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 estim ated using offspring-mother regression and restricted maximum likelihood (R EML) methods for bighorn sheep on Ram Mountain, Alberta. Both methods resul ted in similar estimates of h(2) for adults, but for lambs and yearlings he ritability was underestimated by offspring-mother regression relative to RE ML, possibly because of higher maternal-effects bias for offspring-mother r egression. Heritabilities of body mass in bighorn were similar to published estimates for domestic sheep. Heritability estimated by offspring-mother r egression increased after 2 years of age. The REML method suggested that he ritability was moderate for lambs and yearlings, very low at 2 years of age , and increased afterwards. The increase in heritability with age was attri buted 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 combin ation of high environmental variance and very low genetic variance. Body ma ss 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.