Variation in harem size of red deer (Cervus elaphus L.):: the effects of adult sex ratio and age-structure

The relationships among harem size, adult sex ratio (proportion of males > 5 years in the adult population, i.e. males > 5 years plus females > 2 years) and male age-structure of red deer Cervus elaphus were investigated in La Petite Pierre National Reserve (PPNR) in France. We tested whether: (1) increasing adult sex ratio leads to a decrease in harem size along with an increase in the number of harems within a given rut period; (2) whether participation of sub-adult males in mating activities increases with decreasing adult sex ratio, and as the proportion of adult males decreases. Harem size did not vary over the mating period, suggesting a high turnover of harem-holders leading to an increase in the costs of mating for males. Harem size averaged 1.43 +/- 0.91 and was lower than harem sizes typically reported for red deer (e.g. > 2.5 in Scotland and Norway). In support of the first prediction, a decrease in harem size and an increase in the total number of harems seen with an increasing sex ratio was observed (harem size = 2.08 - 1.26 [ +/-0.43] x (sex ratio); r(2) = 0.25, F-1.18 = 6.19, P = 0.02). Both the uniform distribution of females among harem stags and the small harem sizes observed in PPNR might concur to a smaller variance in male reproductive success than previously reported in red deer. Lastly, in partial support of the second prediction, the proportion of sub-adult males observed during the mating season decreased with increasing adult sex ratio and with increasing proportions of adult males. Whether or not the lower proportion of sub-adults seen when competition among mature males increases means that fewer young males mate cannot be assessed from our study.