Heritability of growth curve parameters and heritability of final size: a simulation study.

Simulations of nestling growth in Great Tits (Parus major) were used to show the consequences genetic variation in the parameters of growth curves could have on the amount of genetic variation in final body weight. This was done for the case of good environmental conditions (i.e. adequate food supply), and for poor environmental conditions (food limitation). A three-parameter process-error growth model based on the Richards curve (Brisbin et al., 1986) was used. The three parameters were the asymptote, the shape, and the time to complete approximately 95% of growth. In simulations, where only the asymptotic value was heritable, genetic variation in final weight was larger under good conditions than under poor conditions. Food limitations decreased the amount of genetic variation and the heritability estimate of final size. Similarly, sib competition, (based on body size differences of nestlings within broods) combined with food limitations lowered heritabilities. Sib competition under good conditions had no effect on heritabilities. In contrast, when the time to complete approximately 95% of growth was heritable, heritabilities for final size were not lower under poor conditions either without or in the presence of sib competition. This model suggests that different amounts of genetic variation in final size under good and poor conditions could be linked to the amount of genetic variation present in the growth curve parameters. The results agreed with an empirical study on body weight in a passerine bird, the Great Tit, where only the asymptote displayed heritable variation and more genetic variance was expressed under good conditions.