Considerable controversy exists on the nature of the relationship between male dominance rank and reproductive activity. The nature of this relationship has important implications for understanding the manner in which males compete for access to limited resources. Behavioral data on mating patterns were collected over a four-year period from one social group of rhesus macaques on Cayo Santiago. Correlations between dominance rank and reproductive activity were not stable over a four-year period, but changed yearly. Positive, significant correlations were present in the first two years of the study while non-significant correlations were found in the second two years. The variation found in the correlations between rank and mating activity could be accounted for by changes in the mating frequencies of different classes of males. The long-term resident males had declines in ejaculation frequencies over the duration of the study. Males who immigrated into the group had yearly increases in reproductive behavior over three consecutive years. Maturing natal males also increased their levels of reproductive activity from year-to-year. Combining these mating patterns over time resulted in shifting the proportions of matings away from the long-term residents and in favor of the new males. High-ranking males had an advantage in reproductive activity over the first two years of the study, as measured by both the total number of ejaculations and the mean number of ejaculation per male. New males, comprised of recent immigrants and maturing natal males, had a greater level of reproductive activity over the last two years. These results suggest that the effect of rank on reproductive activity is variable and that males utilize alternative tactics to attain access to limited resources. Simple one-factor models explaining the relationship between rank and reproductive activity must be replaced with models explaining how alternate strategies affect male competition and reproductive success in primates.

Life history theory suggests that reproduction at one point in time involves costs in terms of energy, reduced survival, or probability of reproduction at a future point in time. In long-lived iteroparous organisms, initiating reproduction at a relatively young age may exact a cost in terms of reduced survivorship, but an early age of first reproduction could be beneficial if it lengthens the breeding lifespan. Data collected over 30 years from one population of rhesus macaques, Macaca mulatta, were analyzed to determine the fertility and survivorship costs of initiating reproduction at a relatively young age. Low population density and high social status increased the chances of accelerating age at first parturition, but high dominance rank was not associated with greater lifetime reproductive success. Rapid reproductive maturation neither reduced short-term survivorship nor decreased lifespan. Fertility costs arose if young females reared a male, but not female, offspring. The fitness consequences of rapid reproductive maturation depend upon longevity, with age at death having a significantly greater impact on lifetime reproductive success than age at first parturition.

Gene conversion and balancing selection have been invoked to explain the ubiquitous diversity of the antigen-presenting proteins encoded in the vertebrate major histocompatibility complex (MHC). In the present study, direct evidence for over-dominant selection promoting MHC diversity in primates is provided by the observation that, in a large free-ranging population of rhesus macaques, males heterozygous at MHC class II locus Mamu-DQB1 sired significantly more offspring than homozygotes (the male-specific selection coefficient s equals 0.34). This heterozygote advantage appeared to be independent of the actual male Mamu-DQB1 genotype. No similar effect emerged for a captive group of monkeys of similar genetic background but under veterinary care.

Researchers have explored the fitness consequences of female dominance hierarchies in many primate populations, with most studies highlighting differences in age of maturation, fertility, and offspring survival. We use resampling techniques and van Tienderen's (2000) elasticity path analysis to identify rank-related differences in finite rate of increase (λ) and their demographic correlates among segments of a semi-free-ranging rhesus macaque population. Higher-ranking population segments grew at greater rates for some portions of the 40-yr study period. The female members of these segments achieved these lifetime fitness differences through higher fertility and especially higher adult survival rates. This is the first clear evidence that social rank influences female primate adult survival, and is a crucial fitness component for any long-lived, slow-reproducing animal. Traditional methods of comparing lifespans, and other life history variables, among rank categories fail to identify most of the rank-related differences primarily because they require completed life histories that are available only on a small number of the females known in the population.

In many mammalian species, male reproductive success appears to climb sharply at young adulthood, form a brief plateau during prime ages, and decline among older animals, a pattern often attributed to reduced physical condition with ageing. However, solid evidence to either substantiate or refute this profile among nonhuman primates is lacking. Here, we combine a decade of genetic analysis of paternity among free-ranging rhesus macaques, Macaca mulatta, with information about body condition in order to evaluate how changes in morphology might govern age-specific reproduction among males. We show that age-specific reproductive success traverses the same life history profile as found in other mammals, but reductions in reproductive output with advanced age were associated with reduced chances of survivorship rather than accompanied by diminished body condition. We demonstrate that variance in male age at onset of reproduction is three times greater than variance in female age at onset of reproduction. We provide the first evidence from primates that age-specific reproductive output among males is not a consequence of age-related changes in body condition, but reflects social and demographic factors.