely and a single stable trait value becomes more likely. Population dynamics also make it possible to hav e dimorphism when individuals have a single pairwise contest and alter native stable trait values when an individual has many contests. Incre ases in the value of the resource being contested may increase or decr ease the evolutionarily stable size of the trait. Competition between very differently sized species will often decrease size in the larger species (character convergence).