Heritability estimation
We estimated narrow-sense heritability (h2 ) as the amount of additive genetic variance divided by the total phenotypic variance (i.e., the sum of the different variance components). Because the model was fitted with binary data, all variance estimates of the model were calculated on a latent scale. It is possible to measure heritability on either the latent trait scale or the observed data-scale, the selection of which depends on inferences being made . Heritability on the latent scale was estimated as\(h^{2}=\frac{V_{A}}{V_{A}+\ V_{\text{SE}}+\ V_{e}}\), where\(V_{A}\) is the additive genetic variance, \(V_{\text{SE}}\) is the variance associated with shared environment, and \(V_{e}\ \)is the residual variance, which was fixed at 1. In this case, we also estimated heritability on the observed data-scale, as this provides parameter estimates which are directly interpretable in relation to the ecology of the species whilst incorporating other factors (i.e., fixed effects and residual variance) which may influence the probability of contracting chlamydia. To convert parameter estimates, we used the QGglmm package in R . This package uses estimates of additive genetic variance, phenotypic variance (sum of all random effects variance + residual variance) and the intercept and converts them to the data-scale, thereby allowing for the calculation of heritability on this scale. We repeated this process for all random effect variances, thus measuring the proportion of phenotypic variance attributed to either shared environment effects or maternal effects (hereafter referred to as intra-class coefficient (ICC)).