Classical work on life history theory traces back to the foundations of evolutionary theory (Charlesworth, 1994; Hamilton, 1966; Stearns, 1992). In any given age-structured population, individuals have a survival rate and a fertility rate that both depend on age. The famous Lotka equation demonstrates that in this scenario the annual population growth rate, λ, is a function of age, survival and fertility. Because these three parameters make up a life history, the Lotka equation provided means to quantify associations between life history components and the corresponding values of λ that these associations yield. The realization that the population growth rate, λ, was a quantitative definition of population fitness – it is defined by fitness components – allowed a direct connection between life history theory and evolutionary theory (Hamilton 1966). Now, estimating changes in fitness λ produced by changes in the life history was made possible. For example, if changes are assumed to arise from random mutations, consequent changes in fitness can be interpreted as selection acting on the mutations (Charlesworth 1994). Crucially, such fitness changes can also be computed from λ and its partial derivatives (i.e., sensitivity; Caswell 2001). The definition of these quantitative associations between fitness and the life history directly linked individual phenotypes, to population and evolutionary dynamics setting the stage for the field of evolutionary demography.
Because of its flexible applicability grounded in general theory, many developments to this approach have been made for both ecological and evolutionary theory (Charlesworth 1994; Caswell 2001). More recent studies addressing the evolutionary demography of animal populations have focused on integral projection models (IPMs) as a flexible method to quantify population and evolutionary dynamics while accounting for continuous phenotypes (Levin et al., 2021). Since the development of IPMs (Easterling et al., 2000), studies addressing size-based demographics by structuring populations into a body size continuum have become common mostly in mammals and birds (Coulson, 2012; Levin et al., 2021; Merow et al., 2014; Rees et al., 2014), however few studies have attempted to incorporate social structures into the model (Kappeler et al., 2019; Paniw et al., 2021), while studies explicitly incorporating health structures have just started to emerge (Vincze et al., 2022).