SEX DIFFERENCES IN THE INTERPLAY OF COHORT AND MOTHER QUALITY ON BODY MASS OF RED DEER CALVES

We investigated the role of individual variation among female red deer (Cer- vus elaphus) on the body mass of their offspring based on the long-term monitoring (1977- 1996) of a Norwegian population. We studied 273 calves weighed in winter, belonging to 114 mothers of known age. After accounting for yearly variation, we found that calf body mass increased with the age of their mother until mothers were 6 years of age, then leveled off until 15 years of age. Body mass varied from year to year in similar ways for male and female calves. Winter harshness, and density when calves were in utero, could account for only a small part of this variation. Variation in calf body mass could be decomposed into a climatic component, determining the mean body mass of a cohort, and a mother com- ponent, determining the position of calves' body mass relative to the mean of their cohort. The mother effect could be further decomposed into an effect of the mother's age and an effect of the mother's quality. The latter explained >35% of the body mass variation in male calves, and -~15% in females, while the proportions of the variation due to cohort effect were 28% and 42%, respectively. The mother's quality effect was therefore sex- specific and accounted for - 1.5 times more of the variance in male than female body mass. Females with relatively long jawbones for their age had larger calves. We supported the hypothesis that female's quality influences the quality of her offspring and that this effect lingered until after the end of the weaning period. Our results suggest that the variance in reproductive success among females is certainly larger than previously thought in red deer.