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] ± (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.