DELAYED DISPERSAL: LIVING UNDER THE REIGN OF NEPOTISTIC PARENTS
The AUK: A Quarterly Journal of Ornithology
THE INCLUSIVE FITNESS concept (Hamilton 1964) formulated consequences of social behavior in gene equivalents. In doing so, it enabled fitness consequences of social behavior to be understood within the framework of genetic inheritance of traits. Delayed dispersal of birds was one system where the inclusive fitness concept was put to test. The key issue was to understand how delayed dispersal could be reconciled with evolution through natural selection, when retained offspring forego personal
... forego personal reproduction while they remain in the natal territory (e.g. Skutch 1961). Cooperative breeding seems to have a secondary role for the maintenance of delayed dispersal, although 96% of bird species where the offspring remain with their parents into adulthood to form family groups also breed cooperatively (Em!en 1995). Although that association between delayed dispersal and cooperative breeding indicates that delayed dispersal is a permissive factor for the maintenance of cooperative breeding, there is not necessarily a causation going in the opposite direction. Cooperative breeding can be seen as an independent decision, and as such it is a consequence rather than a cause of delayed dispersal (Brown 1987, Stacey and Ligon 1987, Koenig et al. 1992, Emlen 1994, Hatchwell and Komdeur 2000), which is consistent with the E-mail: email@example.com observation that dispersal can be delayed without the retained offspring engaging in reproduction. Even if some of the retained offspring in a species participate in cooperatively breeding units, there are usually a substantial fraction of them that do not engage in help-atthe-nest, and only a few species like the White-winged Chough ( Corcorax melanorhamphus) can be classified as an obligate cooperative breeder (Brown 1987). Even stronger support for the fact that delayed dispersal does not require any involvement of retained offspring in cooperative breeding comes from a number of species where retained offspring, as a rule, never help (Gayou 1986, Veltman 1989, Birkhead 1991, Ekman et al. 1994, Newton et al. 1994, Walls and Kenward 1996, Green and Cockburn 1999, Robinson 2000). Therefore, it seems that the maintenance of delayed dispersal requires an explanation that does not have to resort to fitness gains of cooperative breeding (see also Hatchwell and Komdeur 2000). UNRESOLVED ISSUES The fact that cooperative breeding should not be essential for delayed dispersal is consistent with the view that the behavior of remaining in the natal territory is maintained as a product of ecological constraints on dispersal options.