Modelling life-history (co)variation
We used univariate linear mixed-effect models to quantify the sources of
variation in the individual estimates of generation time, age of first
successful reproduction, lifespan, reproductive rate, lifetime
reproductive success and individual growth rate (see Appendix S1B for
model equations). All these models had sex as fixed effect and random
intercepts for population identity (n = eight populations) and cohort
(i.e. birth year; n = 16 cohorts). We assumed normally distributed
errors in these analyses. As a second step, we fitted a multivariate
model to study the phenotypic covariance among individuals in the
different life-history traits (see Appendix S1C for equations). This
model did not contain any random or fixed effects as we were interested
in estimating the overall phenotypic covariance among individual
life-history traits regardless of the cause. We thus estimated the 6x6
“residual” variance-covariance matrix.
We also fitted an animal model (Kruuk 2004) to decompose individual
differences in generation time into additive genetic variation versus
environmental variation (see Appendix S1C for more details). This model
only had sex as fixed effect and cohort year as random effects and we
assumed normal error distribution. The pedigree consisted of 3116
individuals with a mean of 4.6 ancestral generations.