Population density, social behaviour and sex allocation

© Cambridge University Press 2010. Overview Evolution and ecology naturally intersect through birth, death and dispersal rates as they determine both population dynamics and individual fitness. However, we still understand very little about the connections between population dynamics, the evolution of individual behaviour patterns and the resulting social interactions. In this chapter, we first review how density affects individuals and discuss various ways in which population density is expected to influence social behaviour, using local competition for resources, reproductive cooperation and mating systems as illustrative examples. Following a brief introduction to evolutionary theory on sex allocation, we consider a few empirical examples from social insects, hermaphroditic fish, breeding birds and group-living mammals to demonstrate some of the observed patterns of sex allocation and the effect of density and social behaviour on these patterns. We then explore how sex allocation in hermaphrodites and sex ratios in cooperatively breeding animals can be used to demonstrate the links between sex allocation, sex ratio and social behaviours, as well as the difficulty and importance of understanding links between ecological and evolutionary dynamics generally. We finish the chapter with a discussion of directions for future empirical and theoretical research. Introduction Social behaviour takes diverse and fascinating forms in a wide variety of taxa, as the chapters in this book demonstrate. In this chapter, we examine the links between population density, social behaviour and sex allocation as an illustrative example of the general connection between individual-level processes and population patterns.