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

Abstract

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); r²=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.