Female home range size and the evolution of social organization in macropod marsupials

1. Traditional explanations for interspecific variation in home range size are based almost exclusively on analysis of northern hemisphere eutherian mammals. These suggest that variation in body size is the most important factor, while climate and diet are less important. The generality of these explanations is tested using a novel database on macropod marsupials (kangaroos and allies). 2. We found a 490-fold difference in home range size and a 50-fold difference in body weight amongst macropods. In agreement with studies on eutherians, female home range size was strongly correlated with body weight. 3. However, unlike the pattern in eutherians, climate was more strongly associated with home range size than was body weight: annual rainfall explained more than half of the variation, while group size (6%) and population density (8%) were minor explanatory variables, and female body weight explained no further variation. 4. When we used a phylogeny-based comparative method, annual rainfall explained 32% of the variation and body size a further 16%. 5. Sex differences in home range size were not explained by body size dimorphism, but were strongly related to rainfall and less strongly related to the degree of female home range overlap. Species with large sex differences in home range size lived in high-rainfall habitats where female ranges were small. 6. We suggest that climate has been a key factor in social evolution of macropods. In species inhabiting productive habitats, small female home ranges allow males to overlap with more females by increasing home range size. Males often compete for females by searching widely. In less productive habitats, males cannot further increase or defend their already large home ranges, resulting in similar-sized ranges in both sexes. Where female home range overlap is high, males need not increase home ranges as much, but this effect is secondary to that of climate. 7. In agreement with the theory that competitive searching by males is not necessarily related to body size, species with extreme size dimorphism had no sex difference in home range size, and species with extreme home range differences between the sexes showed no particular pattern of dimorphism. 8. Variation among macropod species in home range size and social organization is more closely correlated with variation in habitat productivity, and is less related to body size, diet, density and group size, than predicted by eutherian-based explanations.