Heritability of body mass varies with age and season in wild bighorn sheep

Heritabilities ( h 2 ) of body mass at different ages and seasons were estimated using offspring–mother regression and restricted maximum likelihood (REML) methods for bighorn sheep on Ram Mountain, Alberta. Both methods resulted in similar estimates of h 2 for adults, but for lambs and yearlings heritability was underestimated by offspring–mother regression relative to REML, possibly because of higher maternal-effects bias for offspring–mother regression. Heritabilities of body mass in bighorn were similar to published estimates for domestic sheep. Heritability estimated by offspring–mother regression increased after 2 years of age. The REML method suggested that heritability was moderate for lambs and yearlings, very low at 2 years of age, and increased afterwards. The increase in heritability with age was attributed to declining negative maternal effects. Very low h 2 estimates at 2 years of age, obtained with both methods, appeared to be caused by a combination of high environmental variance and very low genetic variance. Body mass of bighorn sheep has a pronounced seasonal cycle, and h 2 was lower in June than in September for 2-year-olds and older sheep, and associated with both lower V A and higher V E in spring.