J. Zool., Lond. (2004) 264, 1–9 C  2004 The Zoological Society of London Printed in the United Kingdom DOI:10.1017/S0952836904005552 Variation in harem size of red deer (Cervus elaphus L.): the effects of adult sex ratio and age-structure Christophe Bonenfant 1 *, Jean-Michel Gaillard 1 , Franc ¸ois Klein 2 and Daniel Maillard 2 1 Laboratoire de Biom´ etrie et Biologie Evolutive, Unit´ e Mixte de Recherche No. 5558, Bˆ atiment 711, Universit´ e Lyon I, 43 Boulevard du 11 novembre 1918, F-69622 Villeurbanne Cedex, France 2 Office National de la Chasse et de la Faune Sauvage, Direction des Etudes et de la Recherche, 5 rue de Saint Thibault–Saint Benoist, F-78610 Auffargis, France 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 was 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 less young males mate, cannot be assessed from our study. Key words: Cervus elaphus, forested habitat, mating tactics, red deer, sexual selection, rutting season, ungulates INTRODUCTION The type of mating tactic observed in a given population of mammal depends on the spatio-temporal distribution of oestrous females and on group stability (Orians, 1969; Emlen & Oring, 1977; Clutton-Brock, 1989; Davies, 1991), which are both determined by the distribution of food resources (Geist, 1974; Jarman, 1974; Ralls, 1977). Polygyny (sensu Reynolds, 1996) should occur whenever some males have the opportunity to ‘economically’ prevent other males having access to females (Emlen & Oring, 1977; Clutton-Brock & Harvey, 1978; Davies, 1991). At the population level, the mating system can be defined as the sum of all individual mating tactics (Clutton-Brock, 1989; Reynolds, 1996; Apollonio, Festa- Bianchet & Mainardy, 2000). Mating tactics, however, are remarkably variable among and within populations (Hogg, 1984; Lott, 1990; Thirgood, Langbein & Putman, 1999). They can vary among individuals with age, size (Caro & *All correspondence to: C. Bonenfant. E-mail: bonenfant@biomserv.univ-lyon1.fr Bateson, 1986) or environmental conditions, depending on the relative costs and benefits of the amount of energy allocated to fighting and guarding activities, predation risks (Jarman, 1974; Kalas, Fiske & Sæther, 1995; Goldspink et al., 2002) and effective mating success. Alternative tactics (such as kleptogamy) are mostly adopted by subordinate individuals (Byers & Kitchen, 1988; Modig, 1996; for review see Austad & Howard, 1984). Frequent changes of mating tactics according to environmental conditions have led to the concept of optimal mating strategies, which are usually defined by using modelling approaches (Dunbar, Buckland & Miller, 1990; Sandell & Liberg, 1992; Stevenson & Bancroft, 1995). In polygynous species, annual male mating success is closely associated with fighting success and dominance rank (Buechner, 1961; LeBoeuf, 1974; Clutton-Brock, Guinness & Albon, 1982; Gammell, 2001), and is much more variable among individuals than female mating success (Gibson & Guinness, 1980a; Le Boeuf & Reiter, 1988; Rose, Clutton-Brock & Guinness, 1998; Wilson, Olson & Strobeck, 2002). In red deer, the main proximate