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Introduction
Nature has produced, through natural selection, an astounding array of strategies that improve the fitness of animals. These strategies, which include innate life history traits and learned behaviours, stem from the trade-off between energy available in the immediate environment and the phenotypic constraints imposed by evolution (Roff, 2002). Such trade-offs directly impact the allocation of energy towards reproductive output, which is usually measured in terms of associated trait values (Stearns, 1992), e.g. the number and size of offspring. The balance between the impact of past selective pressures and current resource availability ultimately shapes our description of species-wide reproductive traits (de Queiroz, 2007). A classical model that characterizes these traits between species is the r/K selection theory (Reznick et al., 2002), which relates to the selection of traits that regulate population density, also known as density-dependent selection (Bertram and Masel, 2019). Conceptually, a continuum can be drawn between r-selected species that have a capacity for rapid increase in population size and K-selected species that have populations close to environmental carrying capacity (MacArthur and Wilson, 1967), i.e. low and high density-dependent selection, respectively. Therefore, r-selected species are predicted to minimize self-preservation and invest more energy towards reproductive output, while K-selected species are predicted to maximize self-preservation and invest less energy towards reproductive output (Pianka, 1970). Other traits such as growth rate and longevity also differ between r- and K-selected species, e.g. r-selected species tend to have faster growth rates and are relatively short-lived. Albeit a simplified view of natural selection (Stearns, 1977; Parry, 1981), one should be able to quantify the effects of density-dependent selection on the reproductive strategies of species in an r/K continuum (Mueller et al., 1991; Reznick et al., 2002).
For instance, density-dependent selection impacts many ecological traits, including habitat specialization and the life history traits associated with reproductive strategy (Morris, 1987; Bonte et al., 2012; van Beest et al., 2014; Büchi and Vuilleumier, 2016), i.e. the number and size of offspring. It has been shown that habitat specialists exhibit reproductive traits akin to what is expected in K-selected species, e.g. low fecundity and greater energetic investment per offspring. Conversely, habitat generalists appear to show r