Studies of the variance in reproductive success of both sexes are critical to understand demography, genetic structure as well as natural and sexual selection in natural populations.
In most mammals, males maximise their fitness by siring as many offspring as possible, provided that females do not require male assistance to rear offspring (TRIVERS 1972, EMLEN & ORING 1977). Males therefore compete intensely for mates which may result in different reproductive tactics and strategies, where mating success may vary greatly among males. A ‘tactic’ is one of several stated behavioural options (phenotypes) whereas a
‘strategy’ is a set of rules stipulating which alternative behavioural pattern, of several stated options, will be adopted in any situation throughout life (DOMINEY 1984). Strictly speaking, a reproductive strategy can only be understood when the reproductive output paired with the behavioural pattern is known for the whole reproductive life span.
If females are a scacre resource males may compete for access to females by direct or indirect competition. In contest competition (CLUTTON-BROCK et al. 1982, DEWSBURY 1982, VAN HOOFF & VAN SCHAIK 1992) males may compete through physical combats in agonistic encounters, where body condition and body weight may play a key function in achieving dominance (BERCOVITCH & NÜRNBERG 1996). Dominant males have preferential access to limited resources such as sexual partners (WEST-EBERHARD 1975, WILSON 1975, CRAIGHEAD 1995). Preferential access to sexually receptive females often translates into greater rates of reproductive behaviour (SILK 1987). Greater reproductive activity, in turn, has been assumed to result in higher levels of reproductive success (COWLISHAW & DUNBAR 1991). But frequent mating in a relatively short time (e.g. when many oestrous females are available at the same time) may also result in a decreased quantity and quality of the ejaculate and hence in a decreased fertilisation rate (OLDEREID et al.
1984, AUSTIN & DEWSBURY 1986).
Relationships between male mating success and male social status have been extensively studied (review FEDIGAN 1983). Factors such as morphological traits, body condition and age may affect male reproductive success (SPRAGUE 1998). In older males it has been suggested that reproductive success may be influenced by possible reduction in fighting
ability, change in dominance rank, decline in reproductive function or through complex interactions among all these possibilities (TAYLOR et al. 1988, JONES & MENCH 1991).
Some tactics are differentially expressed according to age. The typical pattern for younger males is to adopt “sneak” or submissive tactics while older males are territorial or dominant (DOMINEY 1984). Grey seals with high mating success tend to be older males (GODSELL 1991). In a study of reproductive strategies in the promiscuous soay rams (Ovis aries), paternities per ram increased from juveniles to yearlings and then to adults (COLTMAN et al.
1999). The authors suggested that it may be difficult for young males to defend a female from competitors due to their small body size and limited previous breeding experience. On the other hand, ageing processes affect physiological functions and thus play an important role for their behavioural correlates. It is well recognised that sexual performance in men declines steadily from adolescence to old age (VOM SAAL et al. 1994). In macaques, reproductive suppression is more likely to arise from a failure to obtain access to sexually receptive females than to result from a disruption in endocrine profiles adversely affecting spermatogenesis (BERCOVITCH & GOY 1990).
As low-ranking males are disadvantaged in the access to resources or reproduction (SMUTS 1987), they may be able to compensate partly by forming long-term alliances (SMUTS 1985, NOE 1986, FEH 1999) or short-term coalitions (HARCOURT 1992). The value of male alliances with regard to mating success has been reported in a variety of primates (SMUTS 1987), although in general, alliances are less common in male than in female primates (VAN SCHAIK 1996). This is explained by sex specific differences in sexual selection (DARWIN 1871, EMLEN & ORING 1977): Females should compete mostly for food resources in order to rear their offspring successfully, but male reproductive success should be limited by the number of receptive females to mate with. Whereas food items can often be shared amongst coalition partners without any disadvantage or sometimes may only be exploited by a coalition (e.g. co-operative hunting in chimpanzees, BOESCH & BOESCH 1989), a fertilisation cannot be shared. Even in multiparous species where paternity may be shared in the case of multiple paternity (TEGELSTROM et al. 1991, STOCKLEY et al. 1993, SAY et al. 1999), it is still a reproductive disadvantage for a male when he did not sire all progeny of
a given female. Male kinship is assumed to contribute most to the development of male bonds (GOUZOULES & GOUZOULES 1987, HARCOURT & DE WAAL 1992).
If the number of associated females is too large to be monopolised and defended, multi-male/multi-female systems with a promiscuous mating pattern are more likely to evolve (CLUTTON-BROCK 1989). In a dispersed promiscuous mating system, intrasexual selection may be based on competitive mate searching (scramble competition, WELLS 1977) that is not necessarily correlated to body weight (SCHWAGMEYER 1994). Under these circumstances mate localisation abilities and sperm competition may be crucial for the reproductive success of individual males (FISHER & LARA 1999). Frequent and repeated copulations can be expected under these circumstances. This leads to the evolution of large testes in order to avoid sperm depletion and therefore to the prevalence of sperm competition (HARVEY &
HARCOURT 1984, PARKER 1984, MØLLER 1988). In promiscuous mating systems, factors such as spatial or temporal mating position of male (SCHWAGMEYER 1988, 1994), mating rates (AUSTIN & DEWSBURY 1985, GODFREY & LUNDSTRA 1989) or differential sperm quality (THOMSON 2000, WOONINCK et al. 1998) may bias a male’s fertilisation rate that may not be correlated with dominance rank.
Results purely based on behavioural observations of reproductive activity cannot be used for estimating reproductive success: The actual patterns of parentage may be inconsistent with the observed patterns of reproductive behaviour in mammalian mating systems, and genetic identification of effective breeders is required for the accurate determination of reproductive success (BIRKHEAD & MØLLER 1995; AMOS et al. 1995; FIETZ et al. 2000;
HUYVAERT et al. 2000). Therefore, long-term behavioural and genetic studies are necessary to assess differences in male breeding success (GIBSON & GUINNESS 1980, PEMBERTON et al. 1992, 1999, ALTMANN et al. 1996, COLTMAN et al. 1998, FIETZ et al. 2000, HUYVAERT et al. 2000, LEBAS 2001).
Long-term data are difficult to collect from organisms with reproductive lifespans which exceed the lifetime of most research projects, so that most longitudinal studies on age-related reproductive success concentrate on species such as fruit flies, Drosophila melanogaster (SERVICE & FALES 1993) or sandflies, Lutzomyia longipalpi (JONES et al. 2000). To examine changes in male reproductive success over time, paternity data can be classified by
the relative years in which the fathers and offspring were sampled. Complicated pattern can be expected if most males are sampled either early or late in their careers and if reproductive success changes with age (WORTHINGTON WILMER et al. 1999). Within non-human primates, few studies deal with age-related effects on male reproductive success due to the difficulty of collecting long-term data and the relatively low number of old animals (e.g.
AUJARD & PERRET 1998). In Barbary macaques, neither dominance rank nor sexual activity appeared to be affected by age (KUESTER et al. 1995), whereas prime-aged rhesus macaques (9 to 12 year old) enjoy a greater reproductive success than older or younger animals (BERCOVITCH 1997). To date, nothing is known about these processes in nocturnal primate species, due to difficulties in animal observations and data sampling.