Communal defense of territories and the evolution of sociality.

The evolution of group living has attracted considerable attention from behavioral ecologists working on a wide range of study species. However, theoretical research in this field has been largely focused on cooperative breeders. We extend this line of work to species that lack alloparental care (hereafter termed "noncooperative species") but that may benefit from grouping by jointly defending a common territory. We adopt a demographically explicit approach in which the rates of births and deaths as well as the dispersal decisions of individuals in the population determine the turnover rates of territories and the competition for breeding vacancies thus arising. Our results reveal that some of the factors thought to affect the evolution of cooperative breeding also affect the evolution of group living in noncooperative species. Specifically, high fecundity and low mortality of resident individuals both increase the degree of habitat saturation and make joining an established group more profitable for nonresidents (floaters). Moreover, if floaters can forcefully take over territories, the degree of habitat saturation also affects the chance that residents become targets of takeovers. In this situation, communal defense of territories becomes an important benefit that further promotes the evolution of group living.