BODY-MASS AND SURVIVAL OF BIGHORN SHEEP

In ungulates, body mass is often positively correlated with juvenile s urvival, but little is known of whether body mass affects survival of other age-classes. We studied two marked populations of bighorn sheep (Ovis canadensis) in Alberta, Canada, to determine if body mass affect ed the survival of different sex-age classes. Chest girth at weaning w as correlated (P < 0.0001) with survival of bighorn lambs in the Sheep River population. In the Ram Mountain population, body mass in mid-Se ptember had a stronger effect upon survival than mass in early June, m ass gain in summer, or mass loss in winter. Body mass at weaning was c orrelated with lamb survival (P = 0.004). In both study areas, relatio nships between size and survival of lambs were similar for the two sex es. At Ram Mountain, survival of yearling and adult males seemed to be independent of body mass. Light yearling females were less likely to survive than heavy yearling females. Among females aged 3-6 years, bod y mass had no effect on survival. Among females 7 years of age and old er, mass in mid-September had a weak but significant (P = 0.03) effect on survival. Females were slightly lighter in mid-September in their last year of life than in the rest of their adult life. Maternal expen diture is unlikely to affect the survival of prime-age ewes, but may h ave a detrimental effect on the survival of older ewes.