Population-Level Life History in Italy, Spain and Mexico: The Impact of Regional Climate, Parasite Burden, and Population Density

Theoretical and empirical work in the study of life history evolution has largely shifted away from density-dependent models since the 1980s, focusing on alternative causative processes such as the prominence of extrinsic morbidity and mortality. Nevertheless, recent evidence suggests that density dependent models are integral, even if clearly not sufficient, to account for trajectories of life history evolution. However, little investigation has been conducted among human populations to compare these models. Here we collate data for subnational regions of Italy, Spain, and Mexico to directly compare alternative models of life history evolution, including information on several dimensions of climate, parasite burden, population density, and manifest indicators of life history strategy. We examine direct and indirect effects of potential predictors upon life history speed, and address the hierarchical relations among these variables in relation to the goal of theory integration. Results revealed that ecological indicators had no significant impact on population density. Population density and the brumal factor (a composite denoting higher altitude, latitude and colder mean temperatures) predicted slower life histories, however parasite burden (a measure of morbidity and mortality) failed to exhibit a direct effect upon life history speed. The impact of ecological factors was stronger for Italy and Spain, in comparison to Mexico. We discuss the implications for life history theory.