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In mammals (especially in ungulates), where males usually provide no parental care and have the higher reproductive potential, female reproductive success is directly constrained by access to the resources necessary to breed and to meet the energy requirements of gestation and lactation, whereas male reproductive success is limited more by access to potential partners (Trivers 1972). As a result, female distribution should depend primarily on resource dispersion (modified by predation pressure and the costs and benefits of associating with other individuals), while males should distribute themselves in relation to females (modified by male density), at least during the breeding period, in order to gain access to as many mates as possible (Emlen & Oring 1977).
Males can either directly compete for mates, or indirectly compete for resources that influence female distribution (Emlen & Oring 1977). The economics of these two mating tactics depends mainly on the defensibility of females, which is directly related to female density, distribution, range size, group size and stability, and indirectly to the distribution of resources (Clutton-Brock 1989). When females are spatially and temporally predictable (e.g. when they occupy small stable ranges containing clumped and high-quality food resources), the most economic mating tactic should be to adopt a ‘resource defence polygyny’ tactic, where males gain access to females by anticipating how resources will influence female distribution and competing for resource-rich sites to establish their zone of dominance (Emlen & Oring 1977).
Male territoriality is commonly associated with the resource defence polygyny tactic (Emlen & Oring 1977). In ungulates, in which male territoriality is widespread (see Owen-Smith 1977), the evolution of male territoriality as a mating tactic analogous to resource defence polygyny has been suggested for various species (e.g. in Grevy's zebra Equus grevyi, Klingel 1974; European roe deer Capreolus capreolus, Putman 1988; Wahlström 1994; Johansson 1996; Liberg et al. 1998; hippopotamus Hippopotamus amphibius, Nowak 1991; wildebeest Connochaetes gnou, Estes 1969; Reeves's muntjac Muntiacus reevesi, Dubost 1970; sika deer Cervus nippon, Miura 1984). However, no study has as yet tested this hypothesis by simultaneously assessing the influence of resources on female distribution and the influence of female distribution and resource dispersion on male distribution and reproductive success.
The aim of this study was to investigate this issue in the European roe deer [C. capreolus (Linnaeus 1758)], an ungulate in which only males are territorial and the spatial system of the two sexes is independent, so that a female's home range may overlap several male territories (Hewison, Vincent & Reby 1998; Liberg et al. 1998). First, we expected female abundance within a male's territory to increase with territory size (prediction 1). Second, if male territoriality evolved as a resource defence polygyny mating tactic, females should be distributed in relation to resources. The ‘ideal free distribution’ hypothesis (IFD; Fretwell & Lucas 1970) states that, in the absence of constraints on movement, individuals are expected to be distributed so that differences in local densities reflect differences in habitat quality. As a result, resources are equally shared among individuals and fitness is equal in all habitats (Fretwell & Lucas 1970). While Wahlström & Kjellander (1995) suggested that roe does fit the IFD, Nilsen, Linnell & Andersen (2004) found evidence for fine-scale spatial variance in fitness components induced by environmental heterogeneity. Similarly, Pettorelli et al. (2001) reported higher fawn body weight in the richer northern part of the Chizé forest compared to the poorer south, despite higher local density in the north. Assuming that females are free to distribute themselves in relation to resources, we expected female abundance within a male's territory to increase with habitat quality for a given territory size (prediction 2). Third, territoriality should secure mating access to resident females for the territorial male. Hence, we expected a male's breeding success to depend on female abundance within his territory (prediction 3). Also, males should attempt to enhance their mating success by either enlarging their zone of dominance, or by occupying areas particularly favoured by females, or both. Hence, we also expected male breeding success to increase with territory size and/or territory habitat quality (prediction 4). Finally, we should expect substantial male–male competition to occupy the ‘best’ territories (providing access to the highest numbers of females). Hence, the best territories should be occupied by the best competitors in male–male contests. We therefore expected a positive correlation between the suitability of the territory in terms of access to females, and male attributes which reflect ability to fight and dominate, such as age, body mass and antler size (prediction 5).
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Our results show that female abundance within a male's territory depends on the size of his territory, as expected from prediction 1, but not on its habitat quality, contrary to prediction 2. In support of predictions 3 and 4, territory size and female abundance within the territory, in turn, seem to determine male breeding success, at least partially. Hence, our results do not conform to expectations if male territoriality in roe deer was a conventional resource defence polygyny tactic (sensu Emlen & Oring 1977), because males seem to aim to defend a large territory, irrespective of its attractiveness to females in terms of habitat resources.
We showed that relative female abundance in a male's territory was not related to territory habitat quality, which seems to contrast with predictions under the resource defence polygyny hypothesis. These results also differ from the observations of Johansson (1996) who reported that male mating success in roe deer was related to territory quality, but not territory size: males who occupied a territory which included clover fields were more successful than other males. Strandgaard (1972) also suggested that the quality of crops influenced the number of females on a given territory. However, Johansson (1996) used behavioural estimates of mating success which are certainly poor indicators of true male breeding success, particularly as multiple mating commonly occurs and roe deer are cryptic and often occupy dense forest habitat (Danilkin & Hewison 1996; Liberg et al. 1998). In addition, both studies used a subjective categorical variable to describe habitat quality based on the assumption that clover fields were the preferred resource selected by females. To our knowledge, our study is the first to use a niche analysis to determine the specific resource attributes that reflect the habitat quality of a male's territory in terms of access to females. Several other studies of territorial ungulates have demonstrated a direct relationship between the quality of male territories and access to potential mates (e.g. in American pronghorn Antilocapra americana: Kitchen 1974; puku Kobus vardonii and topi Damaliscus lunatus antelopes: Balmford, Rosser & Albon 1992; impala Aepyceros melampus and waterbuck Kobus defassa: Jarman 1974; in a territorial population of red deer Cervus elaphus: Carranza 1995). However, none of these studies had access to a reliable estimate of male breeding success to fully test the resource defence polygyny mating hypothesis.
The absence of a relationship between the index of habitat quality and relative female abundance in a male's territory observed in our study may be due to several factors:
First, our estimation of relative female abundance within a male's territory was based on the assumption that local female density in winter was a good estimate of local female density in summer, since female home ranges are globally stable between seasons (Hewison et al. 1998). However, some seasonal migrations can occur in yearling and 2- to 3-year-old females (see Wahlström & Liberg 1995). This may potentially result in a redistribution of young females between the winter and rut periods, but concerns only a small proportion of the total number of females. In addition, snow cover may restrict roe deer space use in winter (Cederlund & Lindstroem 1983), but snow cover in our study area is certainly too low to constitute a problem for roe deer ranging behaviour (normal snow cover rarely exceeds 10 cm at Bogesund). To test the reliability of our assumption, we estimated the degree of overlap between winter (January–March) and summer (July–August) home ranges (90% Kernels) of 12 females of 1 year old and older, based on radio-tracking data collected from 1992 to 1995. We found that female home ranges were slightly larger in winter (mean = 42·15 ha, SD = 15·10) than in summer (mean = 25·36 ha, SD = 17·51), and overlap between seasons was variable and quite high, ranging from 17% to 99% of the area of the smallest home range, with a mean of 48·2% (SD = 24·6). This suggests that winter female distribution should be a quite good proxy of female distribution during the rut. Note that in our study, it was not possible to obtain significant behavioural information because of the difficulties of observing roe deer in a fragmented habitat with a large proportion of forest, such as at Bogesund. However, collecting behavioural observations on female distribution, when possible (e.g. in a more open landscape; see Börger 2006), would provide complementary information for understanding male mating tactics.
Second, the use of pellet-group counts to reflect spatial variation in relative deer abundance has not been fully tested at the present time. However, it has been suggested that pellet-group counts provide an informative index of relative animal abundance (e.g. Forsyth et al. 2007), especially in northern areas (such as Scandinavia), where pellet decay rate should be negligible and spatially constant during winter.
Third, although our index of habitat quality integrated a number of habitat variables, it may still not adequately reflect the suitability of an area for females. Fox predation, for example, is known to be a strong determinant of summer fawn survival and variation in female reproductive success at Bogesund (Kjellander et al. 2004a). Hence, predation avoidance may swamp the role of food resources and vegetation cover in determining female distribution. However, a preliminary analysis suggested that the number of fox dens present within a male's territory has no effect on the index of relative female abundance in a male's territory or on male breeding success (Cécile Vanpé, unpublished data).
A fourth possibility is that females may not be free to distribute themselves in relation to resources because of social constraints. Indeed, the IFD model is not appropriate when some individuals can monopolize resources by securing the highest quality areas while forcing inferior competitors into suboptimal habitats (e.g. territoriality or dominance hierarchies). Female roe deer are considered as nonterritorial, except maybe during fawning site defence (Liberg et al. 1998), because they commonly live in overlapping home ranges. However, this overlap often concerns close relatives (see Liberg et al. 1998), with matrilineal associations common due to extensive female philopatry (see Hewison et al. 1998). Hence, we cannot rule out the possibility that dominance relationships between unrelated females impose a constraint on female ranging behaviour. Indeed, female home range size seems to decrease with increasing population density (Kjellander et al. 2004b) and aggressive behaviours between females seem to be common (Börger 2006).
If females do not fully distribute themselves in relation to resources, males will be unable to ‘predict’ the distribution of potential mates by anticipating how resource distribution influences female dispersion. Hence, in contrast to the expectations under resource defence polygyny (Emlen & Oring 1977), males would then show no interest in competing for resource-rich sites to establish their zone of dominance. We showed that, to some degree, local female abundance in a male's territory increased with territory size and, as a result, had a positive influence on male YBS. This suggests that, to maximize fitness, males should attempt to enlarge their territory in order to maximize their chance of encountering receptive females. Interestingly, a male's territory is remarkably stable from year to year (Liberg et al. 1998), with little change of borders and size (e.g. Bramley 1970; Johansson 1996; Linnell & Andersen 1998), even when the distribution of resources or females changes markedly (Strandgaard 1972). Territory borders may even persist after the death of an owner and the takeover of a new occupant (Strandgaard 1972; Cederlund, Kjellander & Liberg 1994), usually an immigrating buck or a nonterritorial subadult (Cederlund et al. 1994; Johansson 1996). These observations suggest that males do not constantly attempt to enhance their mating success by either enlarging their zone of dominance or by occupying areas particularly favoured by females (Hewison et al. 1998), contrary to expectations for a conventional resource defence polygyny strategy.
It should be noted, however, that we did not find very strong relationships between male YBS and territory size and between male YBS and relative female abundance in a male's territory. This could be due to low sample size, but may also be explained by the fact that males probably do not have full mating monopoly inside their territory. Indeed, in roe deer, 30% to 50% of females make short excursions outside their normal home range during the rut, traversing several territories, and potentially mating with other males, before returning (see Liberg et al. 1998; Richard et al. 2008 for more details). In addition, females whose range overlaps several male territories move frequently between them during oestrus, being courted and mated repeatedly by one or more territory holders (Liberg et al. 1998). The above suggests that female roe deer may take an active role in mate searching and mate choice, and this may explain why male breeding success is not more closely linked to the number of females present within the territory.
Our results also showed that, in support of prediction 5, body mass tended to have a positive effect on territory size (90% Kernel) in all males, and interestingly, that residual antler size had a positive effect on territory size in subadults only. Hence, among territorial males, heavy individuals with large antlers are the best competitors, able to defend the largest territories, with the highest access to mates. This also suggests that male phenotypic traits, and especially antler size, which reflect the ability of males to fight are particularly important in subadults, during the key period when males establish their first territory. Because of the strong fidelity of males to their first territory and the positive relationship between territory size and YBS, the establishment of a male's first territory may therefore play a crucial role in determining fitness over his lifetime (note, unfortunately, we were not able to conduct an analysis of the determinants of lifetime breeding success because we had lifetime breeding information and ranging data for too few males).
To sum up, although territoriality of male roe deer has certainly evolved as a tactic for ensuring access to mates (Hewison et al. 1998), our results suggest that it does not entirely conform to a resource defence polygyny strategy (see also Börger 2006 for a similar suggestion). We speculate that social constraints may govern, at least partly, the distribution of female roe deer at the local scale, and that variation in the number of females within a male's territory probably depends mainly on the resultant annual variation in female distribution (Bramley 1970; Strangaard 1972). Our results are therefore in agreement with the view that male territoriality conforms to an ‘always stay’ and ‘low risk–low gain’ strategy in roe deer (see Linnell & Andersen 1998), where males do not compete intensely for territories and mating (at least once they have established their first territory). Rather, males gain the benefits of site familiarity and reduced frequency of dangerous fights with rival males by remaining in the same area where they are dominant (Owen-Smith 1977). As suggested by Liberg et al. (1998) and Hoem et al. (2007), the adaptive function of male territoriality in roe deer is thus not the monopolization of females present within territories for mating, but rather the delimitation of an area in which a buck is dominant and so can court and mate without interference from other males. Roe deer territoriality seems therefore to differ from the territorial systems of most other ungulates, for which previous studies have demonstrated a direct relationship between territory quality and mate access (e.g. in impala and waterbuck: Jarman 1974; American pronghorn: Kitchen 1974; puku and topi antelopes: Balmford et al. 1992), suggesting a better fit with the conventional resource defence polygyny model than for roe deer. We therefore suggest that resource defence polygyny should not be considered as a homogeneous mating tactic. There are actually no discrete distinctions between the different types of polygynous mating tactics described by Emlen & Oring (1977) and, as Jarman (1974) pointed out, ‘any attempt to subdivide a continuum creates problems’. The variability of polygynous mating tactics would be probably more realistically described as a continuum extending from resource defence polygyny to female defence polygyny, with roe deer occupying an intermediate position.