Evolution of redirected help with stepping-stone dispersal

Individuals can be subject to brood failure for many reasons, e.g. egg predation. Some species have adopted a strategy to compensate for fitness loss due to brood failure. In some bird species, individuals that experience brood failure can help a related neighbor care for their offspring. This behavior is known as redirected helping and it compensates for the loss of fitness by improving inclusive fitness. However, inclusive fitness gains associated with redirected helping are counteracted in a 'viscous' population, where individuals remain close to their natal site, by increased competition among kin. In a previous model, we investigated how population viscosity affects the evolution of redirected help by building an infinite-island model that lacked explicit spatial structure and emphasized dispersal on a global scale. Here, we revisit the role of population viscosity in the evolution of redirected help with a spatially explicit stepping-stone dispersal model. The dispersal pattern in this new framework is much more constrained than in the infinite-island case, strengthening the population viscosity we are interested in. In contrast to previous work, we find that localized dispersal can prevent the evolution of redirected help and even lead to the evolution of spiteful behavior in some specific cases. Our predictions are sensitive to the scale of dispersal, survival rate and the benefits provided by helping. We discuss our findings with regard to the previous literature investigating the evolution of helping in a viscous population.