EARLY WEANING IN BIGHORN SHEEP, OVIS-CANADENSIS, AFFECTS GROWTH OF MALES BUT NOT OF FEMALES

Theories of parental investment and parent-offspring conflict assume that investment involves a cost to the parent and a benefit to the offspring, but for herbivorous mammals, behavioral and nutritional weaning are gradual processes that are difficult to define, and little is known about the consequences of individual variation during weaning. To study the effects of late maternal care on offspring fitness, we removed female bighorn sheep (Ovis canadensis) from a marked population in Alberta, Canada, and monitored the survival, growth, and reproductive success of orphan and nonorphan lambs. Mothers were removed when lambs were 3.5-4.0 months, about 2-4 weeks before the suspected time of nutritional weaning. Female orphans and nonorphans had the same weight as yearlings, the same probability of producing their first lamb at 2 years of age, the same lifetime reproductive success (lambs produced or lambs that survived to early autumn), and the same longevity. Male orphans from most cohorts were smaller as yearlings compared to nonorphans from the same cohort. They were unable to compensate for this early weight difference in later life: at 4 years, orphan males had smaller horns and were lighter than nonorphans. Small horn and body size likely lowered the reproductive success of orphaned males compared to nonorphans from the same cohort. We suggest that in this sexually dimorphic species late maternal care is more important for males than for females. Because late maternal care had no measurable benefit for daughters, we suggest that parent-offspring conflict over the duration of maternal care may not exist for mother-daughter pairs. For mother-son pairs it remains to be shown whether late maternal care involves a cost to the mother, but the assumption of a benefit to the son was met.