Alliances I. How large should alliances be?

Males of a number of species form alliances with other males during competition for access to females. There is a great deal of variation in alliance sizes between and within populations. Using individual-based models in which alliances with greater net competitive ability outcompete those with less, and males can switch between alliances based on their expected success, we examined the distribution of alliance sizes that result from different assumptions about how males compete and cooperate. In many runs of the model, the distribution of alliance sizes was quite similar to the distribution of the number of males competing for a receptive female. Results were little affected by the number of males in the population, their range of competitive abilities, how expected success within alliances was allocated, whether there were costs to switching alliances, whether males were able to switch alliances with a partner, or whether a kinship structure was added to the population. However, adding a separate cost of being in larger alliances, or allowing males to leave large alliances as pairs, could reduce mean alliance size. Thus, males would be expected to form alliances except when the number of males competing for a receptive female is very few, there are substantial costs to being in an alliance, or alliances do not outcompete single males. Alliances were found to be quite dynamic except when a cost of switching alliances was introduced.